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Old 25th February 2004, 12:08 PM   #11
Svante is offline Svante  Sweden
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Quote:
Originally posted by IanHarvey


I'm using AudioTester (http://www.audiotester.de/), using its frequency-sweep mode. It does have a waterfall-plot mode (which I've not really played with) - is this what you mean?
No, I don't think so. The method goes something like this:
Play a sound that contains all frequencies (chirp, noise or similar). Record a *long* piece of this signal, longer than the reverberation time of the room. The difference between the send and received signals gives you a frequency response (including phase) that can be transformed into an impulse response by an inverse fourier transform. This impulse response is truncated at a time before the first reflection from the room. Now fourier transform this truncated impulse response, and you will have something that looks like free field.
The limitation is the time it takes to the first reflection from the room, this will put a lower limit on the frequency range, typically it works from ~500 hertz and upwards.
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Old 25th February 2004, 02:46 PM   #12
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Quote:
Originally posted by Svante


No, I don't think so. The method goes something like this:
Play a sound that contains all frequencies (chirp, noise or similar). Record a *long* piece of this signal, longer than the reverberation time of the room. The difference between the send and received signals gives you a frequency response (including phase) that can be transformed into an impulse response by an inverse fourier transform. This impulse response is truncated at a time before the first reflection from the room. Now fourier transform this truncated impulse response, and you will have something that looks like free field.
I understand the waterfall plot thingy plays an impulse through the speakers, and slices the resultant response into chunks of a few ms each. The FFT of each chunk is displayed in a series of 2D slices forming a 3D picture. The very first slice should be the frequency response of the direct sound, and later ones give the room response.

That's how I'd like it to work anyway, but I've not done enough experiments to check whether this is actually the case.

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The limitation is the time it takes to the first reflection from the room, this will put a lower limit on the frequency range, typically it works from ~500 hertz and upwards.
Damn! The balance between < 500Hz and > 500Hz is precisely what we're trying to set with the baffle-step correction.

Cheers
IH
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Old 25th February 2004, 03:52 PM   #13
Svante is offline Svante  Sweden
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Quote:
Originally posted by IanHarvey


I understand the waterfall plot thingy plays an impulse through the speakers, and slices the resultant response into chunks of a few ms each. The FFT of each chunk is displayed in a series of 2D slices forming a 3D picture. The very first slice should be the frequency response of the direct sound, and later ones give the room response.
I might be wrong here, but i had the impression that the waterfall graph represents consecutive FFTs of a stationary (noise) signal that is shut off. The decay of resonances can then be seen in consecutive FFTs. But maybe I am wrong about this. What kind of sound is used for the waterfall graph? Noise? An impulse?

Quote:
Originally posted by IanHarvey


Damn! The balance between < 500Hz and > 500Hz is precisely what we're trying to set with the baffle-step correction.

Cheers
IH
As I said this frequency is set by the time between the direct sound and the reflected sound. Assuming a spkr-mic distance of 1 m and a spkr-to-floor distance of 1.25 m, the distance difference would be 2*sqrt(1.25^2+0.5^2)-1=1.7 m. This in turn corresponds to a delay of 5 ms. So the maximum FFT length that can be used is 5 ms, which would correspond to a 200Hz resolution. Hmm... Not too bad after all, appears as if I was on the pessimistic side.

You could also take the speaker outdoors.
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Old 25th February 2004, 04:38 PM   #14
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Quote:
Originally posted by Svante


What kind of sound is used for the waterfall graph? Noise? An impulse?

It sounds like a click. It might well be an MLS sequence, but I don't know what one of them is...

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You could also take the speaker outdoors.
That was Plan B, which will have to wait until it's (a) lighter and (b) warmer in the evenings.

Cheers
IH
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Old 25th February 2004, 04:57 PM   #15
Svante is offline Svante  Sweden
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Quote:
Originally posted by IanHarvey


It sounds like a click. It might well be an MLS sequence, but I don't know what one of them is...

Aha, so if it is a click (=impulse), you are right. What you will se on the input would be the impulse response, and it the windows size of the fft is 5ms -ish the first frame should correspond to the gated response. Check it up though before relying on it!
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Old 25th February 2004, 07:56 PM   #16
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The speakers look nice----real good job!
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Old 26th February 2004, 02:32 PM   #17
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Originally posted by mulebarn
The speakers look nice----real good job!

Thanks!

I'm still in the middle of listening tests and some final tweaking, but it sounds at least as good as it looks. To give an example, I put on a bit of Bach - one of my regular test tracks, which I must have heard a hundred times on a couple of dozen systems - and was amazed to suddenly hear someone turning over the pages of music! Definitely some detail I hadn't heard before; the stereo image is wonderfully precise - a notch up from my Monitor Audio Gold 10's, even.

At present, the bass is a little light. It's clean and detailed, great for jazz/acoustic stuff, but it doesn't really rock yet. I might fiddle with the ports some more (although I'm not a fan of bass-reflex 'boom'), or the baffle-step correction described earlier would of course boost the bass and low-mids.

Cheers
IH
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