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Old 02-22-2008, 03:34 AM   #1
Big Daddy Big Daddy is offline
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Default Calibrating Your Audio With an SPL Meter

CALIBRATING YOUR AUDIO WITH A SOUND PRESSURE LEVEL (SPL) METER

Prepared by Big Daddy

WHY SHOULD YOU CALIBRATE YOUR AUDIO SYSTEM?

For the past several years, studios have made multi-channel (5.1, 6.1, 7.1) music (SACD, DVD-A, DualDisc) and movies (Dolby, DTS, etc.) available to home theater enthusiasts.

For additional information on the above, check my threads on Home Theater Audio CODECs, SACD and DSD, and DVD-A and DualDisc. Also read my Post #35 in this thread.

Unfortunately, in your home theater room, you may sit at different distances from the left, center, right, surround speakers, and the subwoofer. Since the speakers are at different distances from you, they will have different volume levels. In addition, each speaker may have its own sensitivity, and its placement in the room can affect that. In order to fully appreciate the music or movie audio in the most satisfactory and optimum way, we need to calibrate our audio system. One of the first calibrations that should be done in a home theater involves balancing the volume output of every channel (speaker) in the system to make sure the sound is being reproduced in an accurate manner. After that, we need to compensate for the speaker and room interactions.

Proper calibration ensures that you hear everything the way it was recorded by the sound engineer. Without proper calibration, you may not hear details in the main or surround channels or they may be exaggerated if the speaker levels are too high. You also may be hearing more volume from the right speaker than the left or you may have a hard time hearing the dialog because the center channel is set too low or the subwoofer is clouding the dialog.

WHAT TOOLS DO YOU NEED TO CALIBRATE THE AUDIO SYSTEM?

Test Tone (Pink Noise) Generator
To calibrate the audio speakers, we need to generate Pink Noise (test signals) into the speakers and set the output level of each channel so that they all match. Pink Noise has equal amount of energy per octave. Test tones sound like static on a radio. Most modern receivers or pre-amps have the ability to generate test tones. To access that, you need to go to the setup menu.

Alternatively, you can purchase a calibration DVD/BD such as the Avia Guide to Home Theater or the Digital Video Essentials to calibrate your audio as well as your video system. They can be purchased from vendors such as Amazon:

Sound Pressure Level (SPL) Meter
The human auditory system detects variations in air pressure and converts them into sound. When attempting to optimize a room’s audio system, having the ability to measure the pressure level of a sound wave is extremely useful. Unfortunately, it is virtually impossible to measure the level of sound from each speaker in a home theater by ears alone. A good piece of instrument that does a more accurate job is a Sound Pressure Level meter. An SPL meter is a battery powered device that uses an internal microphone to measure the intensity of sound in decibels (db). Decibels are the relative measure of a sound’s loudness or intensity (as the watt is for electric power). Decibels use a logarithmic scale (see Post #35 in this thread). A 1dB difference in output is barely noticeable. A difference of 3dB is a lot more noticeable. We perceive a difference of 10dB as "twice as loud," or "half as loud".

SPL Meters come in two varieties: with an analog meter or with a digital display. You can purchase an SPL meter from most electronic stores. Radio Shack is a good source for reasonablely priced SPL meters:

Radio Shack Digital SPL Meter ($49.99)



RS Analog SPL Meter ($44.99), (discontinued).


The Radio Shack SPL meters may be off by a few db's at some frequencies. However, the important thing is that they can still be used effectively to set the relative sound level of each speaker with respect to one another. See the corrections to the Radio Shack analog meter in the bottom of this post.

To get better results, you may consider a more expensive SPL meter, such as the Gold Line , Nady, Galaxy, etc. Alternatively, you can buy the test CD from Rives Audio. It is precisely calibrated to compensate for the non-linearity of the analog Radio Shack SPL meter.

