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Infrasound detector

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Anonymous
June 21, 2004 3:52:48 PM

Archived from groups: rec.audio.tech (More info?)

Hi,

Just wondering if anyone has constructed a detector for infrasound (i.e.
below 20Hz). I'm guessing that such a detector would need to be more
like an aneroid barometer than a microphone. Possibly make something out
of a tin can with a membrane stretched on top, with the gubbins of a mic
(or speaker) somehow attached to the membrane. Perhaps put a pinhole in
the can to allow slower changes in atmospheric pressure to even-out. Run
the output into an oscilloscope or via an A/D converter to a PC.

Sound practical? Any better ideas? Any plans online?

Cheers folks.

More about : infrasound detector

Anonymous
June 21, 2004 6:21:01 PM

Archived from groups: rec.audio.tech (More info?)

David Skinner wrote:

> Hi,
>
> Just wondering if anyone has constructed a detector for infrasound (i.e.
> below 20Hz). I'm guessing that such a detector would need to be more
> like an aneroid barometer than a microphone. Possibly make something out
> of a tin can with a membrane stretched on top, with the gubbins of a mic
> (or speaker) somehow attached to the membrane. Perhaps put a pinhole in
> the can to allow slower changes in atmospheric pressure to even-out. Run
> the output into an oscilloscope or via an A/D converter to a PC.
>
> Sound practical? Any better ideas? Any plans online?
>
> Cheers folks.

Since the energy involved will be very small, I'd suggest creating a
motion detector. Use a very large membrane, a laser beam bouncing off it
to an interferometer, and a filter to pass the frequencies you want.

I'm pretty sure similar devices are currently used to measure
loudspeaker distortion. It should not be much of a stretch to measure
subsonic vibration of a membrane with such a device.
June 22, 2004 5:28:59 AM

Archived from groups: rec.audio.tech (More info?)

In article <MPG.1b40ce5c29b9eb01989687@news.individual.net>, David Skinner <branestawm2002@yahoo.co.uk> wrote:
>Hi,
>
>Just wondering if anyone has constructed a detector for infrasound (i.e.
>below 20Hz). I'm guessing that such a detector would need to be more
>like an aneroid barometer than a microphone. Possibly make something out
>of a tin can with a membrane stretched on top, with the gubbins of a mic
>(or speaker) somehow attached to the membrane. Perhaps put a pinhole in
>the can to allow slower changes in atmospheric pressure to even-out. Run
>the output into an oscilloscope or via an A/D converter to a PC.
>
>Sound practical? Any better ideas? Any plans online?

I've seen slow motions of the meter just using a Panasonic electret. Its
a matter of coupling the electronics at a low frequency. You can sometimes see,
vibrations of a few Hertz, caused by ventilation systems.

greg
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Anonymous
June 22, 2004 10:02:36 AM

Archived from groups: rec.audio.tech (More info?)

David Skinner <branestawm2002@yahoo.co.uk> wrote in message news:<MPG.1b40ce5c29b9eb01989687@news.individual.net>...
> Hi,
>
> Just wondering if anyone has constructed a detector for infrasound (i.e.
> below 20Hz). I'm guessing that such a detector would need to be more
> like an aneroid barometer than a microphone. Possibly make something out
> of a tin can with a membrane stretched on top, with the gubbins of a mic
> (or speaker) somehow attached to the membrane.

There are plenty of laboratory condenser microphones that routinely
respond far below 20 Hz. The Bruel & Kjaer capsule have lower limiting
frequencies specifed down to 1-3 Hz (-3 dB) and there specialized
microphon systems that have responses FAR below that. One example
being the B&K 4147, which is specified as being -3 dB at 0.001 Hz
(that's 1 cycle every 15 minutes).

Even some of the cheap electret condenser mics (cheap as in a couple
of dollars) have reasonable responses below 20 Hz.

Electrodynamic microphones won't work: they output a voltage that's
proportional to velocity and thus run out of steam fairly quickly.

> Perhaps put a pinhole in
> the can to allow slower changes in atmospheric pressure to even-out.

In fact, ALL microphones have such a contrivance specifically to
prevent biasing due to slow changes in atmospheric pressure. A great
deal of care is applied in the more expensive laboratory capsules to
ensure the time constant of the pinhole leak is within a small range,
since that leak determins the low-frequency cutoff of the microphone.
Anonymous
June 23, 2004 12:55:09 AM

Archived from groups: rec.audio.tech (More info?)

In article <MPG.1b40ce5c29b9eb01989687@news.individual.net>,
branestawm2002@yahoo.co.uk says...

Thanks for the responses so far folks - gives me a few ideas. I'd
respond to ideas individually but my newsserver is playing up - I saw
three responses when I checked at work today, but I can only see one now
from home.
November 27, 2009 3:48:10 PM

I was interested in a barometer that measures infrasound (>0.1 - 0.5 Hz) anyone know of such intruments as this that doesn't cost a fortune.
Would love any input - research, contacts etc. Thanks Therese
!