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In Reply to: RE: What do you think? posted by hennfarm on February 11, 2016 at 13:43:35:
It's kinda hard to know where to put this - here or under Tom's post."so anything below about 80 is the room I would suppose. I am still wrapping my head around the 1st reflection gating thing."
Making loudspeaker measurements in "regular" rooms is fraught with difficulties, mostly due to room reflections and resonances. But with a gated system, you're creating a window of time, much like the shutter of a camera - it's open for a certain period and then closes, so your measurement system is "listening" to the sound for only the moment you want. Back in 1974, at the acoustics lab of the Institute for Advanced Musical Studies in Montreux, Switzerland, John Curl, John Meyer and Bob Minor developed a system to do exactly that. The computer program also allowed them to average a number of tests, to reduce the influence of ambient noise. This was a big deal in those days. Now, 40 years later, gated systems are readily available.
Now, remember that sound travels at about 1129 feet per second at typical room temperature and humidity. So that's about 1.1 feet per millisecond. So, if your first reflection from a wall, the floor, an object, etc., arrives 5 milliseconds after the direct sound, you've got to close your window before that happens. But wait, it gets even better... You don't want to start the measurement time window until "just" before the desired sound reaches the microphone. So, if your microphone is 10 feet from the speaker, you'll want the measurement to start roughly 9 milliseconds after the sound left the speaker, AND, you'll want to close the window before the first reflection reaches the mic (in this example, 5 milliseconds later).
While all this sounds very cool, there is a limitation: The technique doesn't work below a certain frequency range, due to wavelengths, room size, and first reflection delay time. I've long since forgotten what the "rule of thumb" is for determining where that frequency range is, but IIRC, for "average" sized home living rooms, I think it's in the neighborhood of a couple hundred Hertz. Maybe Tomservo or someone else will refresh my memory on the details.
Hope this helps.
:)
Edits: 02/12/16
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Follow Ups
- Gating - Inmate51 08:15:59 02/12/16 (1)
- RE: Gating - hennfarm 10:35:18 02/12/16 (0)
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