A Guide to Subwoofers (Part II): Standing Waves & Room Modes

[Big Daddy]

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THE SCIENCE BEHIND SUBWOOFER PLACEMENT

Prepared By: Big Daddy

For Part I of A Guide to Subwoofers, Click Here

BASIC FACTS ABOUT SOUND WAVES

Sound is a sine wave that travels through matter such as air, water, and steel. Sound does not travel in vacuum. A sound wave will spread out after it leaves its source, decreasing its amplitude or loudness. The relationship between speed, wavelength, and frequency is: Speed = Wavelength x Frequency.

The speed of sound in dry air is approximately 344 meters/second, 1127 feet/second, or 770 miles per hour, or one mile in 5 seconds at room temperature of 20°C (68°F) and sea level. Sound travels faster in liquids and non-porous solids than it does in air, traveling about 4.4 times faster in water than in air. The speed of sound in air varies with the temperature and humidity such that sound travels slower on cold days, but is nearly independent of pressure. The speed of sound is dependent on air (or any other gas) and is not dependent on the amplitude, frequency, or wavelength.

Longitudinal and Transverse Waves: Sound waves are an example of longitudinal motion and water waves are an example of a combination of both longitudinal and transverse motions. Here is a demonstration of longitudinal and transverse waves.

Longitudinal Waves


Transverse Waves


Water Waves
Animation courtesy of Dr. Dan Russell, Kettering University

Natural Frequency
A sound wave is created as a result of a vibrating object. Nearly all objects, when hit, struck, or somehow disturbed, will vibrate. When each of these objects vibrate, they tend to vibrate at a particular frequency or a set of frequencies. The frequency or frequencies at which an object tends to vibrate with when hit, struck, or disturbed is known as the natural frequency of the object.

The natural frequency of an object depends on the properties of the material the object is made of (this affects the speed of the wave) and the length of the object (this affects the wavelength). Since frequency = speed / wavelength, a change in either speed or wavelength will result in a change of the natural frequency.

Resonance
Assume that two tuning forks are mounted on a box and assume the forks have the same natural frequency (e.g., 256Hz). Suppose the first tuning fork is struck with a rubber mallet and it begins vibrating at its natural frequency of 256Hz. These vibrations set its sound box and the air inside the sound box vibrating at the same natural frequency of 256Hz. Surrounding air particles are set into vibrational motion at the same natural frequency of 256Hz. This sets the second fork into vibration with the same natural frequency. This is an example of resonance - when one object vibrating at its natural frequency forces the second object with the same natural frequency into vibrational motion.

Source: Glenbrook South School, Glenview, Illinois

Formation of Standing Waves
Sound reflects back and forth between two parallel surfaces. At certain frequencies the incident and the reflected sounds interfere to form “standing waves” in which those frequencies can be amplified, and what we hear at those frequencies depends on where we and the speakers are located. Standing wave are non-traveling vibrations (the sound waves themselves are not stationary and are continuously bouncing back and forth).

Let us consider the effect of a sound wave in a small room when it is reflected from the walls. A wave can be thought of as an upward displaced pulse (peak) followed by a downward displaced pulse (trough).The waves from the source and the reflected waves will cause destructive interference (positive cancels negative) in such a manner that there are points of no displacement (standing still). Such point are called Nodes (N). There will also be positions along the medium resulting from constructive interference that will vibrate back and forth between a maximum upward displacement to a maximum downward displacement. These positions are called Anti-nodes (AN). You can examine the following standing wave animations:

Animation courtesy of Dr. Dan Russell, Kettering University


Source: Glenbrook South School, Glenview, Illinois

Here is an animation showing the standing wave patterns that are produced on a medium such as a string on a musical instrument. This type of medium would be said to be fixed at both ends. Standing Waves For A Musical Instrument.

Fundamental Frequency and Harmonics
Resonance is a common cause of sound production in musical instruments. As was mentioned before, when an object is forced into resonance vibrations at one of its natural frequencies, it vibrates in a manner such that a standing wave pattern is formed within the object. These patterns are only created within the object or instrument at specific frequencies of vibration; these frequencies are known as harmonic frequencies. At any other frequency, the resulting disturbance of the medium is irregular and non-periodic (non-repeating). These standing wave patterns represent the lowest energy vibrational modes of the object. While there are countless ways by which an object can vibrate (each associated with a specific frequency), objects favor only a few modes or patterns of vibrating. The favored modes (patterns) of vibration are those which result in the highest amplitude vibrations with the least input of energy. Objects are most easily forced into resonance vibrations when disturbed at frequencies associated with these natural frequencies.

Consider a guitar string vibrating at its natural frequency or harmonic frequency. Because the ends of the string are attached and fixed in place, the ends of the string are unable to move. These ends become points of no displacement or nodes. In between these two nodes there must be at least one point of maximum displacement or anti-node. The most fundamental harmonic for a guitar string is the harmonic associated with a standing wave having only one anti-node positioned between the two nodes on the end of the string. This harmonic will have the lowest frequency and the longest wavelength. This is called as the fundamental frequency or the first harmonic of the instrument. Similarly, the instrument can produce harmonics at higher frequencies and are called second (two times the frequency of the first), third (three times the frequency of the first), and so forth. The following is the simulation of the four modes of a vibrating string.

Animation courtesy of Dr. Dan Russell, Kettering University

The frequency associated with each harmonic depends on the speed and the wavelength of the waves. The speed is affected by the properties of the medium (tension of the string, thickness of the string, material composition of the string, etc.). The wavelength of the harmonic is dependent upon the length of the string. Variations in either factor will result in variations in the frequency at which the string will vibrate. When two sound waves that are one octave apart (i.e., their frequencies have a ratio of 2:1) are combined, they will sound pleasant.

SUBWOOFERS AND THE EFFECT OF STANDING WAVES

Low frequency wavelengths are much longer (e.g., 56.5ft at 20Hz, 22.6ft at 50Hz, and 11.3ft at 100Hz) than higher frequency wavelengths (e.g., 3.8ft at 300Hz, 1.1ft at 1,000Hz, and 1 inch at 13,000Hz). This is important, especially below 150hz or so. Above 150hz, the waves are small enough that they are not affected by the room size as much. They bounce around every which way. Standing waves only become a significant problem at lower frequencies (below 100 Hz) because we normally set the crossover frequency around 85Hz. Long wavelength bass frequencies travel back and forth bouncing off the walls.

