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Accelerometers to measure panel vibrations?
Accelerometers to measure panel vibrations?
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Old 22nd December 2020, 08:47 PM   #111
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
Quote:
Originally Posted by john k... View Post
I don't post much anymore or even read many posts. But I saw this thread and though I might comment. In the far field the sound pressure radiated by the driver's cone, at frequencies where the wave length is greater than the driver's diameter, is directly related to acceleration. Thus, assuming no cone breakup, if you measure the acceleration of the cone and compare it to a far field SPL measurement, any deviation of the SPL from the cone acceleration would represent sound radiated from other sources, such as the enclosure surfaces.
Wow! Very interesting! Makes total sense.
Getting the itch again, or is this a one off?
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Old 22nd December 2020, 09:31 PM   #112
Pano is offline Pano  United States
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Accelerometers to measure panel vibrations?
Yes, it does make sense. But how would one do this in practical terms? I can imagine capturing the acceleration and the sounds, but how would they be compared? A simple subtraction of one from the other?
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Old 22nd December 2020, 09:39 PM   #113
454Casull is offline 454Casull  Canada
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Quote:
Originally Posted by augerpro View Post
classicalfan> how can you look at that plot and see a modest improvement? You seem to only want to recognize a narrow band peak as "only" 6 dB down, what about the much wider band of resonances that are down 15 dB now? I'm sorry but you are seeing what you want to see, not what most unbiased observers would see in the data.
Don't forget the -40 dB reduction around 200-300 Hz! lol
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Old 22nd December 2020, 09:41 PM   #114
454Casull is offline 454Casull  Canada
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Quote:
Originally Posted by Pano View Post
Yes, it does make sense. But how would one do this in practical terms? I can imagine capturing the acceleration and the sounds, but how would they be compared? A simple subtraction of one from the other?
And do we even have measuring equipment available that is precise enough to separate out the wheat from the chaff, as it were?
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Old 22nd December 2020, 09:47 PM   #115
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
I hope John responds. Should be interesting.
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Old 22nd December 2020, 10:10 PM   #116
scottjoplin is offline scottjoplin  Wales
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Didn't he explain?
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Old 22nd December 2020, 10:16 PM   #117
Pano is offline Pano  United States
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Accelerometers to measure panel vibrations?
Well about 6 years ago we did a "Direct vs Reflected sound" test here on this forum. I can never remember the title of the thread but a number of people did it and it was successful.
Perhaps something akin to that cancellation technique?
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Old 23rd December 2020, 04:07 AM   #118
hifijim is offline hifijim  United States
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Default Measuring cabinet signature - B&W approach

What is the sound radiating from a speaker cabinet? How much, and at what frequency? At what point does it become audible with program material?

It is a question that comes up repeatedly, and the same points and counterpoints are rehashed. … and no wonder, it is a complicated subject, with too little data. I think that there are two reasons we struggle with understanding the unwanted sound which may or may not radiate from a cabinet.

(1) The cabinet sound is very difficult to directly measure. The sound from the driver(s) usually overwhelms the radiation from the cabinet, so it is hard to segregate the two. Augerpro’s thread has shown that it is possible, but only by constructing dedicated experimental boxes. It does not seem to be possible to take an existing speaker system and measure it in-situ, and simply determine how much of the sound output is due to cabinet resonance.

(2) It is even more difficult to assess cabinet signature with real program material. Swept sinewave testing can certainly reveal resonances, but this testing does not distinguish audible from inaudible sounds. The amount of energy in program material is maximum in the 200 – 300 Hz range, and it decreases rapidly above that. It decreases below that as well, but not as rapidly. It takes energy to excite cabinet resonance. There is evidence that high Q cabinet resonances are less audible because program material is less likely to excite them. One of the many cabinet design philosophies is to make cabinets stiff enough so that resonances are pushed up high (where there is less energy), and simultaneously making those resonances higher Q so they are more difficult to excite. I myself have used this approach, and I advise others to use it as well… But how effective is this, really? It is hard to say without some kind of measurement, and we just don’t have any using real program material. The prototypical BBC approach is to dampen resonance and lower the resonance frequency because low frequency cabinet resonances are less audible, according to some research. How effective is this…? Again, it is hard to say.

This AES paper from B&W may offer an interesting new way of measuring cabinet radiation. This was posted by Augerpro in post #97 of this thread
A simple (but maybe not easy) bracing question.

