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Old 6th January 2008, 05:25 PM   #11
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It's pretty easy to calculate the Helmholtz resonance knowing the orifice diameter and length, along with the free volume between the back of the orifice and the mike diaphragm. It will (for practical geometries) be at very high frequencies (10k or higher).

I actually had to work this out for a B&K 4002 mike for my dissertation back in the days when dinosaurs ruled the Earth. The number ended up being in the 20k range for that particular setup. If you're interested in detailed analysis, it might be worth digging up old papers on photoacoustic spectroscopy, where this is all pretty well known.
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Old 6th January 2008, 07:04 PM   #12
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Wow, I used to work for a place where we made PAS cells with little Knowles microphones. A blast from the past.
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Old 6th January 2008, 07:13 PM   #13
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Where was that? Not many commercial PAS cells available, which is why I built my own. MTEC?
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Old 6th January 2008, 07:30 PM   #14
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Quote:
Originally posted by Conrad Hoffman
................with little Knowles microphones. A blast from the past.
Knowles makes also little accelerometers (available from Farnell) that go up to over 10 kHz and these can also be used as a contact microphone.

Other option is to use a piezo disk from a music postcard. They will work pretty good as a microphone up to their resonant frequency (somewhere between 3 kHz and 4 kHz). These will survive very high spl IMO.
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Old 6th January 2008, 08:36 PM   #15
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Hi Sy, that was back in another lifetime at Burleigh Instruments. At the time we were also making "F-center" infrared lasers and the physicist involved with that thought it would be a good companion product because they were so few and far between. The thing was beautifully made, but also expensive, so most people probably made their own. The whole laser line was discontinued decades ago, and Burleigh was eventually bought by Exfo and moved out of the country. I have a couple of the Knowles mics, which still have competitive specs today.
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Old 6th January 2008, 10:43 PM   #16
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Quote:
Originally posted by Charles Hansen
Nothing will work for that. I think that at something like 163 dB the pressure varies from 2 atmospheres to a vacuum. That will destroy any microphone ever made, let alone a cheap $2 electret capsule.

There are two things to consider -- one is the diaphragm excursion, and the other is the maximum output level of the circuitry. I would be surprised if you could find an electret capsule that would handle more than 110 dB or so.

Good luck.
Charles, I have to disagree with you on this one. It's unfortunate how the usual cheap capsules are limited by the interface amp. The Linkwitz source follower mod routinely gets you >120dB dynamic range, and there are modest mikes that quote 138dB at 1% distortion. The usual one FET circuits don't even come close to testing the limits of the actual capsule.

But to stay on point, the latest monster car stereo record is ~182dB SPL. These guys are limited by blowing bullet proof glass right out of the vehicle body. They must have some way of verifying their measurements, so looking here probably will lead to an answer.
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Old 6th January 2008, 11:30 PM   #17
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Quote:
It will (for practical geometries) be at very high frequencies (10k or higher).
I take that back. I dug out my dissertation, looked up the formulas, and plugged in a few reasonable numbers based on EC8010's suggested 1mm aperture. For a guard plate thickness of 1mm, a 1mm aperture, an 18mm diameter diaphragm, a 3mm dead space between aperture and diaphragm, and assuming diaphragm motion is negligible compared to 3mm, there's a Helmholtz fundamental at about 7200 Hz.
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Old 7th January 2008, 03:13 AM   #18
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Quote:
Originally posted by scott wurcer
...the latest monster car stereo record is ~182dB SPL. These guys are limited by blowing bullet proof glass right out of the vehicle body. They must have some way of verifying their measurements...
They probably use a thermocouple attached to the car body.
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Old 7th January 2008, 03:16 AM   #19
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Originally posted by scott wurcer
The Linkwitz source follower mod routinely gets you >120dB dynamic range, and there are modest mikes that quote 138dB at 1% distortion. The usual one FET circuits don't even come close to testing the limits of the actual capsule.
You are correct that gains can be made on specific suitable electret capsules by modifying the common source FET into a common drain circuit. But you are still limited by the voltage rating of the FET, which is typically pretty low. I believe your numbers, but that is still not enough for the original poster's project.

There are mics that will measure that high. The only ones that I know of are the Bruel & Kjaer true condensers. These use a metallic (nickel?) diaphragm that is tensioned, as well as a 200 volt polarizing voltage. They are made in 1", 1/2", 1/4", and 1/8" sizes. The smaller sizes can handle higher SPL's. Probably the 1/4" and 1/8" sizes would work for the original poster's project.

I didn't suggest this because of the expense. For the 1/8" capsule you need the capsule, a 1/8" to 1/4" adapter, a 1/4" preamp, and a power supply. Buying all of this new would be *well* over $5,000. ACO makes some less expensive clones (possibly in the Far East, I'm not sure), but I don't think for the 1/8" capsule. You can also find this stuff on eBay if you are patient. But it will still cost over $1,000 to get everything you need. But I guess for completeness I should have mentioned it. Who knows how much the OP is willing to spend on such a project? I assumed not very much, since his starting point was trying to use $2 electrets. But I could be wrong. I often am...

Quote:
Originally posted by scott wurcer
But to stay on point, the latest monster car stereo record is ~182dB SPL. These guys are limited by blowing bullet proof glass right out of the vehicle body. They must have some way of verifying their measurements, so looking here probably will lead to an answer.
You know what they say about idle hands. Don't those guys have anything better to do???

I must be wrong about the 163 dB figure as +/- 1 atmosphere. Now I am curious, so I will see if I can find the correct number. Aha, Wikipedia to the rescue:

http://en.wikipedia.org/wiki/Sound_pressure_level

The correct number is 194 dB giving +/- 1 atmosphere.

The loudest thing I ever measured was some equipment destined for the first launch in the Space Shuttle back in the mid-'70s. They wanted to make sure it would withstand the noise and vibration of takeoff. This was at NASA Langley. We had a bunch of B&K mics nearby and B&K accelerometers mounted on the apparatus. Everything was recorded on FM tape recorders, which go down to DC.

They placed the DUT near the exit of what was referred to as a "wind tunnel". It was actually a rocket engine mounted horizontally in a building. The DUT was about 100' away from the rocket nozzle, off at an angle of 45 degrees or so. We were inside the equipment vans with all the windows rolled up and a combination of foam earplugs inside over-the-ear muffs. The van shook like an earthquake, and it was ridiculously loud, even with all the hearing protection. If I recall correctly, it hit around 155 dB. Luckily it was largely very low frequency content, so we didn't even get a temporary threshold shift. That was the closest I ever got to a rocket launch...
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Old 7th January 2008, 03:44 AM   #20
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Hey, here's a chance to test for component microphonics. Maybe all you need is a ceramic capacitor for a mic. Even coax is microphonic- how about a foot of coax into an FET amp?
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