In general, at most frequencies, the decay of sound waves is rapid, but when a sound’s wavelength is precisely twice the size of a room dimension (e.g., length), the waves from both directions reinforce each other at the wall boundaries and cancel each other in the midpoint of these two boundaries, creating a resonant condition. Like most other resonant conditions, standing waves produce a fundamental tone (the lowest-frequency resonance the space will support) and a series of harmonics. If the fundamental frequency is 25Hz, there will be other, progressively weaker ones at 50Hz, 75Hz, 100Hz, 125Hz and so on. Each of these harmonics causes a high energy peak points in the room, with a null (low energy) midway between each adjacent pair of peak points.

Standing waves in a room are called room modes or room resonance modes. The crests (high points) of the standing waves and the troughs (low points) between them represent what happens when a single subwoofer generates the long wavelengths of bass. Those peaks and dips in bass energy do not change unless you change the dimensions (length, width, and height) of the room and the frequency of the bass tones. Even if you did alter these, you would be left with a whole new set of standing waves to deal with.

Type of Room Modes
The sound waves interact with the room boundaries (walls, floor, and ceiling) and create standing waves or room modes. The standing waves are different between floor and ceiling, side walls, and end walls, unless any of these dimensions are the same (the worst kind of room is a perfect cube). There are three basic types of modes: axial, tangential, and oblique. Examples of these modes are shown in the following diagrams:

Source: Marktaw.com

It is important to remember that these diagrams are over simplification. Remember sound waves will zig-zag around the room, and that sound sources are not directional like flashlights. A speaker is more like a bare lightbulb, or a light bulb in a box. Because of this zigzag, room modes are actually a range of frequencies centered around the number given in our calculations. In addition, tangential and oblique room modes are the most difficult to visualize.

To gain some understanding of the room modes and standing waves, it will be very helpful to consider a one-dimensional acoustic space like a long narrow pipe. If both ends of the pipe are closed, then it becomes similar to a one-dimensional room. Unless otherwise stated, all diagrams are created by Big Daddy.

Now position a subwoofer at one end of the pipe and connect it to a frequency generator. At the other end of the long pipe, put an SPL meter to measure the sound pressure. Start by feeding very low frequency signals to the subwoofer, you will notice no reading on the SPL meter. However, as you increase the frequency of the sound waves fed to the subwoofer, you will reach a point where the reading on the SPL meter jumps to a high point. This is the first mode and is called the fundamental resonant frequency or the first harmonic frequency of the one-dimensional room (pipe).

Continue raising the frequency of the signals and the meter drops back to normal for a while, but finally peaks again. This next frequency is evidence of the second resonance mode and is called the second harmonic frequency. The frequency of this second resonance will be exactly twice that of the first resonance. If we increase the frequency of the signal some more, we will find the third resonance mode which will have exactly three times the frequency of the first fundamental resonance mode. This harmonic series can continue as we increase the frequency.

If we move the location of the speaker or the SPL meter, we will get a new set of harmonic frequencies. However, if the subwoofer is moved to the middle of the pipe (low pressure zone), the odd numbered resonances will not be stimulated and will disappear. If we move the subwoofer to a position one third from either end of the pipe, only the third, sixth, ninth, and so on harmonics can be stimulated. If we move the speaker to the one quarter point of the pipe from either end, we will find only the fourth, eighth, twelfth, and so on harmonics.

In a closed pipe, which has been stimulated into its first resonance condition, we will find that the sound is very loud at either end of the pipe and very quiet at the halfway point, the middle. These loud areas are called sound “pressure zones”. If the subwoofer is placed in either of these pressure zones, it can pump up the resonant condition. However, if it is not placed in a pressure zone, it cannot pump up the resonant mode.

The second harmonic of a closed pipe has three pressure zones, one at either end and one in the middle. If we place the subwoofer in any three of these pressure zones, we will stimulate the second harmonic. However, if we place the subwoofer in the middle pressure zone, we cannot stimulate the first resonance but we can still stimulate the second one. Let us now plot the sound pressure as a function of distance, and remember that one wave moves from left to right and the other moves from right to left and polarity changes each time we cross a null.

Important Facts About Subwoofers, Listeners, and Standing Waves

• Subwoofers are sound pressure generators. They will reinforce the room modes when they are located in high pressure regions of the standing waves.
• Ears respond to sound pressure. When our heads are located in the high pressure regions of the standing waves, the room modes will be most audible.
• If the subwoofer is placed in the null areas, the corresponding modes will disappear.
• If you move your head to the null areas, you will not hear a lot of deep bass.
• Room dimensions and subwoofer location create room modes.

There is another factor that limits the remaining options for speaker placement. The pressure zones are spread out and not pinpoint-sized. For all practical purposes, the subwoofer should be located at least 25 percent away from the end of the pipe to best avoid stimulating any of its first three harmonics. There is no location towards the middle of the pipe that suits a subwoofer position, as the pressure zones there are overlapping.

Calculating the Resonance Modes of a Home Theater Room and Subwoofer Placement
Axial Modes are the strongest and the most important, and the easiest to compute. Tangential Modes are about half as loud, and Oblique Modes are about a quarter as loud. They tend to be the least important, but if an oblique room mode occurs near another mode, that frequency may still be a problem. It is best to calculate all room modes to see where any overlap may be.

A room can be approximated by three intersecting pipes. These pipes would lie along the three room axes: front to back, side to side, and floor to ceiling. For most rectangular home theater rooms, it may be sufficient to calculate only the axial modes of the room.

Since a room can enforce a wave twice as long as it is, the first fundamental frequency can be calculated by using the formula: Standing Wave Frequency = Speed of Sound / 2*Distance Between Boundaries. If we multiply this frequency by 2, we will get the second harmonic frequency and so on. Usually it is necessary only to look at the first three or four modes because the crossover frequency for most home theater rooms are set around 80Hz-100Hz. Let us now calculate the axial modes for a 15ft W x 20ft L x 8ft H room.

Width
The first resonance frequency: 1130ftps / 2x15ft = 37.7Hz.
The second resonance frequency: 37.7 x 2 = 75.4HZ.
The third resonance frequency: 37.7 x 3 = 113.1HZ, ignore, because it is above the roll-off frequency of 85Hz.
The subwoofer has to be placed at least 25 percent away from the wall (15x0.25=3.75ft) because of the first harmonic, but that is the point of minimum of the second harmonic. Therefore, the subwoofer can be placed anywhere between 3.75ft (minimum of the second harmonic) and 7.5ft (minimum of the first harmonic) away from either wall.

Length
The first resonance frequency: 1130ftps / 2x20ft = 28.3Hz.
The second resonance frequency: 28.3 x 2 = 56.6HZ.
The third resonance frequency: 28.3 x 3 = 84.9HZ.
Since all three harmonics are below the roll-off frequency of 85Hz, we should place the subwoofer in a position that avoids the maximum and minimum of the three waves – at least 25% (20 x0.25=5ft) from either end walls.