SPL Meter for iPhone:
http://appshopper.com/music/spl
http://acousticsoftwarereviews.com/r...iphone-review/

SPL Meter Components
There are five major components on a Radio Shack SPL Meter.
Microphone: There is a microphone at the top of the SPL meter to pick up sound pressure.
Range: This indicates the range of sensitivity for the microphone, which has corresponding values usually from 60db to 120db.
Weighting: The weighting affects how sensitive the microphone will be over a given frequency range. It lets you choose between Weighted A and C measurements. “A-Weighting” reduces the frequency response of the SPL meter to the 500Hz - 10,000Hz range, where human ears are more sensitive, while the “C-Weighting” will make the meter respond to the frequency range from 32Hz to 10,000Hz. For Home Theater use, the wider C-Weighting is recommended.
Display: The readout on analog meters contain some numerical markings and a little needle. The markings will vary in meaning depending on the range of the dial. If the dial is set to 80, the 0 marking will mean 80, the -4 marking will mean 76, and the +4 marking will mean 84. Digital meters have a 3-digit LCD display which shows both the average and peak sound pressure level in db. It also has a bar-graph which gives you a visual representation of the measured sound.
Speed: This adjusts how quickly the meter will respond to changes in sound pressure level. It lets you choose a “fast” or “slow” response. The “slow” setting will make the meter less sensitive to rapid changes in sound level and can be used for measuring average pressure levels. The “fast” setting will respond to rapid changes in pressure level and is more useful when peak sound levels are being measured.

HOW DO YOU USE AN SPL METER TO CALIBRATE YOUR AUDIO SYSTEM?

Speaker Setup (5.1, 6.1, and 7.1 Setup)
Depending on the number of speakers you plan to use in your home theater, the setup should follow one of the diagrams below: http://www.smartbuyspeakers.com/speakersetup




Setting Up the System
  1. Verify that the speaker wires are correctly attached from the receiver to the speakers (i.e, positive to positive and negative to negative). If the speaker wires are attached incorrectly, the sound will be diffuse and hard to locate.
  2. Verify that your receiver has been configured properly for the number of speakers and the type of cables used (e.g., component, HDMI, optical, etc.).
  3. Verify that the speaker distance to the listening area is set correctly.
  4. Verify that the speaker size is correctly selected in the receiver’s menu.
  5. Verify that the crossover setting for your subwoofer is set according to the manufacturer’s recommendation (usually 80Hz, if you have full-range front speakers).
Positioning of the SPL Meter
To use an SPL meter, it should be placed as close to the normal position of the head of the primary listener. If the primary listener sits in multiple locations, you should find an average location between those positions. Aim the SPL meter toward the front of the room at an angle of about 45 degrees in the direction of the ceiling. You can also point the SPL meter straight up toward the ceiling. Remember that your ears are not directly aimed at the speakers when you listen to them.

Holding the SPL Meter
It is best if you use a tripod, high chair, or step ladder. Make sure nothing blocks the microphone. Stand off to the side of the SPL meter when you are using it as human body absorbs sound. Standing behind the meter can distort the results.

Setting Up the SPL Meter
  1. Check the battery. A weak battery can result in errors.
  2. Use the “C-Weighting” setting.
  3. Use the “Slow” speed setting.
  4. Set the db range on the meter to “75" or “80". You can use other settings such as 70 or 90. 70db may not be too sensitive and 90db may be too loud and tiring to listen to.
Reference Level
Reference level is defined for film mixing and movie theaters. Every studio and movie theater is calibrated according to this level. It represents an average of 85dB for the regular speakers on the SPL meter (set on C weighting and Slow) using a band limited (500Hz to 2,000Hz) pink noise at the listening position. The peak level is set 20dB higher at 105db and the LFE peak level is set +10dB higher to a maximum of 115dB. The purpose of the +10 dB gain for the LFE channel is to increase the dynamic range of bass sound such as explosions and crashes. This means when the receiver master volume is set to 0dB, the regular speakers are expected to play a peak level of 105dB and the subwoofer is expected to produce a peak output level of 115dB. This is louder than most people can tolerate, so people normally set the master volume much lower than 0 when watching movies or listening to music. Furthermore, such loud bass level places a heavy burden on the subwoofer and requires multiple high-end subwoofers to produce it accurately.

Because 85dBC test tones can be very loud in a small home theater room and can damage hearing, receiver manufacturers through the encouragement by Dolby and THX decided that a reasonable test-tone level is 75dB and that is the level that most receivers use.

