ETF5 small gate time

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ETF5 small gate time

Postby Bob » Sat Dec 18, 2004 5:00 am


I found this over at ... ost4826130

This post is not a question. Just one of those "it's not common sense until someone tells you". It should have been obvious to me that with a gate time much smaller than the time it takes the wave to pass the microphone it might be hard to detect, but it wasn't. :)

Since Dennis brought up the SBIR (Speaker-Boundary Interference Response), some of you may be wondering how to measure it. You can do this with a program like ETF.

The idea is based on the fact that SBIR is a short-time effect. It happens within the first tens of milliseconds of the sound, where 1 millisecond = about 1 foot of sound travel. So if you take the frequency response using a short impulse-response time window (20 milliseconds or so), you will see what is going on during the time period when the direct sound from the speaker interferes with its reflection from a nearby wall.

On the other hand, if you look at a long-term frequency response (time window of a hundred or more milliseconds) it will span several wall reflections back and forth through the room. You now see the effects of room modes added to the SBIR, since the modes need time to build up over several wall reflections. Get it?

Now, there is one gotcha. With a small time window, you automatically get less detail in the frequency domain. It is a fundamental trade-off in the signal processing world. So if you set the time window to 20 milliseconds for example, you won't see any frequencies below 50 Hz. ETF will show you a smooth response above this, but there is still limited low frequency information, and you will not see a lot of detail. Nevertheless, you still should be able to spot any dip corresponding to the 1/4 wave cancellation which Dennis described.
Bob Golds
"The only thing we regret in life is the love we failed to give."
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