Originally Posted by GGA
I had read that because the wavelengths of a test signal in the subwoofer range (say 20-80Hz) are so long (say 15'-50') that one cannot measure the distance of a subwoofer (from sub to listening position) using a program. Yet REW and others will happily measure the distance.
That's incorrect. It can definitely be measured, but whether what the programs tell you is the best distance to use is another question.
With subs the acoustical
distance measured is always longer than the physical
distance. It's the acoustical distance that is the important one because this is the one you need to properly time align and integrate your subs and other speakers.
There are a few different techniques that can be used for measuring distance. These include (amongst others) largest peak of the impulse response, first peak of the impulse response, first deviation from zero on the impulse response and group delay. In my opinion they all have some challenges in use.
I think REQ uses peak of the impulse response. However I can tell you from experience this may not be the best distance to use for integrating say subs and mains. For example the other day I was calibrating some JL subs. REW said the delay was 34ms. In reality a 5.4ms delay to the subs resulted in the best integration with flattest frequency response and phase. The 5.4ms was equivalent to the first deviation from zero on the impulse response.