Can subwoofer distance be measured with a program? - AVS Forum
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post #1 of 6 Old 01-21-2013, 08:35 AM - Thread Starter
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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.

What is it they are measuring?
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post #2 of 6 Old 01-21-2013, 09:26 AM
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Originally Posted by GGA View Post

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.

What is it they are measuring?

Wherever you 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, it was in error.

When measuring the location of something using signals with very long wavelengths, you use information about phase shifts.

For example the length of an electrical wave whose frequency is 20 Hz is about 9,000 miles. If you divide 9,000 miles by 360 degrees you get about 25 miles per degree. Old time electronic navigation systems like Loran used phase shifts like these to measure distances from reference radio transmitters. Actually multiple radio transmitters that were precisely in phase with each other were used this way.

Programs like REW have access to both the electrical input to the speaker and its acoustical output, and use that information to make an estimate of its location.
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post #3 of 6 Old 01-21-2013, 01:05 PM
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Quote:
Originally Posted by GGA View Post

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.

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post #4 of 6 Old 01-21-2013, 01:19 PM
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Quote:
Originally Posted by Nyal Mellor View Post

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.[/quote

How much was the actual physical distance without calculating any electronic delay??
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post #5 of 6 Old 01-21-2013, 02:11 PM - Thread Starter
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Quote:
Originally Posted by Nyal Mellor View Post

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.

Thanks Nyal. That is very helpful, but I am still confused.

What does the 34ms REW result mean?

Should one always use the first deviation from zero on the impulse response with REW if one is integrating subs and mains?
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post #6 of 6 Old 01-21-2013, 05:47 PM
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Quote:
Originally Posted by GGA View Post

What does the 34ms REW result mean?

34ms is the value REW returns for system delay (this is for a different measurement BTW than the one I previously referred to ):


Quote:
Originally Posted by GGA View Post

Should one always use the first deviation from zero on the impulse response with REW if one is integrating subs and mains?

No, one should use the value for delay which results in the highest SPL at and around the crossover frequency. This will tell you that the subs and mains are most in phase. Trial and error is in my experience the best way to find this in small rooms. Comparing the impulse responses of mains and subs when swept over the same frequency range (otherwise the IR looks different) will give you a nice starting place for experimentation including delay and polarity. Generally I then try adding or subtracting delay to find the highest SPL around the crossover.

Alternatively you can use XTZ Room Analyzer II which has a delay finder function and use this to integrate your subs and mains. I wrote a blog article about how to use XTZ Room Analyzer to setup your home theater in terms of speaker levels, distances and phase.

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