Height
The first resonance frequency: 1130ftps / 2x8ft = 70.6Hz.
The second resonance frequency: 70.6 x 2 = 141.2HZ, ignore, because it is above the roll-off frequency of 85Hz.
The third resonance frequency: 70.6 x 3 = 211.8HZ, ignore, because it is above the roll-off frequency of 85Hz.
The vertical position for a subwoofer is anywhere in the middle half of the room, keeping it at least 25% (two) feet away from either the floor or ceiling.

So, a 15ft W x 20ft L x 8ft H room will have the smoothest bass if the subwoofer is located 2ft from the floor or 2ft from the ceiling (6ft from the floor), between 3.75ft and 7.5ft from the side walls, and five feet from the end walls. This is done to avoid the coupling of the subwoofer to room modes.

Unfortunately, most people do not have an unlimited options with regard to the placement of a subwoofer in their living room, so the benefit from one of these calculations is limited. However, if one is building a dedicated home theater room, then one should pay more attention to these calculations.

Dr. Floyd Toole, formerly of National Research Council of Canada and currently a Vice President and researcher at Harmon International has developed a simple Excel Program to calculate axial room modal frequencies.

Here are other calculators: Calculator1, Calculator2, or Calculator3.

Also, see animations at the beginning of Post #2.

Location, Location, Location
The location of your subwoofer in the room creates the standing wave modes. And the modes are what determine whether your listening position gets great bass or poor bass. If your chair or sofa happens to be located in one of the troughs of the standing waves, you are not going to hear much deep bass. But if you get up and walk a few feet back, or to the left, or to the right, chances are you will hit one of the peaks and the bass will be very strong, perhaps too much of a good thing. An equalizer will solve some problems, primarily those related to peaks. A null is an entirely different situation and no amount of boost can fill a room-induced null. Think of it as a water drain. No amount of water can fill a drain.

The following analysis is based on the work of Dr. Floyd Toole. All diagrams are created by Big Daddy.

One Subwoofer
Let us consider the width modes. One subwoofer close to a wall is in the high pressure region of all the width modes and energizes all of them.

What happens if the subwoofer is moved to the location of the first pressure minimum (green minimum)? That particular mode is not energized and will disappear. What then happens if it is moved to the next null (magenta minimum)? That mode will disappears, but the other one returns. Subwoofer location determines which of the room resonances is activated, and which ones are not activated.

Optimum Position for One Subwoofer: If the subwoofer is placed in the wrong position in the room, we hear “room booms” instead of music. Bad speaker positions are those that allow the speaker to stimulate room resonance (modes). The best position for one subwoofer would be in the anti-mode region of all the room resonances.

Position of the Listener
Similarly, the location of the listener determines which modes will be heard. Just like a subwoofer against a wall energizes all room modes, a listener with his/her head against either wall hears all of the modes. There will be probably be too much bass and very tiring.

If the listener is moved forward to a null position, no sound will be heard from that mode. Different positions mean that different frequencies will be heard with very different loudness. This is true for subwoofers as well as listeners.

Other Techniques for Placement of One Subwoofer: Subwoofer Crawl Placement
This is an effective technique for the placement of a subwoofer. Play a CD or a DVD that has lots of low bass and move your subwoofer to the normal listening position. Go to the spot where you would like to place the subwoofer. Now sit down in that place and listen to the recording. If it sounds reasonably good in that position, go ahead and put your subwoofer there. If the bass does not sound good, try other available locations to find the location that provides the best sounding bass. Remember that moving the subwoofer as little as 6"-12" can have a noticeable impact on its performance. The following video by Axiom Audio demonstrates the subwoofer crawl technique.

Using an SPL Meter and Test Tones: It is more accurate to use an SPL meter and low frequency test tones. You can buy a digital or an analog SPL meter from Radio Shack for less that $50. If you do not know how to use an SPL meter, read the thread on Calibrating Your Audio with an SPL Meter, Sticky Under Receiver Discussion.

You can download free test tone generators from the following sites:
RealTraps - Test Tone CD
Test Tone Generator Free Download
Tone Generator Software - Create Audio Test Tones, Sweeps or Noise Waveforms

Similar to the listening subwoofer crawl technique mentioned above, start with the subwoofer located in the listening position and set the meter where you want to put the subwoofer. Set the SPL meter to 75dB setting and play the test tones from 20Hz to 120Hz in increments of 1/6 octave and write down the SPL levels. Move the subwoofer and repeat the experiment. Use an Excel worksheet and plot the SPL readings versus frequencies for different locations. Use these measurements to find the best possible placement. Of course this means having the flattest response.

Problems Associated with One Subwoofer and Standing Waves
With some care in placement of a single subwoofer and the listening location, one listener can experience fairly smooth and deep bass in a rectangular room. Unfortunately, other listeners seated elsewhere in the same room will hear different bass response, which may be significantly irregular. Trying to reduce some of the largest peaks (too much bass) at one or two frequencies is possible with careful placement and equalization for one location and one listener. But attempting to apply equalization for multiple locations is usually ineffective. There are far too many problems in a small home theater room that cannot be solved with one subwoofer. Using two subwoofers is preferable as you will get a better bass performance and will have less of a problem with standing waves, since the bass will originate from two locations.

MULTIPLE SUBWOOFERS

Considerable research has been done by Dr. Floyd Toole, Todd Welti, Sean Olive, Allan Devantier and others into the behavior of deep bass in different rooms. The following suggestions are based on their work.

Two Subwoofers Against the Opposite Walls Across the Width of the Room
Two subwoofer against opposite walls will cancel the odd-numbered modes, leaving only one active mode. In the diagram below, blue and green modes will be eliminated, leaving behind only the magenta mode.

Remember that the sound pressures on opposite sides of a null in a standing wave have opposite polarity. If one side is decreasing , the other will be increasing. If we use one subwoofer in a room, this does not matter. A single subwoofer against a wall will energize all the room modes. However, if we place another subwoofer with the same polarity against the opposite wall, the first and third modes will have opposite polarities at the subwoofer locations and the subwoofers will behave in a destructive manner, cancelling the odd-numbered modes. This will leave only the second (magenta) mode across the width of the room.

Two Subwoofer Move to the Null Position of the Second (Magenta) Mode
If we move the subwoofers to the null locations for the second mode, they will still be in opposite polarity regions for the odd-numbered (blue and green) modes, and as a result width modes are significantly reduced, if not eliminated. What this means is that everyone across the width of the room will hear the same bass sound.

