If we all lived in a perfect world, there would be no need for equalization, room correction, room treatment, bass management, crossover settings, level adjustment, etc. Sound systems would be perfect and would reproduce perfect signals in all rooms. Also, everyone had perfect hearing and would like exactly the same amount of bass, midrange and high frequency, and there would be no such thing as standing waves, feedback, and phase cancellation. Unfortunately, the world of home theater audio is not perfect. The main purpose of this thread is to concentrate on why we need room correction and equalization. Bass management, crossovers, and room treatment are covered in other threads.
How Do Equalizers Work?
Imagine the frequency range of the sound you hear as a highway with 10 lanes. Each of these lanes represents a single octave of the sound wave spectrum. The first five lanes, labeled 31.5Hz - 500Hz contain the really low frequency sound content, mainly bass, bass vocals, and the kick and tom drums. Most musical instruments and male and female vocals fall in the next three lanes labeled 1kHz, 2kHz and 4kHz. The higher 8kHz and 16kHz frequency range cover the cymbals, snare drums, and percussion instruments.
Octave Bands
One-Third Octave Bands
Under normal circumstances, the traffic in the lanes are not even. Some lanes are ahead of the other lanes, a few by a significant amount and a few by a small amount. Out of nowhere a cop (equalizer) appears in front of the lanes and the traffic is equalized like magic. All the lanes will be perfectly equal. If they try to get ahead in a lane (octave), the cop will give them a ticket (slow down the lane). When properly applied, equalization makes it possible to hear all of these frequency ranges equally, or as closely as the sound engineer intended for them to be heard.
In general, there are several reasons why we need careful placement, room treatment, bass management, calibration, and equalization:
Speakers are not perfect
Flaws in their design
Even speakers with perfect +/-3dB frequency response are different
On-axis and Off-axis response is significantly different
Interaction between frequency waves with the boundaries, furniture, and other obstacles
The interaction between lower frequencies and boundaries causes the most problem
Inappropriate positioning of speakers
Standing Waves & Room Modes
Overtones & Undertones
Comb Filtering and interaction with one another
Frequency Masking
Feedback in recording studios
Equalization can restore old recordings
Protection from loud music
Problems with human hearing
Equal loudness contours
Different preferences
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Speakers are not perfect
Flaws in their design
There are many reasons why speakers are flawed. Inadequacies in the design, drivers, crossovers, ports, cabinets, and interaction between drivers and ports can cause major problems. Poorly designed and low quality speakers exhibit uneven frequency response. An equalizer can help even out the frequencies up to a limit.
Even speakers with perfect +/-3dB frequency responses have problems
Until recently it was not possible to achieve high levels at frequencies down to 20 Hz without high levels of harmonic distortion, and even today the best speakers are likely to generate around 1 to 3% of total harmonic distortion, corresponding to 30 to 40 dB below fundamental.
The sound quality of a speaker cannot completely be quantified. Frequency response, sensitivity, timbre, type of drivers, type of cabinet, cabinet damping, etc. etc., etc. play a role. Although we can make two speakers sound alike in an anechoic chamber, it is not unusual for the right front speaker and the left front speaker to sound differently depending on their position in a home theater room.
Look at these three speakers. All three have relatively the same frequency response of 20Hz-20kHz +/-3dB, but they don't sound the same.
Speaker C will have the so-called one-note bass and will make the voices and some musical instruments a bit unnatural. Speaker D has smoother amplitude variations and sounds a bit more natural. Speaker C has the rapid changes in amplitude and experience has shown that these types of speakers are less pleasing and more fatiguing. Based on frequency response, speaker D is preferred to speaker C.
Speaker E also has relatively large amplitude variation, but it demonstrates a much smoother response curve. It will sound more natural and pleasing even though the bandwith of frequency variation is large (+6dB). Speaker E will have rich bass, good treble, and relatively laid-back midrange. Despite its inadequacies, some people may prefer speaker E.
Even the best sound system can benefit from the use of good equalization.
On-axis and Off-axis response is significantly different
The high frequency sound waves have a narrower off-axis response and low frequency sound waves have a much wider off-axis response. That is why you can stand completely to one side or behind a speaker and still hear bass frequencies like you were in front, but lose midrange or higher frequencies.
It is said that the higher frequencies sound waves behave like headlights or flashlights and low frequency sound waves behave like ripples of water waves in a pool. This is why tweeters are shaped like domes. A curved diaphragm pushes high frequency sound waves in a way that aides dispersion.
In the following diagram, compare the on-axis (purple curve) versus the off-axis (green curve) where the speaker is facing a microphone or a person. Notice the significant changes in frequency response just because two people do not sit exactly in the same place.
Consider what will happen if the same speaker produces different frequencies in 360 degrees, and reflecting from many different surfaces, resulting in cancellations and additions, and finally entering your ears.