Mathematically,
  • Reference Level is 1.85v line level = 0dB VU meter = 85db playback level.
  • 105dB Peak level = 0dB (Full Scale).
  • 85dB Average Level = -20dB (Full Scale).
  • 75dB Average Reference Level = -30dB (Full Scale).
Quote:
  • dBFS (Full Scale) = unit of measure for the amplitude of digital audio signals.
  • The reference level is "0 dBFS, which is also the maximum signal amplitude that can be stored digitally in a typical digital audio recording system.
  • Signals louder than 0dBFS just produce clipping (truncation of the waveform, hence distortion).
When calibrating your audio system, the receiver plays pink noise that is recorded at 75dB (-30dB FS). When the individual speaker levels are set to 75dB at the listening position, as measured by an SPL meter, the effects of speaker sensitivity and room acoustics are accounted for and the speakers are all level-matched against the Reference Level.

You can use either the internal test tones of a receiver or an external disc. The internal test tones of most receivers are band limited and recorded at 75dB level (-30 dB FS). External calibration signals on most discs are typically full-range pink noise and recorded at 85dB (-20dB FS). It really doesn't make a lot of difference which method you use as long as all the speakers are balanced.

Playing Test Tones
The A/V receiver should let you adjust the level for each speaker, ranging from -10db to +10db, with 0db being the default. Before you proceed, make sure that each speaker is set to the default value. Engage the test tone generator from the menu on your receiver. This function is usually found under the speaker setup menu. You will hear a static sound in one of the speakers (usually left front speaker). This static sound moves from speaker to speaker in a clock-wise direction.

Measuring and Evaluating the Response
When you go into the receiver's manual setup mode and play test tones, the master volume on some receivers will be disabled and the test tones are played at 75dBC reference level for home theater. You can use the gain level for each speaker inside the menu and adjust their levels so that you get 75dB on the SPL meter. It is ok to adjust them a little higher in order to get 78dB or 80dB.

The master volume is not disabled on some receivers when you go to the manual setup menu. For these receivers, you can choose either the left front channel or the center channel and adjust the master volume until you read 75dB on the SPL meter (+5dB on the analog meter), or the level that the calibration disc specifies. Once you have completed this step, do not change the master volume.

As the test tones go through each speaker, use the adjustment level for that speaker (not the master volume) to cut or boost the output for that speaker to 75db. When you are finished, every speaker should output the same volume level to your listening position. Your subwoofer should also be adjusted to the same level, but if you like a little extra bass, calibrate it 5db more than the other speakers.

If you want to calibrate your system more fully, you may want to purchase the AVIA, DVE, or AIX discs. They are designed to work with an SPL meter and do a much better job of generating test tones. They also have specific instructions on how to calibrate your audio system. Most experts recommend that you use them instead of the calibration system built into your receiver.

If you are trying to double check the results of the built-in auto calibration program, using the internal test tones of the A/V receiver/processor will not be entirely accurate since the test tone mode bypasses all post processing. It is better to use calibration discs and with your desired EQ mode engaged to fine tune your system.

Levels of the Surround Speakers
You may want to set the levels of the surround speakers behind you a couple of dBs higher than the levels of the front speakers. The researchers at Audyssey have found out that human perception of loudness falls off faster behind us than it does in front. So,we need to compensate for that using a human loudness spatial model that looks at the master volume setting and makes the appropriate adjustment to the level of the surrounds. This is designed for content where the overall surround impression should remain the same at all volume levels: i.e. surround movies and music.

Finishing Your Calibration
Once all of the speakers read the same SPL level, your system is calibrated. After you have completed your calibration, you will be able to listen to movies and music as it was intended. Even if you set the master volume of the receiver to a different setting, the sound will still be balanced. Anytime you change your place of residence or purchase new speakers, you should re-calibrate your audio system.

HOW DO YOU ADJUST YOUR SUBWOOFER?

For additional information on Subwoofers, check my threads: A Guide to Subwoofers and A Guide to Subwoofers, Part II

As the receiver's test tones move around the room calibrating the levels on all the main speakers, you will end up at the subwoofer. Receivers switch their test signal for bass, typically from 40Hz to 80Hz. It sounds like a dull rumble. Make sure you adjust the subwoofer channel level until you read the same number on the SPL meter as the main speakers. Because the low frequency sounds are very much room dependent, you should move the SPL meter around the listening area to get an average value.