What we have accomplished is the elimination of room modes by selecting different locations for the subwoofers and the listeners. We can do this for length, height and other mode types. The distribution of the room modes will look a lot less cluttered. We may still need to equalize in the sense of changing the frequency response of the system for everyone in the room to hear smoother and more uniform bass.

Watch this video on the benefits of using Multiple Subwoofers by Axiom Audio.

Multiple Subwoofer Placement (Rules of Thumb)
In most circumstances two subwoofers will perform better than one. While you might assume this is for added SPL, the greatest benefit will actually be smoother bass response. Two subwoofers are easier to place and result in a flatter frequency response and creation of a much larger “sweet spot” for everyone in the room to hear smoother and more consistent bass.

For maximum output, some experts suggest that you put a single subwoofer in a corner for maximum output and place a second one in a less reflective area to smooth out the response. You can use the “crawl around the room” technique as described above for determining the location of the second subwoofer, except in this case, look for the minimum amount of bass output.

Dr. Toole suggests that in a rectangular room you should put one subwoofer close to the front wall in the middle, and another subwoofer at the back of the room in the same relative position. THX recommends placing them in the middle of the left and right walls. Dr. Toole also recommends some equalization to flatten the bass response so that all the seats in the primary listening area hear solid and even bass.

For Better Results, Use Four Subwoofers
Four subwoofers were found to be most effective when two subwoofers are placed at the middle location of the front and back and two subwoofers at the middle location of each sidewall, opposite each other. Placing one subwoofer in each of the room’s four corners was also found to be similarly effective.

The 25% Subwoofer Positioning: This solution is suggested by Todd Welti at Harmon International: “You shrink the whole room by 25% and put the subwoofers at the corners of that virtual room. Of course you get incredible performance, but that is not practical for most people. But if you use two or four subwoofers in the corners or the wall midpoints, you can get pretty good performance.”

Cont'd in the next post.

[Big Daddy]

I exceeded the number of characters allowed in my original post. Therefore, I moved some of the information here.

http://www.acs.psu.edu/drussell/Demo...s/driving.html

Example: If the room has a length of 5m, then the resonance frequencies of the first five room modes are: 34.3Hz , 68.6Hz, 102.9Hz, 137.2Hz, 171.5Hz .

Response of the One Dimensional Room as a function of Source Location

In the animations below, the red dot represents the source as it moves from the left wall (at x=0) towards the right wall (at x=5). The sine curve in the animation represents the amplitude of the pressure wave as a function of position in the room.

Driving the room at a resonance frequency

As the source moves along the length of the room, the resulting room response (as indicated by the amplitude of the pressure standing wave pattern) is a maximum when the source is located at an antinode for that given frequency. Notice that when the system is being driven at a resonance frequency the pressure amplitude is always greatest at the walls of the room. Furthermore, the maximum pressure is twice (2) the source amplitude, because the incident and reflected waves are in phase at the wall boundaries. When the source is placed at a node for that frequency standing wave pattern, the maximum room response drops to zero at that frequency. With a speaker located at a node, the room will be "dead" no matter how loud the speaker is. Moving the speaker a small distance away from the node brings back the room's response

Mode 1 (34.3 Hz)



Mode 2 (68.6 Hz)



Mode 3 (102.9 Hz)

Animations courtesy of Dr. Dan Russell, Kettering University

Driving the Room at a Frequency which is Not a Resonance Frequency

Standing wave patterns only occur when the room is being driven at a resonance frequency. At any other frequency, the pressure waves radiating outwards from the source reflect from the walls, but do not combine to produce a standing wave. As a result, there are no nodes and antinodes, and the pressure can go to zero at a wall. The maximum pressure never exceeds the source level (1) and the location of the pressure maximum moves with the source.

Driving frequency = 51 Hz

Animations courtesy of Dr. Dan Russell, Kettering University

Even Number of Subwoofers Works Best
Two or four subwoofers deliver the greatest benefits in smoothing out irregular bass for multiple listening seats. According to Todd Welti, an associate of Dr. Toole’s, whose white paper, "Subwoofers: Optimum Number and Locations," explored in detail multiple subwoofer performance: “The conclusion I came to was that two subwoofers give you about 90% of the performance that is possible, and that four take you about as far as you can reasonably expect to go. Anything more than four is not going to get you much in the general sense -- and these are general conclusions.”

DIFFERENT OPTIONS FOR PLACEMENT OF MULTIPLE SUBWOOFERS

In a square or rectangular room, the center of the room is the worst location for the listening chair or for the subwoofer. I am not an artist. The following diagrams only demonstrate the best positions for the subwoofers. Ignore the position of the listening chair. That is just bad drawing.

Diagrams Created By Dig Daddy

Stacking Two Subwoofers Versus Placing Them in Different Locations
If you stack (co-locating) two subwoofers on top of each other and place them in a corner, you will get up to 6dB's of additional output. If you place the two subwoofers in different locations (e.g., the middle of the two side walls or one in the middle of the front wall and the other one in the middle of the rear wall), you may not get as much additional output, but two subwoofers on two sides of the room will eliminate many room modes and create a more even and smoother bass across the room for all listening positions. The advantages of placing two subwoofers in different locations far outweigh any additional output that you may gain from stacking them on top of each other. Read THIS interesting post on stacking subwoofers. There is more information on stacking HERE.

MULTIPLE SUBWOOFERS

Todd Welti at Harman International:

You shrink the whole room by 25% and put the subwoofers at the corners of that virtual room. Of course you get incredible performance, but that is not practical for most people. But if you use two or four subwoofers in the corners or the wall midpoints, you can get pretty good performance.

Under this solution, low frequency support is compromised a bit in mid-wall placement, in favor of a flatter overall frequency response.

Dr. Floyd Toole:

That said, no matter how many subwoofers and how many listeners we’re talking about, equalization should be the final step to make it sound right. A single subwoofer can entertain a single listener with equalization -- good sound is possible. But once you have more than one listener, then you need multiple subwoofers.

Based on his experiments with subwoofer placement, Dr. Toole recommends that it is easier to place subwoofers in corners and equalize out the room modes, than to try and avoid the modes and sacrifice low frequency support in favor of a flatter overall response without EQ. Corner placement adds to the overall SPL of the subwoofer without placing any additional demands on the amplifiers.

Dr. Floyd Toole:

When a full-range signal is panned to each of the loudspeakers in turn, and measurements are made at the listening position, we find hugely different bass responses for each of the loudspeakers. The differences are a large as 40dB in this room, and the biggest ones are all at low frequencies. The reason, the woofers each have very different acoustical coupling to the room resonances because they are in different locations. This will be different for every different room. Again, referring back to the circle of confusion the bass that was heard in the control room will not be the same as that heard at home. It cannot be.

