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Old 14th December 2011, 12:49 AM   #41
RevMen is offline RevMen  United States
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Quote:
Originally Posted by Conrad Hoffman View Post
With the new system you can do impulse or use the pink noise and do an FFT.
You might use FFT to approximate a filter, but generally you would still use filters to break out your bands.

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Is the output of all four methods theoretically identical?
I think so.

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The reason I ask is let's say there was a big brass church bell in the room. As the sine sweeps through the resonance, the bell starts building up a tone. You'll see it in the sweep. This doesn't happen with random noise.
Assuming that by "random noise" you mean pink noise, you're mistaken. Pink noise will excite the bell continuously while a sine sweep will only excite the bell for the moment the sweep is in the bell's resonant frequency.

Pink noise is equal power in all octaves (or 1/3-octaves, or 1/n-octaves). That means it has power in whatever frequency excites the bell. So the bell will be excited.
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Old 14th December 2011, 12:51 AM   #42
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Originally Posted by RevMen View Post
You could drop the word perfectly and the sentence still has the same meaning.
Why then the "perfect" this and "perfect" that in your comments?

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I don't see a $20 noise source with known sound power in each band as existing.
Perhaps it doesn't . . . that was the question. But a squirel cage on a motor need not be expensive, and there might be alternate ways that you don't know about as well. Things might exist that "[you] don't see" . . .

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You are. Let's just walk through the process and maybe you'll be able to see the problem.
"Walking through" your process I see your problem . . . it is different from (and doesn't address) what I'm asking.

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Suppose you discover a fan on Craig's List that looks similar to the one in the article
I'm not, as the original post should have made clear, looking for "looks similar". "Looks" are not the issue, or the desired function. Ultimately, I suppose, addressing the flawed power response of most DIY speakers, and the flawed absorbtion profile of most listening rooms, is. Discussing a "constant power noise source" (and all the implications and issues surrounding it) may lead people to think about those issues in useful ways that "impulse response" almost certainly does not.
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Old 14th December 2011, 01:06 AM   #43
dewardh is offline dewardh  United States
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Originally Posted by Conrad Hoffman View Post
Is the output of all four methods theoretically identical?
Yes. But whether (or not) the results are identical in practice is a different question, as each method imposes its own "flavor" on the data. "Noise" will excite the bell, but since it is of random phase I suspect it will not excite it in the same way as a sine wave sitting on the resonant frequency.
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Old 14th December 2011, 01:22 AM   #44
RevMen is offline RevMen  United States
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Originally Posted by dewardh View Post
Why then the "perfect" this and "perfect" that in your comments?
I really am trying to be helpful here. I don't understand why you can't just quote what I said. This is silly so let's just go ahead and nip it in the bud now. Here's everywhere in this thread I used the letters p-e-r-f-e-c-t together.

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It requires that you have a perfectly flat broadband noise spectrum.
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REW does perfectly valid room response measurements
...and that's it. So where are all the "perfect" this and "perfect" that that you're referring to? Nowhere. So knock it off.

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I'm not, as the original post should have made clear, looking for "looks similar". "Looks" are not the issue, or the desired function.
You're missing the point of a detail that doesn't even matter that much. Pedantic, much?

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Discussing a "constant power noise source" (and all the implications and issues surrounding it) may lead people to think about those issues in useful ways that "impulse response" almost certainly does not.
Decay time is decay time. What difference does it make what the nature of the source is for measurement?

You don't have to use an impulse source if you don't want. I've explained multiple times already that it's common practice to use a pink noise build-up and cut off to measure room response. You don't have to go anywhere near a balloon if you don't want to. Maybe they scare you due to a childhood event involving a clown? It doesn't matter. Use pink noise and you won't have to worry about it.

OK, original question...

This is what you asked for:
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Something "electrical" rather than mechanical would be fine, as long as the acoustic power remains uniform, and the directivity doesn't change too much . . .

Simple, portable and gotta be cheap . . .
And it doesn't exist, as far as anyone who has contributed to this thread knows. People had a few ideas but you dismissed them all.

I didn't get to this thread until post #27. It seemed appropriate to indicate, based on my professional experience as someone who does these kinds of tests for a living, that there's no point in you seeking out this magical device. If you really want such a device, don't let me stop you from looking, but I can't imagine you'll ever find it. And here's why: There's no reason for anybody to make one. Not that I can think of.

If you can't find one, but you still really, really want one, you can build/measure your own. Pretty much any fan will do, as long as you can get it to spin at a known speed. Shouldn't be a problem, since electrical grid frequency is reasonably constant.