Subwoofer Positioning
Room and furniture have an enormous impact on the sound of speakers. A well placed subwoofer’s bass integrates with the sound of the main speakers and produces a natural reproduction of music. A few guidelines for subwoofer positioning are listed below.

Corner placement: This is the advice that is given most often. Although corner placement will yield loud bass, it may make the music sound boomy. You should place your subwoofer in a corner only if it is not capable of producing deep bass. It is also important to note that corner placement will not always make the subwoofer boom. In most cases, it depends highly on the geometry of the room. According to some experts, you should always place a sub in a corner, and use equalization to deal with audible peaks of the subwoofer's frequency response at that position.

You should not sit against the wall: Your movies and music will sound heavy and tiring when you are sitting against a wall. If you must sit against the wall because of the room’s layout, turn down the volume of the subwoofer to compensate.

Do not place the subwoofer in a symmetrical position in the room: Avoid putting a subwoofer in a location that is the same distance from the walls. Subwoofers sound better if they are placed in a location where their distances to the front, side, and rear walls are different.

Put the subwoofer close to the main speakers: Even though bass sounds are not localized, you will get a better blending between the main speakers and the subwoofer if they are on the same side of the room.

Two subwoofers are better than one: Two subs can reinforce each other’s bass response and will yield a smoother and more dynamic sound. If using two subwoofers, you must use two identical subwoofers. Two different models, even from the same manufacturer, may cause uneven response.

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.

Setting the Low Pass Crossover Frequency
You need to set the crossover on the receiver’s menu and not the subwoofer. If your main front speakers are full size with good bass response, set the high pass filter to 60Hz-80Hz. If your main speakers are small, bookshelf, satellite, or in-wall, set the high pass filters in the 100 to 120Hz range.

Run the test tone generator for intervals between 30Hz to 200HZ and measure the output level with an SPL meter. If different output levels are read by the SPL meter for different frequencies, it is quite normal as different frequencies interact differently with the room acoustics. Increase or decrease the low-pass frequency to achieve the smoothest response. Decrease the crossover frequency if there is too much output around the crossover point, increase it if you notice a drop in the response.

You can download free test tone generators from the following sites:
http://www.realtraps.com/test-cd.htm
http://www.cleansofts.com/softdownlo...Generator.html
http://www.tasignal.com/Software/MiniDSGe.html
http://www.nch.com.au/tonegen/index_b.html

Setting the Subwoofer’s Phase (Polarity)
When the woofer on the sub and the woofers of the front speakers move in and out in sync with each other, the system is said to be in phase. When the speakers and the subwoofer are moving out of sync with each other, the subwoofer and the front speakers’ bass overlap and cancel each other. In this case the system is said to be out of phase, resulting in less bass.

Unfortunately, there may be another problem between the main speakers and the subwoofer. If the main speakers are producing bass at the same time as the subwoofer, at some points the bass will reinforce each other. At other points the bass will cancel each other. The solution is to allow only the subwoofer to reproduce bass by setting the front speakers to small in the receiver’s setup menu. This can yield a smoother bass response throughout the entire room.

To get the best bass response, you should set the phase (polarity) of the subwoofer(s) to deliver the highest output at the listening position. This can be achieved with the help of a test signal at the crossover frequency and an SPL meter. You should run this test several times by changing the polarity of the subwoofer and measuring the bass response on the SPL meter. Select the phase option that results in the highest bass response.

Fortunately, most subwoofers have a switch to change their polarity. If the subwoofer does not have a phase switch, you can change the polarity of the main speakers by switching the positive with the negative speaker wires (the black wire goes to the red terminal and the red wire to the black terminal). Some subwoofers have a “variable phase control”. This control can be set continuously between 0 and 180 and allows for a more precise phase control of the subwoofer.

When setting the subwoofer phase by ear, play some music (not a movie) that has a repetitive bass line. Switch the polarity several times and choose whichever setting sounds “faster” or “fuller”. If you do not hear any difference, leave the phase switch at “0" or “normal”.

If you are using two subwoofers, put one in the front and the other in the rear diagonally opposite from the front sub and set the phase of the rear sub to 180.