Use Identical Subwoofers
As a general rule, if you are using multiple subwoofers, it is best to use identical ones from the same manufacturer. Although using different subwoofers may work, equalization, level matching, and adjustments become much more difficult. In all the research on the use of multiple subwoofers, the experts have used identical subwoofers.

The Science Behind Identical Subwoofers: Assume we have a device with several metallic poles with a bob attached to them. The bobs are colored red, blue, and green. Each pole has a different length, thus having a different natural frequency of vibration. When the red bob is disturbed, it begins vibrating at its natural frequency. This in turn forces the attached bar to vibrate at the same frequency; and this forces the other attached red bob into vibrating at the same natural frequency. This is resonance - one bob vibrating at a given frequency forcing a second object with the same natural frequency into vibrational motion. However, the green and blue bobs would not resonate. This is because the frequency of the red bobs share the same natural frequency. The result is that the second red bob begins vibrating with a huge amplitude. Identical subwoofers will have the same natural frequency and can work together a lot easier to generate smoother low frequency sound.

Source: Glenbrook South School, Glenview, Illinois

The following experts recommend using identical subwoofers:

Read Robert Harley's Books. He is the editor of the Absolute Sound Magazine. Robert Harley, Introductory Guide to High-Performance Audio Systems & The Complete Guide to High-End Audio, 558 pages. Also read Richard Hardesty’s articles in Audio Perfectionist.

Polk Audio: Polk Audio - Education, FAQ & Advice, Technology Overviews and Technical White Papers

Using two asymmetrically placed subs will minimize the effects of standing waves in your room, yielding smoother bass response as well as better dynamic range. If using two subwoofers, you must use the identical model of subwoofer. If two different models are used, even from the same manufacturer at some points they will help each other, at others they will fight each other causing a uneven response.

Subwoofer Connection Guide For A Multi Subwoofer System — Reviews and News from Audioholics

We get a lot of questions about what types of subs to purchase for a multiple sub setup. When choosing the right subwoofers for your system, it’s a good idea to choose identical subs for optimal cancellation of room resonances, or ones of similar output level and design. Don’t for example mix and match a high quality 15” servo subwoofer with a cheap dual 6” bandpass sub that came with your “cubed” speaker system. Implementing an inferior subwoofer with a good one will limit your systems dynamic range and bandwidth to the weaker sub making your ears focus on the one that is breaking up and running out of steam. Mixing different subs will also reduce the success of canceling out room resonances since they will exhibit different amplitude and phase responses. Always select two well designed subwoofers (preferably the same) that are each in their own capable of filling your theater room with ample bass without bottoming out or running out of gas. If you can’t afford two subwoofers at the moment, buy one quality sub for now and add a similar capable one down the road when you’ve got the cash.

http://www.rivesaudio.com/files/spkr_plcmt.pdf

Two subwoofers are easier to place and result in a flatter frequency response in almost all situations. If you can afford a second matching subwoofer, this is generally preferred to a single more expensive subwoofer. You will almost always achieve a flatter frequency response and a more realistic overall integration with the main system.

http://pdf.outlawaudio.com/outlaw/docs/lfmmanual.pdf

In most circumstances two subwoofers will perform better than one. While you might assume this is for added SPL, the greatest benefit will actually be smoother bass response. Two properly positioned subwoofers will distribute the bass throughout the room with greater accuracy than a single sub. If near perfect bass response is your goal, consider using two LFM Series Subwoofers: the Outlaw Audio Scattered Subwoofer Systems.

Wendy Carlos Surround1

In this case I made a trade off for two smaller subwoofers instead of one larger one. With careful A/B comparisons I learned that the bass was nearly the same when the two smaller units were working together as a team as with the single larger unit. But there was, contrary to what I had read, a small amount of additional directionality present with the two subwoofers compared to one.

Sir Terrence, Audio Insider at Blu-Ray.com:

The main reason for using identical subwoofers would be identical performance irrespective of room acoustics. With two identical subwoofers, there is a much smaller chance that one will overdrive before the other. If two different size woofers in two different subwoofers are used, the smaller one would have more distortion, more cone motion, and a subtly different sound than the larger one at higher volumes. Even if you use two identical woofers with a different cabinet size, amplifier size, and a different lower frequency limit, one will reach its peak output at a different volume than the other. You want all of your subwoofers to have identical performance to prevent one from dragging another down at high volumes. That is why two identical subwoofers from the same manufacturer are preferred to two different ones.

ADDITIONAL TOPICS

Forced Vibration
If you were to take a guitar string and stretch it to a given length and a given tightness and pluck it, you would hear a noise that you can barely hear. On the other hand, if the string is attached to the sound box of the guitar, the vibrating string is capable of forcing the sound box into vibrating at that same natural frequency. The sound box in turn forces air particles inside the box into vibration at the same natural frequency as the string. The entire system (string, guitar, and enclosed air) begins vibrating and forces surrounding air particles into vibrational motion. The tendency of one object to force another interconnected object into vibrational motion is referred to as a forced vibration. This causes an increase in the amplitude and thus loudness of the sound. A louder sound is always produced when an accompanying object of greater surface area is forced into vibration at the same natural frequency.

Combining waves
If you are listening to waves from two sources at the same time:

1. A high pressure from one will cancel out a low pressure from the other.
2. Two high pressure waves will reinforce each other.
3. Two low pressure waves will reinforce each other.

When two or more waves with the same frequency reach the ear, the ear interprets these waves as one wave with amplitude as big as the sum or difference of the initial waves. Two sound waves sound good when played together, if one sound has twice the frequency (one octave higher) of the other.

Transient Response
In physics: a short-lived oscillation in a system caused by a sudden change of voltage or current or load.

Transient response: The ability of a component to respond quickly and accurately to changes. Transient response affects reproduction of the attack and decay characteristics of a sound.

Echo and Reverberation
When a wave reaches the boundary, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary. Reflection of sound waves off of surfaces can lead to either an echo or a reverberation. A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Why the 17 meters? The human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations. Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 seconds for a sound to travel the length of a 17 meter room and back.

Reflection of sound waves also lead to echoes. Echoes are different than reverberations. Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard. In this case, the arrival of the second sound wave will be perceived as a second sound rather than the prolonging of the first sound. There will be an echo instead of a reverberation.

