Wanted: acoustic "noise" source

[tvrgeek]

But was not their goal to be able to extract the room from the measurement?

[dewardh]

Originally Posted by tvrgeek

But was not their goal to be able to extract the room from the measurement?

What does that mean?

Their "goal" appears to have been to measure the power response of loudspeakers. Their proceedure (using the fan as a "flat" power source, and thus the "reference") also reveals the "reverberant room's" absorbtion curve, but I don't find any reference to that being a "goal", or to their using it to measure the absorbtion of other rooms. Which isn't to say that they didn't . . .

[Chris Hornbeck]

These days wouldn't it be easiest just to sum multiple mic locations from a small source? This also gets around the otherwise thorny issue of a single mic location's universality/validity.

Thanks,
Chris

[Conrad Hoffman]

Wow, I have that same GR Sound & Vibration Analyzer with the coupling to the GR high speed chart recorder- they sold it as a system. Underneath is the beat frequency oscillator that could give 20-20k in one turn of the dial; got one of those too for heating the basement in the winter. It was neat gear and I used it to design several speakers, but the whole mess probably weighs a couple hundred pounds and can be replaced and far outclassed by a laptop and sound card. My lab is however, incomplete, as I've no squirrel cage fan.

[maxseeley]

This board is capable of being a very robust noise generator and has numerous other features. Plus, it has a 7 part article to go along with it that has been featured in Elektor magazine over the past 7 months.

Audio DSP Course (110001-91) - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement

[Pano]

How about a gun? You know, like a starter pistol - if it's the room that interests you and not the speaker. That's how a lot of impulse measurements of rooms are done.

That's what I wanted to do, until I found out you have to have a permit for a starter pistol in this stupid county.

[dewardh]

Originally Posted by Chris Hornbeck

These days wouldn't it be easiest just to sum multiple mic locations from a small source? This also gets around the otherwise thorny issue of a single mic location's universality/validity.

It would take a lot of locations, I suspect, and weighting the summation wouldn't be particularly easy. Of course the "fatal flaw" of all box loudspeakers (baffle step) is fairly simple to model, but how would you account for lobing (around the crossover frequency) on the forward vertical axis? The McIntosh technique actually measured the overall power respones . . .

For measuring a room it would probably be reasonable to take several measurements within the listening area, and to look selectively at absorption from particular walls or surfaces.

[dewardh]

Originally Posted by Conrad Hoffman

My lab is however, incomplete, as I've no squirrel cage fan.

As is mine <g>. The electronics is all easily replaced (and improved on) with more "modern" equipment . . . but without an acoustic source for reference and calibration . . .

[dewardh]

Originally Posted by maxseeley

This board is capable of being a very robust noise generator[/url]

There is no shortage of electronic pink noise sources, the problem is getting from that to "flat" acoustic noise (rather than the non-flat "power response" of almost all loudspeakers).

I'm looking for a flat (or accurately defined) acoustic source . . .

[dewardh]

Originally Posted by Pano

How about a gun? You know, like a starter pistol - if it's the room that interests you and not the speaker.

That is, of course, the old "standard source" for reverberation time . . . but I've never seen it characterized for power distribution in a way that would make it easily used for absorbtion profile. And there is that "problem" you mention . . . you probably wouldn't be allowed to carry a fan onto an airplane (they'd find a reason), but it probably wouldn't be an issue in the studio or performance space (or my listening room <g> . . .). And a steady-state noise source is easy to measure and visualize (with a spectrum analyzer).