e)3. Why is it a bad idea to use your AVR test tones and a SPL meter to check trim levels?
Many newcomers to Audyssey seem to like to 'double check' their final channel trims by using a SPL meter and the AVR's internal test tones to see if each channel measures the same, or measures the expected 75dB. If the channel trims do not measure the same or differ from 75dB, these users will often change the AVR trims to match the readings given by their SPL meter. This is not a good idea and the following answer explains why.
Since the AVR's internal test tones are not (and cannot be) processed by Audyssey, when you measure the AVR's test tone levels, you're measuring what your room is doing to unprocessed audio. Audyssey is designed to correct those inequalities for you. It is therefore possible that if Audyssey has had to perform a lot of correction on one or more speakers, relative to the others, then the SPL readings of unprocessed tones will be different to the reading that Audyssey has made, post-correction. It is also possible therefore that the SPL readings will vary from speaker to speaker for the same reason.
Use an external test signal instead.
If you use audio level test tracks from a good, properly recorded audio calibration disc (or other reliable external audio signal source), they can be processed by Audyssey. If Audyssey's calibration has done its job correctly, when Audyssey is enabled (and Dynamic Vol and Dynamic EQ are disabled) those externally-supplied calibration sounds should all produce the same sound level from each speaker, at the Main Listening Position.
Why this is important.
When Audyssey applies a set of filter taps to a channel, it implements a group of boosts and cuts at various frequencies. If, for example, the cuts are greater over a larger number of frequencies than the boosts, the "average" level of the whole signal will be reduced. Audyssey will then raise the average level of the whole signal to compensate. Audyssey does this by looking at the entire bandwidth of the signal or "chirp.".
When you measure the SPL of a bandwidth-limited "noise" signal, (the receiver's internal test tones), if Audyssey has made changes to the frequency response within that bandwidth, but you're not measuring those changes because Audyssey is not engaged in the signal path, you end up with a measurement that is different from what Audyssey measured. Therefore, if you re-set the levels based on these measurements, and then re-engage Audyssey, you now have a different calibration than Audyssey's, but one that doesn't take Audyssey's filters into account.
Why would anyone feel the need to do this? Audyssey measures the levels for all channels using the same mic, in the same spot with the same test signal. It then calculates the filters for all the channels. Finally it compensates for the average level changes the filters induce, and sets the level trims to ensure that all the channels are outputting equal average signal levels. It calculates them all the same way, using the same algorithm. Why would anyone think they're wrong... and then re-set them without taking the filters into consideration?
If you make changes to the trim levels after running Audyssey, using the receiver's internal test tones, it means you are uncorrecting the corrections Audyssey has made to compensate for its filters. If you change a channel level by 1.5 dB using the internal test tones, when you turn the test tones off and the Audyssey filters are re-engaged, that channel's calibration is off by 1.5 dB from the rest of the channels.
The way to confirm this is to use external test tones that run through the Audyssey filters. I have 2 such test tone discs, Avia and The 5.1 Audio Toolkit. I have checked the calibration with the internal test tones and both test discs.
Here are the results:
Channel Test Tones (dB) AVIA Disc (dB) 5.1 Audio Toolkit (dB) Left 76.0 76.0 76.0 Centre 76.0 76.0 76.0 Right 77.5 76.0 76.0 R Surround 76.0 76.0 75.5 L Surround 76.0 76.5 76.5 Subwoofer 73.0 75.5 75.0
Using the internal test tones results in a 4.5 dB difference in calibrated levels between channels. Using 2 different test discs, with Audyssey engaged for both, results in 1 or 1.5 dB of difference. If we could measure and average the exact same way Audyssey does, (sweeps + averaging), I'm sure we would see that Audyssey correctly compensated for the EQ filters it applied, and set the levels correctly, and that using the test tones afterwards results in un-correcting the corrected levels. I know it seems intuitive to want to change the levels when your SPL shows that they don't appear to be correct. Nonetheless, they ARE in fact correct and it's the reading you get from the SPL meter that are not. There's nothing wrong with your SPL meter; you're just not using the proper test signals.
Audyssey doesn't get everything right. However, one thing it does *very* well is to get the level calibration correct. To override that, based on measurements taken with a different mic, different test signals and without the Audyssey filters engaged is... well, let's just say it's not recommended!