ADDITIONAL INFORMATION AND REFERENCES

http://www.harman.com/EN-US/OurCompa...ndRoomsPt1.pdf
http://www.harman.com/EN-US/OurCompa...ndRoomsPt2.pdf
http://www.harman.com/EN-US/OurCompa...ndRoomsPt3.pdf
http://www.harman.com/EN-US/OurCompa...s/multsubs.pdf
http://www.harman.com/EN-US/OurCompa...ions/13680.pdf
http://www.harman.com/EN-US/OurCompa...dioScience.pdf
http://www.harman.com/EN-US/OurCompa...dioScience.pdf
http://www.aes.org/tmpFiles/elib/20080825/13680.pdf
Sound & Vision Magazine - Why You Need Four Subwoofers
Sean Olive
http://www.audioperfectionist.com/PD...journal2rl.pdf
ASC Home Theater Acoustics
http://www.glenbrook.k12.il.us/gbssc.../soundtoc.html
Acoustics and Vibration Animations
Speed of Sound
onhometheater.com - Features Archives
http://www.school-for-champions.com/science/sound.htm
ESP - Frequency, Amplitude and dB
Subwoofer Placement For Deep Bass Nirvana
http://www.highemotionaudio.com/whiteppr.pdf
Acoustics Crash Course 1 - Modes
SubwooferSetup and EQ page 2
http://www.harman.com/about_harman/t...eadership.aspx
An Easy Solution To Subwoofer Calibration — Reviews and News from Audioholics
Subwoofer Placement - The Place for Bass Part 1 — Reviews and News from Audioholics
Crawling for Bass - Subwoofer Placement — Reviews and News from Audioholics
http://www.audioholics.com/tweaks/ge...or-bass-part-1
http://www.audioholics.com/tweaks/ge...-bass/two-subs
http://www.audioholics.com/tweaks/ge...-part-1-page-4
http://www.audioholics.com/tweaks/ge...-calibration-1
http://www.baudline.com/erik/ht/news...positions.html

[Big Daddy]

http://www.klausaudio.com/subwoofer-...slot-ports.php

Slot versus Flared Round Ports
Many people question what the difference is between flared ports and slot ports. There are fairly basic differences between the two, and depending on your situation, one might suit you better than the other. The common purpose of the two ports is that they must be the correct length for the particular subwoofer box to achieve the desired sub box tuning. There are a few differences that may help in your decision between flared and slot ports for your custom subwoofer box.
Flared ports are very good at virtually eliminating port noise. Port noise is the noise created by the friction of air moving through the port. If you have sharp edges on a port (as with standard round ports, which we do not use), you are going to have significantly more port noise than with a flared port. A flared port requires the smallest acceptable port area for a subwoofer box. Since the ends are flared, and the inside of the port is round (no sharp edges), it is unlikely you will be able to notice any port noise from the subwoofer enclosure. However, if you are running a large, high-powered subwoofer, you will want to use two flared ports. A single flared port is still capable of creating port noise when very large amounts of air pass through it.

With a slot port, you can use as much port area as needed for your application, which is especially helpful in SPL applications. A slot port requires more port area than a flared port to minimize port noise. We generally use a rule of 12 square inches of port area per cubic foot of internal volume of the custom sub box. You may go with a larger port area, but this will increase the total volume and length of the port. In turn, a large port area will mean an increase in the total size of your subwoofer box. Slot ports are the standard for our ported custom subwoofer boxes.

If you are not limited on space, we would recommend going with a slot port for your subwoofer box. If space is tight, and you have a few extra bucks, you'll probably want to use a flared port (or two, depending on your subwoofer size and power). It is possible to achieve the same result from both subwoofer boxes, so the decision between flared ports or slot ports depends on your specific needs for your custom subwoofer box.

http://www.klausaudio.com/subwoofer-...ort-tuning.php

Port Tuning
Tuning of a ported subwoofer box is determined by a combination of port area, port length, and net volume of the subwoofer box. Tuning changes the frequency at which the frequency response peaks (is loudest) and can change the way a subwoofer box sounds in your setup. Your tuning choice will be determined by the goal of your system install.

If you're looking to achieve an SPL setup your car (getting as loud as possible), you'll want to tune fairly high. High tuning for SPL is usually somewhere around 45Hz or possibly higher. At this high of tuning, the sound quality will not be very good, but it will be louder than if you were to tune low.

A SQ (sound quality) based setup will call for a tuning fairly low - even down around 25Hz. This will also allow you to hit the ultra-low notes with your system. Lower-tuned boxes produce better sound quality, more like a sealed box. The downside to lower tuning is that it flattens the frequency response, which will cause the box to lack the boost around the tuning frequency.

To achieve a good mix between SPL and SQ with a ported box, we have found that it's good to be in the lower 30's for tuning. If you tune up around 35Hz, you'll get a fair amount of SPL out of the box and still have fairly good sound quality. If you tune closer to 30, the box will yield better sound quality but have a little less output than the 35Hz. A range between 30 and 35 Hz is generally good for most general setups.

If you want SPL, tune high. If you want SQ, tune low. If you want somewhere in-between the two, we have found a good tuning is right at 33Hz. This is why we default the port tuning to 33Hz on our customization pages for our ported enclosures. When choosing a ported subwoofer box, the freedom to tune the enclosure is a great way to get your system sounding the way you want it to sound.

http://www.klausaudio.com/subwoofer-...vs.-ported.php

Sealed vs. Ported
When choosing a subowofer box, the decision between sealed and ported will determine how the final install will sound. The general rule for this choice is fairly simple.

Sealed subwoofer boxes generally are much smaller than ported enclosures. If you're limited on space, it would probably be a good idea to go with a sealed enclosure. Not only is there a size difference, but there is also a difference in sound between the two enclosures. Sealed enclosures tend to produce tight, accurate bass and have a flat frequency response curve. They are also generally the enclosure of choice when looking for a SQ (Sound Quality) oriented setup.

Ported boxes can get very large and have many varying factors that will determine their size and sound output. Ported enclosures produce louder bass than sealed enclosures, and allow you to tune the box to a specific frequency to determine how the bass will sound. Higher tuning on a ported box will get louder, but at the expense of sound quality. Lower tuning will still get louder than a sealed box, and at the same time will yield fairly good sound quality.

Port area and port type also plays a role in ported enclosures. Too little port area for the subwoofer box can result in port noise, which can severely detract from the sound quality of your setup. To much port area can also have a negative impact on the sound quality. However, if your goal is to get loud and not care about sound quality, then a ported box, tuned high, with a large amount of port area is the way to go.

Personal preference and size limitations will both play a role in the decision between sealed or ported enclosures. If you have the room, and you want to get loud, go with a ported box. If you're limited on space or are looking for the best sound quality possible, it would generally be best to go with a sealed box. Either way, as long as the specifications of the subwoofer box are within the subwoofer manufacturer's recommended box specifications, you should be safe with either choice.

