Wanted: acoustic "noise" source

[Pano]

Yes - a fan is certainly less threatening than a pistol.

I wonder what sort of motor the fan used? It looks substantial, wonder is it was special in any way. It's handy that it puts out noise at a good round figure, like 1mW.

[benb]

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'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.

I've read of a high voltage discharge through a spark gap used to generate a good, wide-band acoustic impulse.

[dewardh]

Originally Posted by Pano

I wonder what sort of motor the fan used? It looks substantial, wonder is it was special in any way.

Just one of many questions. I've got a nice little DC motor (that's easy to speed control . . . it has a separate set of windings that serve as a tachometer) that's suitably quiet . . . finding a fan that produces uniform broad spectrum "noise", and the correct speed to spin it . . . not obvious to me . . .

Originally Posted by Pano

It's handy that it puts out noise at a good round figure, like 1mW.

I suspect that was "around' 1mW, not "exactly" 1 mW . . .

[EssB]

Originally Posted by Pano

Yes - a fan is certainly less threatening than a pistol.

I wonder what sort of motor the fan used? It looks substantial, wonder is it was special in any way. It's handy that it puts out noise at a good round figure, like 1mW.

Probably a synchronous motor. That way it would run at a constant speed locked to power line frequency and independent of power line voltage fuctations.

[dewardh]

Originally Posted by EssB

Probably a synchronous motor.

Maybe. AC induction motors running with constant load (which the fan would be) are very close to immune to normal line voltage changes . . . and "constant speed" within a few percent even with variable load. It could well be a synchronous motor, but that would not be a design requirement. There might be an advantage to a variable speed (adjustable) motor, as it would allow for optimizing the noise profile of the fan. I'm sure there are studies somewhere of the physical mechanism of noise production ("quiet" fans being a marketable commodity), I just don't know where to find them . . .

[maxseeley]

Originally Posted by 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

Alright ignore my post - should have read the original post more closely.

[RevMen]

I don't understand why you need a flat, constant noise source to measure room response. Can you explain that?

Back then they needed the fan because they weren't able to measure all 1/3-octave bands simultaneously, so they had the tedious process of measuring decay in each band separately. A known noise source was a shortcut for them to save time. Now you can measure decay in all bands simultaneously from either an impulse source (balloon pop is what we use, but starter pistol works too) or a build up and cut of pink noise. As long as you have sufficient energy in all bands, their relative levels don't matter. You can look at the decay in each band independently to determine your room response.

[Pano]

A balloon pop! Will have to try that.

Looking at the MacIntosh fan, it has an end plate with a hole. Maybe that's how it was calibrated?

How would you use a noise source like this? I understand the impulse measurements because I've used them, I'm not sure about the continuous noise source, tho some software will convolve and impulse from a noise signal.

[RevMen]

Balloons are light, inexpensive, simple to operate, and allowed on all aircraft (as far as I know).

Once I was measuring a room's response on an out of town job and I forgot to bring a tripod, meaning I had to hand-hold my meter for the test. Popping a balloon is usually a 2-handed job, plus I didn't want to pop the balloon right next to my meter, so I had a challenge on my hands.

I happened to be measuring a kitchen, so I set the balloon on the island, took a few steps back, started my meter, and gracefully threw my pencil at it point first like a dart. It worked... pop! Except I had a second impulse in my data from the sound of the pencil hitting the floor. Luckily that happened more than a second after the 60 dB decay I needed so it didn't effect my measurement.

[dewardh]

Originally Posted by RevMen

I forgot to bring a tripod, meaning I had to hand-hold my meter for the test.

What meter do you use? How does it display the room's absorbtion frequency response? Can it measure less than third octave bandwidths (identify narrow resonances)?