Then measure it. You'll have to go through the process of characterizing the room you're using through RT tests, but fear not, you'll only have to do it just this once. Measure the room you're in, then measure the fan. Back-calculate the sound power in each band and you'll have your device.

You will not find a device that has even sound power across the spectrum. I will try to save you time and tell you not to look for it, we'll see if you listen. But you don't need it to be even across the spectrum. I don't know why you think you need that, but you don't. As long as levels in adjacent bands are within ~10 dB of each other, they will behave independently.
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Old 14th December 2011, 03:14 AM   #45
dewardh is offline dewardh  United States
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Originally Posted by RevMen View Post
I really am trying to be helpful here.
Thank you for that . . .

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Originally Posted by RevMen View Post
Decay time is decay time. What difference does it make what the nature of the source is for measurement?
Since you imply "none" then why the objection to using a known noise source?

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Originally Posted by RevMen View Post
it doesn't exist
Well, perhaps not a "perfect" one, but how about one "as good as" the one they had at McIntosh? Do you have any reference to the performance (how "good" or "not good") of that one?

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you can't find one, but [if] you still really, really want one, you can build/measure your own.
That was the point of the original question. In the absense of any help, thanks at least for the encouragement. But, as I commented to Pano, I would rather skip the "project" (of making something that "doesn't exist", even if it once did) and get straight to the use . . .
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Old 14th December 2011, 03:30 AM   #46
Pano is offline Pano  United States
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Quote:
Originally Posted by dewardh View Post
Since you imply "none" then why the objection to using a known noise source?
His objection seem perfectly clear to me. It isn't needed with today's hardware and software and will be more trouble than it's worth.

Would be fun if you found one, but that doesn't look likely.
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Old 14th December 2011, 03:38 AM   #47
Ron E is offline Ron E  United States
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Originally Posted by dewardh View Post
I would rather skip the "project" (of making something that "doesn't exist", even if it once did) and get straight to the use . . .
I guess the point is, how would you use it?
Say you have the exact same fan as used by macintosh, what next?
What kind of microphone do you plan to use with this setup, and what kind of measurement instrument? What are you even measuring?

How can you measure RT60 shorter than the fan takes to slow down? I think a loudspeaker source is going to be much more useful for real room measurements because it stops faster than a fan. Directivity doesn't matter much in a reverberant test room, but it does in a real room, and I doubt that fan is effectively an omnidirectional point source at anything but very low frequencies.
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Old 14th December 2011, 03:43 AM   #48
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I asked the same thing back in post #28, but got no answer.
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Old 14th December 2011, 04:04 AM   #49
Ron E is offline Ron E  United States
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Deward,
take a look at Room Equalization Wizard and Holm Impulse.
Two free tools that are extremely powerful and easy to use.
Speaker workshop can also do impulse measurements.

You could even do measurements with Audacity - record a balloon pop, or a hammer blow on a piece of stone, or a squirrel cage fan

I have SoundEasy and would not recommend it for a beginner, it is really a bewildering kluge of tools with no discernable workflow - it works if you essentially understand how to write a program to do measurements yourself...
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Old 14th December 2011, 04:06 AM   #50
RevMen is offline RevMen  United States
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Originally Posted by dewardh View Post
Since you imply "none" then why the objection to using a known noise source?
The RT60 measurement is different than the known sound power measurement. You use the sources in a different way.

For the RT60 measurement, you excite the room with your source (whether impulse or pink noise), remove the source (automatic for an impulse, switch off the source for pink noise), and watch the SPL drop in each band with respect to time. Your room response is a function of the time it takes the SPL to die down by 60 dB in each band.

For the McIntosh known sound power measurement, you simply leave the noise source on and measure the resulting steady state SPL. Your room response is a function of the known sound power and the measured SPL. It's an easier measurement to take, but you have to know the sound power of your source, and it's not likely to be as accurate. I also think this would require a very reverberant room to have a chance of working correctly, whereas RT60 can be measured in live rooms and dead rooms.

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Well, perhaps not a "perfect" one, but how about one "as good as" the one they had at McIntosh? Do you have any reference to the performance (how "good" or "not good") of that one?
I don't, but in the 7 years I've been doing mechanical noise analysis I've never seen a fan with a flat Lw spectrum. It's possible they tweaked it somehow to get a more specific Lw spectrum, but that would take a lot of effort and that unit is almost certainly 1 of a kind.
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