In conclusion, I need to make a few general statements:

• We need to distinguish between a subwoofer's output (SPL) and its sound quality. People with a background in car audio tend to be interested mainly in SPL, while people interested in home audio and home theater applications tend to care more about sound quality.
• Our ears are more sensitive to upper bass frequencies than the lower bass frequencies. We generally feel the ultra low bass frequencies. We do not hear them.
• In most averaged-sized rooms, upper bass (mid-bass) is best reproduced when the subwoofer is near the listener.
• The ultra low bass is most efficiently reproduced when the subwoofer is in the corner of the room and not near the listener. See the following diagram.
  
  
  
  
  
  
• When a subwoofer is placed near the listener, the high direct to reflected sound ratio of nearfield placement helps to reduce room effects, will have excellent headroom (high output with relatively low power), and low distortion.
• A subwoofer driver that is optimized for low bass reproduction is not necessarily the best for mid-bass to upper bass reproduction.
• A driver with a heavy cone material is best for low bass, but this type of driver will have reduced mid to upper bass efficiency.
• A subwoofer driver with a very light cone material, a low inductance voice coil, and a strong magnet will have superior transient response with high efficiency (relatively low power will be needed).
• Also, let us not to ignore the effect of the cabinet on a subwoofer's performance, but that is another story.

Having said all of that, manufacturers specializing in HT equipment do not make large (15" or 18") subwoofer drivers to optimize their mid-bass to upper bass performance. They tend to do that with smaller drivers. We can therefore conclude that as a general rule, large drivers are better in generating ultra low bass frequencies, but they may not be as good in generating the upper bass frequencies. In that respect, 12" drivers are considered to be better than 15" drivers.

[maseo503]

Wow! Thorough post, BD. Quite the information overload... but worthy effort in deed. Good to see The Speaker Company listed in subwoofers, often overlooked company offering a lot of value for their products. I would request that Home Theater Direct (HTD) also be listed. http://www.htd.com/cabinet-speakers/powered-subwoofers

[Big Daddy]

Originally Posted by maseo503

Wow! Thorough post, BD. Quite the information overload... but worthy effort in deed. Good to see The Speaker Company listed in subwoofers, often overlooked company offering a lot of value for their products. I would request that Home Theater Direct (HTD) also be listed. http://www.htd.com/cabinet-speakers/powered-subwoofers

I added Home Theater Direct (HTD) to the list of subwoofer manufacturers. Thank you.

[welwynnick]

Magnificent effort to explain in plain English one of the most common stumbling blocks to understanding audio.

For completeness I'd like to comment on this though.

In general, at most frequencies, the decay of sound waves is rapid, but when a sound’s wavelength is precisely twice the size of a room dimension (e.g., length), the waves from both directions reinforce each other at the wall boundaries and cancel each other in the midpoint of these two boundaries, creating a resonant condition. Like most other resonant conditions, standing waves produce a fundamental tone (the lowest-frequency resonance the space will support) and a series of harmonics. If the fundamental frequency is 25Hz, there will be other, progressively weaker ones at 50Hz, 75Hz, 100Hz, 125Hz and so on. Each of these harmonics causes a high energy peak points in the room, with a null (low energy) midway between each adjacent pair of peak points.

As you say, sound waves are longitudinal travelling waves, where the wave perturbation is a small displacement in the direction of travel (unlike water waves or light waves for example).

Standing waves are caused by reflections from a discontinuity in the medium. A wall is a discontinuity, simply because it’s solid and stationary, and doesn’t allow the air at the boundary to move in the direction it wants to go - in a perpendicular direction to the wall.
There are other types of discontinuity, but this one is a node because there can be no amplitude at the boundary (because the wall doesn’t move, so neither can the air right next to it). Simple, huh?

By contrast, the middle of the room may be an anti-node, because there is nothing stopping the standing wave from having its maximum displacement there. Equally, for higher order standing waves it MAY still be a node, but that just depends on the geometry of the standing waves – it can be either or anything in between.

However, the wall is always a boundary that forces the standing wave to a node. In the first resonant mode, the room will be a half-wavelength across, with nodes at the walls and an anti-node in the centre, where the amplitude is highest. In this situation, all the air in the room is rushing up and down the room altogether – all in phase – all the air molecules start and stop together and move in the same direction. The mass of the air is simply bouncing against it’s own compliance. Of course, there will usually be other resonant modes in other directions as well, but that’s basically what happens.

BR, Nick

[jaejw1]

great read..

[Big Daddy]

Originally Posted by jaejw1

great read..

Thank you. I spent a couple of months researching the material, creating the graphs, and putting the information in Posts #1 & 2 together. When I hear that someone found the information useful, it makes me very happy.

[crazyBLUE]

Well I just learned Quit A bit from the first 2 posts ! Thanks Big Daddy for the information that explains questains I had on A couple of different things pertaining to multiple (4) subwoofers .

[sheedoe]

I cannot remember where, but I've read somewhere that cylinder subwoofer does not produce standing waves. Is this true?

[Underdog Audio]

WOW! I must say your reputation certainly does precede you Big Daddy. I heard about you on the AVS Forum, but I ran into this post looking for the reasons why my dual subwoofer placement has fixed my issue with my bass. I had both of my subwoofers at the front of the room and was pretty disappointed with the frequency response I was getting in the very low end from like 35Hz to 20Hz. I had just moved into a house with a totally new room and it's kinda been a nightmare just trying to tame the modal ringing from my front speakers because they're so close to the wall. After I took care of that will some simple bass traps in the front corners of the room I found that all of my low bass was heard only in the back right corner of the room. I took one of my subwoofers and put it in the spot where I could hear all the bass and now I can hear bass that starts at 17Hz! Even though I was glad that I had remedied the problem, I wanted to know why. After reading your post it all makes sense now.

[tacanmj]

I'm curious to know if my set up is ideal for proper bass management. I would take any advice. I set the towers to large and Xover to 80. Reciever set to LFE+

[nikostis69]

Hello! This thread is really awesome!

I have added a sub recently to my PC audio setup in my room. The sound within the room is as I want it to be.

There is a side effect, though... Due to the room acting like a huge speaker cabinet, the LF exit the room to the rest of the house quite amplified.

Is there any way I can reduce this effect without affecting sound experience inside the room?

[Mike7300]

Originally Posted by tacanmj

I'm curious to know if my set up is ideal for proper bass management. I would take any advice. I set the towers to large and Xover to 80. Reciever set to LFE+

I could be wrong but i would think you want your towers set to small if you have a subwoofer hooked up so that the sub is handling the bass.

