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In 2004, I presented a modified version to CEDIA.
I am posting a slightly revised version here for those that need a quick how to on audio calibration. It is not meant to be the complete be all to end all technical paper, but rather a quick summation of how I recommend to calibrate your HT system using just a SPL meter. This is based on almost 16 years of HT experience hopefully will hepl others achieve a better listening experience...
Excerpt from "UNDERSTANDING SURROUND SOUND - 5.1 And Beyond..." writen by Mark Techer, July 2000
SYSTEM CALIBRATION.
AUDIO ADJUSTMENTS.
A well designed controller will have a few standard items. Apart from different surround sound decoding algorithms, your audio processor should have level trims for all channels including the FRONT LEFT and FRONT RIGHT.
For those systems that do not, you will need to find the level where the Left channel that reads +75dB and remember or mark the master volume at that point. That number will your 00dB level, even though it may not be close to 00dB.
Also, not all volumes display in relative (negative dB figures), some display in “absolute” (number that increase with the loudness). If your system is such, then choose a “round” number and then adjust each channel trim to read +75dB on the meter.
The worst case scenario will be a unit that does not offer either trims on the front left and right channels or a master volume displayed in absolute values…
In this digital age, usually both the trims and the master volume control will be digital and the trims should be in half dB adjustments. Full dB adjustments are too course and may not allow total precision.
All well designed decoders will also have an internal noise generator. The tone emitted from the noise generator will be equivalent to an audio tone recorded at –30dBFS for film sound (18BIT) which is the same as 105dB – 30dB or +75dB. Using a Sound Pressure Level meter and these tones, you will be able to precisely adjust the acoustic output level of your system to equal that of a cinema or dubbing stage.
The end result will be that you will hear the sound track they way it was heard in the studio that created it both from channel to channel sound as well as absolute volume level.
The SPL meter should have an analog display. The use of the analog model is preferred as reading can be under 1dB and depending on the type of level trim on your system, totally accurate.
The radio shack SPL meter is the same type used by the film industry. Independent tests revealed that this meter was the most accurate model from a group of meters tested including more expensive models.
The new meter from JAYCAR is more expensive, but it has a calibrate feature. Whilst this meter is digital, it does display in 1/10th of a dB.
USING THE METER.
The meter has a rotating “range” dial and two switches. The range dial provides the SPL range you would like to measure in decibels starting form 60dB to 120dB. There is also a battery check “BATT” position just next to the “OFF” position.
The two switches are labeled “WEIGHTING” and “RESPONSE”. Set the “WEIGHTING” switch to the “C” position and the “RESPONSE” switch to the “SLOW” position.
Set the dial to the 70dB range. This will now allow you to read SPL from 60dB to 76dB.
EDIT: Position the meter in the prime listening position.
Hold the meter at arms length from the body. The meter should be at seated ear height, pointed up and slightly forward.
DO NOT POINT THE METER AT ANY ONE SPEAKER.
I use a tripod to hold the meter and move well away so that reflections from my body are not read by the meter and give false readings.
Activate the test tone generator on your surround sound processor. Well designed processors will default to a 0dB reference level position regardless of the actual volume position and start at the LEFT front channel.
Observe the level reading. It should be reading at +75dB/C/SLOW. If the audio level is too high, the meter’s needle will “peg” and you will have to reduce the trim level for that channel to read the desired +75dB/C/SLOW. If the audio level is too low, adjust the trim to read the correct level.
PEGGING OF THE NEEDLE IS NOT ADVISED AND MAY ACTUALLY DAMAGE THE METER.
Once you are satisfied with the level, proceed to the next channel, working in turn around the room.
SUBWOOFER AND LFE LEVEL.
While the standard level for all main channels (including the Surrounds) is +75dB/C/SLOW, setting the level for both the subwoofer and the LFE channel is a little more complex.
Ideally you would use a Real Time Spectrum Analyzer. The level of the Subwoofer can be set with the SPL meter by the following method. The level of the Subwoofer should also read +75dB on the meter, however the tone for the Subwoofer is a “Warble” and the level will very along with the frequency. It will be difficult to set the level at the 70dB range.
Turn the dial to the 80dB range. If your processor has a Sub test warble, adjust the trim to read an average of +79dB/C/SLOW with the lowest part reading at –5dB on the 80dB scale. The level difference will be around just 4dB and is equivalent to the average level of +10dB when using the RTA.
DO NOT SET THE SUBWOOFER TO READ 10 DECIBELS HIGHER THAN A MAIN CHANNEL WITH THE SPL METER.
The LFE channel is a fixed value in reference to the subwoofer level. That is if you increase the Subwoofer level, you will also increase the level of the LFE channel. Some decoders offer a separate trim for the LFE channel. It should be set to the same level as the Subwoofer. If the SUB trim is 0dB, then so is the LFE trim.
THE LFE TRIM WILL NOT GO ABOVE 0 REF DECIBELS. IF THE LEVEL IS NOT HIGH ENOUGH AT THE 0 REF DECIBEL LEVEL, INCREASE THE GAIN ON THE SUBWOOFER WHERE POSSIBLE.
PLAYBACK.
Once your system has been calibrated, you can enjoy a film. The 0dB reference level is the level you should watch films at. You may of course reduce this level if it is too loud.
DIALNORM may be seen on some format decoders. It is a part if the Dolby Digital coding and will advise you of the average level of dialogue in a program. If the DIALNORM reads a plus figure EG “DIALNORM +4”, you must reduce the level of the master volume by 4dB. When used correctly, you will find that the level of dialogue remains consistent across a wide range of program material. The peak levels however will vary occasionally resulting in some very high Sound Pressure Levels.
Enjoy your films and music…
Mark
Edited by MarkTecher, 02 March 2006 - 09:00 PM.