Interaction between frequency waves with the boundaries, furniture, and other obstacles
Probably the majority of the problems in audio are due to the effects of the room acoustics. All rooms distort the sound. Room distortion is caused by reflections from the walls, ceiling, floor, furniture, and other obstacles in a room or in a car. Using exactly the same equipment and playing exactly the same music in different rooms will sound different. Each enclosed space affects your sound differently. Read the following article:
The sound that we hear from the loudspeakers in our home theater reaches our ears directly, but also reflected off the surfaces mentioned above. The unwanted reflections arrive a bit later than the direct sound and this causes distortion. Early reflections are those which reach the listener within a delay of 15 ms relative to the direct signal. The direct sound and its delayed reflection can cause phase cancellations and additions. This phenomenon is called comb filtering.
Each room is different in its dimensions, the distance from you and your equipment, the paintings on the walls, windows, doors, curtains, cabinets, type of floor, and many other factors. Calibration/equalization programs such as Audyssey/YPAO/MCAC/DCAC analyze these reflections and try to correct the problems they create so that the sound waves that arrive to your ears are as closely as possible to the way they were created. However, before you apply equalization and room correction, you should make every attempt to place the speakers appropriately in the room and use as much room treatment as possible.
Please note that Denon/Marantz and Onkyo/Integra use Audyssey, Sony has DCAC (Digital Cinema Auto Calibration), Pioneer uses MCAC (Multi Channel Acoustic Calibration), and Yamaha uses YPAO (Yamaha Parametric Acoustic Optimizer).
Equalization can also help with some of the room’s more troublesome features. If the room is very bright you can roll-off the high end or increase the low end to help offset it. If the room tends to be boomy, you can try to reduce resonance by lowering the the low end. Another way that equalizers can be quite effective is in controlling troublesome feedback. Feedback is the terrible squealing sound from the loudspeaker that gets picked-up by one of the microphones and is re-amplified and goes back to the speaker, only to be picked-up again by the microphone, and and so on. Most often, this happens when the system is playing too loud. An equalizer can be used to cut the troublesome frequencies frequencies that want to squeal and still allow the system to play loud.
The audio signal is composed of different frequencies. Proper equalization enables a signal that is originated from point A to arrive at point B with the same shape and amplitude as it was orignated.
The interaction between lower frequencies and boundaries causes the most problem.
One of the most important results Dr. Floyd Toole and his associates at NRC and Harman International found was that when all your speakers are running full-range in the room, you will experience a huge difference in the level of bass that is generated by each speaker. It is best to quote Dr. Toole from one of his scientific articles.
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 as 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.
TooleBassResponse.jpg We have five very different bass sounds, one for each channel!
Attempting to improve the situation by panning the bass to pairs of loudspeakers changes things, but does not remove the problem. Anybody think that an “ideal” room can help this? An anechoic room would, but none of us would wish to live in one.
And this is why bass management and subwoofers make sense. Now we can place the woofers where they perform optimally for a specific room with a specific listening position. We can place the satellites (a term that seems inappropriate for some of the large capable loudspeakers that we use in the high-passed channels) where they need to be for directional and imaging effects. In other words, we design the low-frequency portion of the system separately because rooms force us to do so. This is the only way that we can get good bass in any room, and have any hope of having similarly good bass in different rooms. Remember about preserving the art?
MultEQ corrects the subwoofer in every seat providing precise bass reproduction.
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.
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.
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.
Dr. Floyd Toole:
Quote:
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.
Sub Performance Without Equalization
Sub Performance With Equalization
Inappropriate positioning of speakers/subwoofers
In the following, you will find some guidelines on how to position the speakers appropriately so that the problems associated with room reflections, phase interaction, and delays are minimized:
Make sure the front and surround speakers are positioned symmetrically with respect to your main listening position. The speakers have to be precisely positioned to avoid amplitude and signal arrival delays. Keep the distance between the right and the left speakers and the listening position equal.
Early reflections from the boundaries can interact with the direct sound from the speakers. This can smear and compromise the sound coherence and its localization.
If the speakers are placed too close to the boundaries, the lower frequencies will be emphasized and the speakers will sound boomy. If they are placed too far from the boundaries, there will be back wall cancellation effect generated by reflections from the wall behind the speaker. The cancellation depends on the distance and the reflectivity of the wall. It is highly recommended that you use absorption wall treatment behind the speakers.
The side surround and rear surround speakers should not be placed too far above the ear level. Otherwise, coherence of the front to the surround field is lost. Unfortunately, in many cases, people have to place the surround speakers too high because of space limitations in their rooms.
For multi-channel music, if the surround speakers are pointed toward the listener, it may be preferred. For movies, the more diffuse sound from bipolar/dipolar speakers may be preferred.