[ChromeJob]

Originally Posted by tacanmj

I'm curious to know if my set up is ideal for proper bass management. I would take any advice. I set the towers to large and Xover to 80. Reciever set to LFE+

Originally Posted by Mike7300

I could be wrong but i would think you want your towers set to small if you have a subwoofer hooked up so that the sub is handling the bass.

He's got his AVR set to "LFE+" which is sending bass to the sub and the mains.

This is a relatively uncontrolled duplication of bass creation, IMHO not "ideal for proper bass management." Set the mains to "small," set crossover to 80Hz or 100Hz (lower if your mains can handle low bass consistently and accurately), then turn LFE+ off. You can get test files at audiocheck.net to determine any nulls in your mains' response, etc. When set up well, I believe you should hear a nice smooth descending tone from all the speakers without a clear dip or boost anywhere near the cross over point.*

Originally Posted by nikostis69

Hello! This thread is really awesome!

I have added a sub recently to my PC audio setup in my room. The sound within the room is as I want it to be.

There is a side effect, though... Due to the room acting like a huge speaker cabinet, the LF exit the room to the rest of the house quite amplified.

Is there any way I can reduce this effect without affecting sound experience inside the room?

Your sub is clearly interacting with the structure. Try doing the "subwoofer crawl" (Google for it) and determine the ideal location for the sub, then place it there, tune it down so that it's blending in with the other speakers but not resonating with the walls. *

* There are several articles on how to do this stuff at audioholics.com. Some are old (8 years) but still relevant.

[Driver_King]

Originally Posted by Big Daddy

For maximum output, some experts suggest that you put a single subwoofer in a corner for maximum output and place a second one in a less reflective area to smooth out the response. You can use the “crawl around the room” technique as described above for determining the location of the second subwoofer, except in this case, look for the minimum amount of bass output.

Dr. Toole suggests that in a rectangular room you should put one subwoofer close to the front wall in the middle, and another subwoofer at the back of the room in the same relative position. THX recommends placing them in the middle of the left and right walls. Dr. Toole also recommends some equalization to flatten the bass response so that all the seats in the primary listening area hear solid and even bass.

Currently, I have my Elemental Designs A2-300 subwoofer near the middle of the front wall about 6" or so away from the wall. Similarly on the rear wall, I have my DIY subwoofer squeezed between a dresser and my nightstand about 3" away from the wall. The DIY sub is not completely aligned with the sub near the front wall. After reading the quoted section above, I saw some experts suggest that having one subwoofer along a less reflective area (the middle of the front wall for me) and then another subwoofer elsewhere in a corner would be the best. I can move my DIY subwoofer the the back right corner of my room beneath the right back surround speaker. Would I be better off moving the DIY subwoofer to the corner or leaving it in its current spot, which is also an ideal spot?

[Big Daddy]

Originally Posted by Driver_King

Currently, I have my Elemental Designs A2-300 subwoofer near the middle of the front wall about 6" or so away from the wall. Similarly on the rear wall, I have my DIY subwoofer squeezed between a dresser and my nightstand about 3" away from the wall. The DIY sub is not completely aligned with the sub near the front wall. After reading the quoted section above, I saw some experts suggest that having one subwoofer along a less reflective area (the middle of the front wall for me) and then another subwoofer elsewhere in a corner would be the best. I can move my DIY subwoofer the the back right corner of my room beneath the right back surround speaker. Would I be better off moving the DIY subwoofer to the corner or leaving it in its current spot, which is also an ideal spot?

Most experts believe the middle of the side walls or the middle of the front and rear walls are the best positions. However, rooms and the boundaries are different and your listening position also affects the performance of the subs. You can move the DIY sub to a corner and see what kind of results you will get. The corner is not exactly a less reflective area.

I have my two Hsu subwoofers in the middle of the side walls, the DIY subwoofer in the middle of the rear wall and the AA sub in the rear corner. I prefer to move it to the middle of the front wall, but there is absolutely no room. The results are very good and the bass is fairly even, but there is always the possibility of phase cancellations. Your next item on the agenda should be an equalizer like the Velodyne SMS-1.

Did I tell you that ED is building me a big box for my 15" DIY subwoofer?

[Driver_King]

Recently, I re-ran Audyssey do to some older settings that bottomed out the DIY's driver too fast since the settings were very old and were made when I calibrated the old DIY's subwoofer driver and old amplifier (which has since been replaced with a significantly better driver and amplifier). I found that to get results that are even remotely close to what they used to be, I've had to go through loads of settings and controls to try to get it back to where it was before. Before, I had extreme low end impact (10-30hz range used to be much more audible). Although I have much more headroom and a much more even frequency response, I think that the corner with the much improved settings and modifications will yield even lower frequency response and possibly better frequency response in the corner. I will try this. Will I need to make any settings modifications like phase if I move the sub to the corner?

[Big Daddy]

Originally Posted by Driver_King

Recently, I re-ran Audyssey do to some older settings that bottomed out the DIY's driver too fast since the settings were very old and were made when I calibrated the old DIY's subwoofer driver and old amplifier (which has since been replaced with a significantly better driver and amplifier). I found that to get results that are even remotely close to what they used to be, I've had to go through loads of settings and controls to try to get it back to where it was before. Before, I had extreme low end impact (10-30hz range used to be much more audible). Although I have much more headroom and a much more even frequency response, I think that the corner with the much improved settings and modifications will yield even lower frequency response and possibly better frequency response in the corner. I will try this. Will I need to make any settings modifications like phase if I move the sub to the corner?

You will definitely need to adjust each sub separately. If you have an SPL meter, adjust the amp level of each sub first by turning off the other one so that each subwoofer gives you approximately the same level. You don't want one subwoofer to overpower the other one.

Next, you need to adjust the phase of either subwoofer to get the maximum output at your listening pposition.

After that, run Audyssey for multiple positions and tweak a little with an SPL meter.

[Driver_King]

Oh my god. Big Daddy, you are the man. I put the subwoofer in the corner and went to adjust some tone levels and settings to turn it the hell down! I was playing some Frequency Sweeps and started to sweat when it went down to the 10hz range!! I think the tuning of the DIY subwoofer is probably 16 or 17 hz with all of the Poly-fill in it. I can't believe what I'm hearing. I am truly terrified of the subwoofers now. Before, the bass was drowning out the other speakers and it was just too high for my tastes. Now I need to make sure to turn that volume down! I have been listening to music at well over 0dB on my system and going that high now will hurt your ears! Huge difference! I checked my SPL meter and I'm still averaging 75dB on the receiver test tone. I'm worried about tweaking any more right now because it might get better! I feel like Chevy Chase in National Lampoon's Christmas Vacation right now where he's on a rant about his boss.