similar (in effect) to the one shown here;
McIntosh Reverberant Room
to use for measuring/calibrating "room response". 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 . . .
McIntosh Reverberant Room
to use for measuring/calibrating "room response". 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 . . .
Not all "squirrel cage" fans are created equal (I have several, and they "sound" different). Finding the "right" one (or one with a "calibration curve") would be fine with me . . .
Too many resonances, probably, and I'm not sure how one would deal with the power response irregularity when they start beaming (in other words, how would one calibrate it?). The wind turbulence noise may show more spectral uniformity?
I'd like a tool . . . not another "project" . . .
I have to admit, I don't quite get what they were trying to achieve with the secondary noise source. Is it a reference floor that is extracted from the DUT output? We have far better ways to understand polar radiation.
If one had the noise source, then you would need to build the room. A far harder and more costly project.
If one had the noise source, then you would need to build the room. A far harder and more costly project.
Without a more clear explanation of *what* you don't understand it's hard to help. In the McIntosh lab the noise source provided a reference standard for power response measurements. What I'm looking for is that, and a way to (easily) measure the absorbtion profile of a room. Anything with a known power output spectrum would do, but a flat spectrum makes it easier, since the room can then be read directly with a spectrum analyzer (without having to apply a correction curve).
So, you are looking for basically a perfect omni-directional pink source and known power. Not sure one exists. Well, a fan is pretty clever for the 50's. Probably a bit more white than pink. Might explain some of Mac's speakers 
I am guessing they were trying to save the absurd cost of an anoeic chamber.
I believe the current thoughts are to use very short bursts that are less that the reverb time, and to gate the measurement. This basically eliminates the room from the DUT. I have never been a fan of broadband noise testing. About as useful as pure tone. I am sure others have their views and preferred methods. I am afraid the "easy" way we are all searching for still eludes us. It might be worthwhile to note, Joe D'Appolito gets buy with his basement and a few old surplus cotton mattresses.
I am guessing they were trying to save the absurd cost of an anoeic chamber. I believe the current thoughts are to use very short bursts that are less that the reverb time, and to gate the measurement. This basically eliminates the room from the DUT. I have never been a fan of broadband noise testing. About as useful as pure tone. I am sure others have their views and preferred methods. I am afraid the "easy" way we are all searching for still eludes us. It might be worthwhile to note, Joe D'Appolito gets buy with his basement and a few old surplus cotton mattresses.
Doesn't have to be "perfect" . . . that makes it a quest for the Holy Grail. "Known power" is more important that directionality (within reason, of course).
No, they were looking to directly measure power response, since "guestimating" it from averaging multi-axis measurements was even more tedious then than it is now.
That's fine for single axis measurement of loudspeakers, not so fine for power response measurement, and all but useless for measuring a room's absorbtion spectrum.
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 . . .
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.
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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
Audio DSP Course (110001-91) - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement
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.
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 . . .
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 . . .
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).
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