How to Check the Polarity of Speakers: Take the grill off a speaker. Apply a wire from the (-) terminal on the speaker to the (-) terminal of a AA battery. Then touch a wire from the (+) terminal on the speaker to the (+) terminal of the battery and look at the cone of the speaker. The speaker cone should move forward. If the cone moves backward, the speaker is wired out of phase. In some instances midrange drivers are intentionally wired out of phase with their woofers. This test is intended to check the phase of woofers and subwoofers.

Conclusion
Getting the optimum bass response from a subwoofer is a tricky business. There are no easy solutions. Don’t be afraid to experiment.

OTHER APPLICATIONS OF SPL METERS

Building a Recording Studio or Sound-Proof Room
If you are building a recording studio, having an SPL meter is important as you can measure how much sound enters the studio from outside. You can also calculate how much sound proofing you will need to keep your own noise private. Auralex Acoustics has a website called “Bothering Your Neighbors”. It shows how much sound in decibels you can reduce with each layer of additional building materials:
http://www.auralex.com/profiles/profiles.asp?Pro=Byn

Checking the Frequency Response of Speakers
This test is a good way to determine the frequency response of your speakers. With the help of a test disc, play different test tones at different frequencies and measure the response of your speakers with an SPL meter. Chances are that most loudspeakers do not have a flat frequency response from 20Hz to 20,000Hz and their output will drop dramatically at high and low frequencies.

Safety: You Only Have Two Ears
Measuring sound pressure levels in your listening room can help prevent damage to your hearing from long term exposure to high sound levels. Continuous high pressure levels (> 100 dB) can permanently damage your ears and your loudspeakers.

The following table from the U.S. Department of Labor gives data regarding safe sound level exposure.

A-Weighting, Slow response
Hours per day, Sound Level (dB)
8hrs, 90db
6hrs, 92db
4hrs, 95db
3hrs, 97db
2hrs, 100db
1.5hrs, 102db
1hr, 105db
0.5hrs, 110db
0.25hrs or less, 115db

To make sure your speakers are operating at safe listening levels, use the SPL meter. While playing music at your normal listening level, hold the meter up at a 45 degree angle and take a reading with the meter set to “A-Weighting” and the speed switch to the “Slow” position. Adjust the volume according to the table above. Although you can safely listen at 90db for about eight hours per day, use this table only as a guide and turn the volume a few decibels down.

Corrections to Radio Shack SPL Meter
When using a Radio Shack SPL meter, you need to adjust for the fact that it's not accurate at all frequencies. Add according to the numbers listed below. These frequencies are 1/6th octave apart.

10.0 hz: +20.0 db
12.5 hz: +16.5 db
16.0 hz: +11.5 db
20.0 hz: + 7.5 db
25.0 hz: + 5.0 db
31.5 hz: + 3.0 db
40.0 hz: + 2.5 db
50.0 hz: + 1.5 db
63.0 hz: + 1.5 db
80.0 hz: + 1.5 db

100.0 hz: +2.0 db
125.0 hz: +0.5 db
160.0 hz: -0.5 db
200.0 hz: -0.5 db
250.0 hz: +0.5 db
315.0 hz: -0.5 db
400.0 hz: +0.0 db
500.0 hz: -0.5 db
630.0 hz: +0.0 db
800.0 hz: +0.0 db

1.00 khz: +0.0 db
1.25 khz: +0.0 db
1.60 khz: -0.5 db
2.00 khz: -1.5 db
2.50 khz: -1.5 db
3.15 khz: -1.5 db
4.00 khz: -2.0 db
5.00 khz: -2.0 db
6.30 khz: -2.0 db
8.00 khz: -2.0 db
10.0 khz: -1.0 db
12.5 khz: +0.5 db
16.0 khz: +0.0 db
20.0 khz: +1.0 db

Some critics believe that meter calibration offsets are not that useful because all Radio Shack meters are not the same. Moreover, they claim the low frequencies are usually accurate enough even with inexpensive microphones. See THIS ARTICLE and THIS ARTICLE by Ethan Winer at RealTraps. However, THIS STUDY by AudioXpress found inaccuracies between the old and new RS SPL meters.


For List of Additional SPL Meters and additional References, see Post #9.

Last edited by Big Daddy; 10-22-2012 at 11:16 PM.
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