The human brain has much higher capability to localize information in the horizontal than vertical plane. That is mainly because we have two ears on both sides of our heads. That is why placing a center speaker below or above the screen does not hurt the sound localization that much. If the center speaker is placed too close to the floor, you must use acoustic treatment or a very thick carpet on the floor to minimize the speaker/floor interaction. For the same reason, if the center speaker is placed too close to the ceiling, you must use acoustic foam on the ceiling. In either case, it is not a good idea to place the center speaker inside a cabinet as the cabinet will smear the sound.
As far as the subwoofer is concerned, some experts recommend corner placement. A subwoofer in a corner will excite all room modes and will gain output. This will not put any additional stress on the sub amplifier and as a result, it will increase the subwoofer’s headroom. The disadvantage of corner placement may be that you may not be able to significantly reduce the subwoofer’ level during calibration and there is the possibility that subwoofer may become too boomy. If corner placement is not an option, a subwoofer should not be placed in a symmetrical position from the boundaries such as the middle of the room. In the center of the room, the standing waves will create a null and you may not be able to hear any bass. For the same reason, you should not sit in the middle of the room.
The acoustical adjustment between the speakers and the room is very important and should be performed before doing any kind of calibration, equalization, or crossover adjustments so that the frequency response of the whole system is reasonably consistent across the room without major peaks and dips in the listening area.
Overtones & Undertones.
All tones when produced excite other, different tones that vibrate along with them. Every sound consists of the actual sound that is produced, called the fundamental, as well as the overtones and undertones. The number of overtones and undertones and their loudness, relative to the fundamental, determines the character of a sound. These overtones and undertones are generally referred to as harmonics.
Those other sounds, the harmonics, may be louder or softer in volume or greater or lesser in quantity.
When sounds are recorded and played back, all the sounds that emanate from the loudspeaker become fundamentals, even those that are the overtones and undertones of the sounds that were recorded. This means that they all produce their own set of harmonics. The result is a massing of these over/undertones.
THE ADDITIONAL HARMONICS IN ALL SOUND REPRODUCTION DISTORT THE ORIGINAL
The harmonics that are added by the playback process blur the sound textures, emphasizing the high frequencies (sometimes called adding shrillness). Unfortunately, the harmonic of most sound peak in the same frequency range to which our ear is most sensitive. This adds an additional distortion and that is another reason why sound reproduction has to be equalized.
It is important to note that without equalization, recorded music will not sound natural. In relation to all recorded sound (sound-reproduction), the Fletcher-Munson curves (for explanation, see below) together with the massing effect of overtones show that equalization is necessary in all sound-reproduction.
Comb Filtering and interaction with one another
Comb filtering occurs when two signals arriving at the same location at different times. Because of the differences in the arrival times, the sound waves will have additions when they perfectly overlap and reinforce each other, and also have cancellations and destructive interference where they cancel each other out. This occurs in all speakers where both drivers are reproducing the same sound and whose frequencies overlap.
The following tests were performed at Axiom Audio:
Quote:
A standard frequency sweep from 20 Hz to 20 kHz was played back over the two M2 speakers and we recorded the test sweep with the measurement microphone. The purple curve in Figure 1 shows the frequency response with the microphone exactly centered in the sweet spot between the two M2 speakers.
Then we moved the measuring microphone 1/2-inch to the side, off center from the sweet spot, and recorded another frequency-response curve. The green curve in Figure 1 shows the first comb cancellation effect at 15 kHz.
Then we moved the microphone 1 inch off center and ran another curve. In Figure 2, the green curve shows the next comb filter cancellation at 5.5 kHz. In Figure 3, the measurement microphone was moved 8 inches off center from the sweet spot. The dark greenish curve shows the pronounced comb-filtering cancellations beginning just below 1.5 kHz and extending all the way up to 18 kHz. The dips in response resemble the downward teeth of a comb, hence the name "comb filtering".
The precedence effect (previously known as the Haas Effect) dictates that our brain and ears pick out the location of a sound source that reaches our ears in the first few milliseconds of a sound's arrival. The first sound to arrive at the ears enables you to determine the direction of the source. After hearing an initial signal, the brain will suppress any later-arriving signals, up to about 30 milliseconds. These later-arriving signals that show up with steady-state pink noise (within the 30-millisecond window) do not disrupt the brain's precise localization mechanism. What occurs is that you do not "hear" the contributions of the later-arriving sounds from the adjacent drivers that are responsible for the measurement artifact of comb filtering. Or rather, your brain hears and processes them but disregards them lest they confuse our directional acuity; in fact all they do in the listener is create a sense of added spaciousness.
12-29-2010, 11:54 AM
Why Do We Need Equalization and Room Correction?
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