Not if the taps are random (like noise) . . . sometimes you reinforce the swinging, sometimes you cancel it. It averages out to lots of pokes in the back for not much fun . . .
I suspect that you are confusing "sines and cosines" as mathematical functions with continuous waves. Not the same . . .
6 of these on a cube
Put a 1/4"-20 insert on one corner of the cube and put it on a tripod. Feed it with MLS and measure Aurasound NS525-255-8A 5.25" Paper Cone 8 Ohm: Madisound Speaker Store
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Nasty response curve, and definitely beaming even with six of them. NSW2 on a pipe works better, flatter and still omni to well above 2kHz (pretty good at 3, even 4k). Already have, and have tried, that. Might work up to over 5k with more of them (but expensive). Getting smooth and omni from a cone or dome above 4k is difficult. The 100-10,000 range of a fan (if they really can do that), and the absense of peaks and dips, is pretty attractive. I have a shelf full of drivers that I'd try before the 5" Aura . . .6 of these on a cube
Here are some sound power levels for typical fans according to the ASHRAE applications book.
Fan Type
63Hz - 125Hz - 250Hz - 500Hz - 1kHz - 2kHz - 4kHz - 8kHz - BFI
Centrifugal Airfoil, Backward Curved or Inclined Blade Under 30 in.
42 - 42 - 40 - 36 - 31 - 25 - 21 - 16 - 3
Centrifugal Forward Curved Blade
50 - 50 - 40 - 33 - 33 - 28 - 23 - 18 - 2
Centrifugal Radial Blade
Lower Static, Under 40"
64 - 56 - 50 - 40 - 39 - 36 - 31 - 28 - 7
Vaneaxial
Hub-Ratio 0.3-0.4
46 - 40 - 40 -45 - 44 - 42 - 35 - 31 - 6
Hub-Ratio 0.4-0.6
46 - 40 - 43 - 40 - 38 - 33 - 27 - 25 - 6
Hub-Ratio 0.6-0.8
56 - 49 - 48 - 48 - 46 - 44 - 40 - 37 - 6
Tubeaxial under 40"
45 - 44 - 46 - 50 - 49 - 48 - 40 - 37 - 7
Propeller
45 - 48 - 55 - 53 - 52 - 49 - 43 - 39 - 5
Add the BFI number to whatever octave band contains the blade passage frequency.
You are now well equipped to select your reference noise source. Choose a fan type that gets you closest to your desired spectral shape, whatever that is. Then find a fan of that type. Then either perform your own sound power measurements or send it off to an acoustics lab to have measurements done for you.
You can find fans/motors in many places. Furnaces, window unit air conditioners, kitchen vent hoods and fan powered VAV boxes are things to come to mind.
Fan Type
63Hz - 125Hz - 250Hz - 500Hz - 1kHz - 2kHz - 4kHz - 8kHz - BFI
Centrifugal Airfoil, Backward Curved or Inclined Blade Under 30 in.
42 - 42 - 40 - 36 - 31 - 25 - 21 - 16 - 3
Centrifugal Forward Curved Blade
50 - 50 - 40 - 33 - 33 - 28 - 23 - 18 - 2
Centrifugal Radial Blade
Lower Static, Under 40"
64 - 56 - 50 - 40 - 39 - 36 - 31 - 28 - 7
Vaneaxial
Hub-Ratio 0.3-0.4
46 - 40 - 40 -45 - 44 - 42 - 35 - 31 - 6
Hub-Ratio 0.4-0.6
46 - 40 - 43 - 40 - 38 - 33 - 27 - 25 - 6
Hub-Ratio 0.6-0.8
56 - 49 - 48 - 48 - 46 - 44 - 40 - 37 - 6
Tubeaxial under 40"
45 - 44 - 46 - 50 - 49 - 48 - 40 - 37 - 7
Propeller
45 - 48 - 55 - 53 - 52 - 49 - 43 - 39 - 5
Add the BFI number to whatever octave band contains the blade passage frequency.
You are now well equipped to select your reference noise source. Choose a fan type that gets you closest to your desired spectral shape, whatever that is. Then find a fan of that type. Then either perform your own sound power measurements or send it off to an acoustics lab to have measurements done for you.
You can find fans/motors in many places. Furnaces, window unit air conditioners, kitchen vent hoods and fan powered VAV boxes are things to come to mind.
To measure the sound power of your source, you need not do it indoors. If you go outside when it's quiet (late at night), you can take advantage of the near perfect anechoic environment. I think this would give you better results than trying to characterize a normal room.
Set your source up on a driveway, parking lot, or road that's a good distance from any buildings or other faces that can reflect sound. Take your measurements far enough away from the source that it approximates a point source (20' should do it). Back calculate sound power using this relationship:
Lp = Lw + 10 log(Q) - 20 log(d) - 0.7
Lp is the measured sound pressure (use Leq)
Lw is sound power
Q will be 2 for hemispherical radiation.
d is the distance between the source and your microphone in feet
I suggest taking several measurements with the source in different orientations and averaging the results (logarithmic, not arithmetic). If it was me I'd spin it 90 degrees for each measurement, including laying it on its side to capture the top.