What B&W did, which was very innovative, was their test method of isolating cabinet acoustic radiation from the much larger driver radiation: They first demonstrated that above Fs, the cone is decoupled from the motor structure. Then by analysis they showed that the virtually all of the force transmitted to the cabinet comes from the motor alone, transmitted through the driver chassis. This knowledge allowed them to cut the cone away from the voice coil. The motor, voice coil, and spider remain intact. They were doing this to validate their FEM and other numerical methods, but it is a very interesting technique.

Now they have a driver which produces very little acoustic radiation, but which can induce fully representative vibration forces into the cabinet. This solves what I believe is the biggest hurdle with measuring cabinet radiation.

So now we can take an existing speaker system and test it. It will cost us a sacrificial driver (nothing is free it seems), but we can get a pretty definitive answer as to what kind of sound is coming from the cabinet.

Step 1 would be to make a frequency sweep of the speaker with a fairly high voltage, something like 5 V. This FR would establish the baseline. Step 2 we cut the cone out of the driver, leaving the dust cap and spider untouched. We need to replace the mass of the cone, so we add a little mass to the dust cap (silicone caulk) until the Fs is back were it was originally. Step 3 we make a sine sweep at the same voltage level. Since there is no cone, the driver will make little sound, but the full vibration energy of the motor will remain, and it will be transmitted into the cabinet. Resonances will now be easily revealed and measured. If, for example, a strong resonance at 500 Hz produces 85 dB, and the baseline measurement of the intact driver was 95 dB at 500 Hz, then we know the cabinet is 10 dB below the driver. Step 4 would be (ouch) buy a new driver.

It will be necessary to rotate the cabinet and take FR from all four sides and above. I would conservatively assume that maximum output from any direction is the representative signature at that frequency.

What about program material? Well it would be interesting to play real program material and test the assumption that high frequency high Q resonances are rarely excited. This technique would make that pretty straightforward. The theory is that an 800 Hz high Q resonance is much less intrusive than a low Q (well damped) 200 Hz resonance. Testing a high stiffness cabinets and lower stiffness CLD cabinets with real program material will reveal the truth, at least for those two cabinets and that program material…

Thoughts?

J.
Attached Files
File Type: pdf B&W AES paper on cabinets.pdf (1.41 MB, 10 views)

Last edited by hifijim; 23rd December 2020 at 04:10 AM.
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Old 23rd December 2020, 08:03 AM   #119
andy19191 is offline andy19191  Europe
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Quote:
Originally Posted by hifijim View Post
Thoughts?
The B&W paper is poor. It came up earlier w.r.t. the simulations which contain a gross systematic error in failing to correctly predict the easiest largest scale resonance of the speaker nodding on its feet. One can speculate how but if this is wrong what else is?

I don't think the measurements presented are what you seem to think. If you look at the photo of the drivers without the cone the suspension remains and is obviously going to radiate a lot of sound. Less than the cone but far more than the cabinet. If you look at the presented measurements they are not taken with a microphone but a laser vibrometer. They measured the deflection over the baffle and the side panel and integrated to get the SPL. This is the normal way in industry (and in practise likely the only reasonably accurate and reliable way) to obtain the cabinet radiation. There is of course no need to mess about removing driver cones to do this.

Conclusion: the intended experimental approach was so flawed the results could not be presented but the authors wanted to publish something regardless. Their supervisors/managers let them despite what it might portray about the technical competence of the company.
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Old 23rd December 2020, 01:59 PM   #120
john k... is offline john k...  United States
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My comment was just a thought. May not be practical. As noted the problem is that the acoustic output from cabinet surfaces is generally well below the driver's output. Right there, if that's the truth, it's not a big issue. But if it were significant, using a 2 channel measurement system you could attach the accelerometer to the driver and then use it's signal as a calibration signal for the SPL. The SPL measurement has to be anechoic. But given that both mic and accelerometer would be pretty flat at low frequency the result should be a flat curve at low frequency. Any sharp discrepancies in amplitude would be an indication of cabinet resonances, or so I would hope. Well, you would still have the baffle step in there too.

But if you really want to isolate panel resonances the way to do it would be with a laser scan of each surface. You could generate a plot of x-y position on the surface with amplitude that evolved as a function of frequency, f(t). Capture it on video and then play it back to watch the resonances evolve (in time) as the frequency changes. Like looking at the surface of a lake as the ripples change.

Maybe a simpler way would be sprinkle salt on the panel and see what happens. As soon as I though of that I figured someone on YouTube would have done it. Sure enough, here is a video showing doing that. Amazing Resonance Experiment! - YouTube

Doesn't give amplitude info but shows panel modes.
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