Set your source up on a driveway, parking lot, or road that's a good distance from any buildings or other faces that can reflect sound. Take your measurements far enough away from the source that it approximates a point source (20' should do it). Back calculate sound power using this relationship:
Lp = Lw + 10 log(Q) - 20 log(d) - 0.7
Lp is the measured sound pressure (use Leq)
Lw is sound power
Q will be 2 for hemispherical radiation.
d is the distance between the source and your microphone in feet
I suggest taking several measurements with the source in different orientations and averaging the results (logarithmic, not arithmetic). If it was me I'd spin it 90 degrees for each measurement, including laying it on its side to capture the top.
That might be useful were it referenced to some specific fans, but . . .
Yes, ranging from DigiKey and Grainger to Surplus Center and e-bay (and parts suppliers for, well, almost anything that has a fan in it). We probably all knew that already. Do you have any useful information regarding cage blowers specifically suitable for the application at hand? Like an inexpensive source for something functionally like what Larson Davis uses? Size, blade type, etc.?
All pretty obvious to anyone who has ever measured loudspeakers, indoors or out, or taken sould level readings in a studio or concert hall (or read anything prior to this on the subject). But I suppose there may be some novices in this thread for whom the advice would be new and useful . . .
As stated, it's a starting place to give you an idea of what spectra you can expect from certain fan types. You're not supposed to use those sound power levels in an actual analysis, only in helping you select a fan.
What else do you need to know? The data I posted tells you relative sound power based on type, including for all 3 main centrifugal types in the size range you'd be interested in. All of those fan types will work for your application, though the centrifugals are probably better in terms of radiation pattern.
Blade pass frequency is rpm x (# of blades) / 60. Use that to determine your acceptable range of blade counts if you have certain frequency bands you're trying to avoid or enforce.
You have more than enough information to do what you want to do.
Find a fan.
Measure it under controlled circumstances.
Back-calculate sound power.
Use it to do room analysis for in-situ sound power calculations.
Be happy.
I mean, yeah, most of us have a lot of experience doing sound power measurements, but, you know, maybe someone might wander along that could use that information, eventually.
http://www.ee.bgu.ac.il/~acl/Equip/OMNI.pdf
If you search omni noise source you will find many more examples.
I have heard of people using spark gaps and I wonder if compressed air could be "pinkified" somehow with an acoustic filter of some sort?
There is quite a lot of useful information regarding this subject in the REW “help” manual, as in this explanation of why one might prefer sweeps (or broadband noise) as a test signal for room measurements:
“The impulse response is actually exactly the same signal we would see if we could emit a very short but loud click at the source position and record what the mic picks up afterwards ("very short" meaning lasting just the time of 1 sample at the sample rate we are using for our analysis, so for a 48kHz sample rate that would be just 1/48,000 of a second which is 21 millionths of a second). You might ask why we don't just use a click then. One difficulty is that the click, because it is so short, needs to be extremely loud for us to be able to pick up what happens after the initial click over the background noise of the room. We could no longer use a speaker to generate that, we would need something like a starting pistol or to pop a balloon. We would also need a mic that could cope with both the extremely loud click itself and the much quieter echoes of the click produced by the room. You are likely to find that your family and neighbours are not that keen on you repeatedly firing a pistol to figure out what your room is doing, and even if they put up with it your results would not be as good as using a sweep. To be technical about it, you can achieve a much higher signal-to-noise (S/N) ratio by the sweep method. The S/N is determined by the background noise level and by how much energy is in the test signal, which in turn depends on how loud the signal is and how long it lasts. An impulse is extremely short, a few millionths of a second, so to get any signficant energy it needs to be very loud. A sweep can last for many seconds, so even at a modest volume its total energy can approach as much as a million times more than an impulse.”
Getting Started with REW
or the more extended discussion at
Signals and Measurements
Perhaps worth noting is that the OmniMic system uses both sweeps and noise . . .
“The impulse response is actually exactly the same signal we would see if we could emit a very short but loud click at the source position and record what the mic picks up afterwards ("very short" meaning lasting just the time of 1 sample at the sample rate we are using for our analysis, so for a 48kHz sample rate that would be just 1/48,000 of a second which is 21 millionths of a second). You might ask why we don't just use a click then. One difficulty is that the click, because it is so short, needs to be extremely loud for us to be able to pick up what happens after the initial click over the background noise of the room. We could no longer use a speaker to generate that, we would need something like a starting pistol or to pop a balloon. We would also need a mic that could cope with both the extremely loud click itself and the much quieter echoes of the click produced by the room. You are likely to find that your family and neighbours are not that keen on you repeatedly firing a pistol to figure out what your room is doing, and even if they put up with it your results would not be as good as using a sweep. To be technical about it, you can achieve a much higher signal-to-noise (S/N) ratio by the sweep method. The S/N is determined by the background noise level and by how much energy is in the test signal, which in turn depends on how loud the signal is and how long it lasts. An impulse is extremely short, a few millionths of a second, so to get any signficant energy it needs to be very loud. A sweep can last for many seconds, so even at a modest volume its total energy can approach as much as a million times more than an impulse.”
Getting Started with REW
or the more extended discussion at
Signals and Measurements
Perhaps worth noting is that the OmniMic system uses both sweeps and noise . . .
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