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.

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.

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.

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.

In article , 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?

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

In article , 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?

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

In article , 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?

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

David Skinner wrote in message et...
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.

David Skinner wrote in message et...
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.

David Skinner wrote in message et...
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.

In article ,
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.

In article ,
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.

In article ,
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.

David Skinner wrote:

Hi,
Just wondering if anyone has constructed a detector for infrasound

Sound practical? Any better ideas? Any plans online?

Although I've seen plans for building several configurations on-line, I
don't have a URL handy.

Here's a good starting point.
http://www.etl.noaa.gov/et1/infrasound/

Be sure to get the pdf doc:
http://www.etl.noaa.gov/et1/infrasou...infrasound.pdf


-Greg

David Skinner wrote:

Hi,
Just wondering if anyone has constructed a detector for infrasound

Sound practical? Any better ideas? Any plans online?

Although I've seen plans for building several configurations on-line, I
don't have a URL handy.

Here's a good starting point.
http://www.etl.noaa.gov/et1/infrasound/

Be sure to get the pdf doc:
http://www.etl.noaa.gov/et1/infrasou...infrasound.pdf


-Greg

David Skinner wrote:

Hi,
Just wondering if anyone has constructed a detector for infrasound

Sound practical? Any better ideas? Any plans online?

Although I've seen plans for building several configurations on-line, I
don't have a URL handy.

Here's a good starting point.
http://www.etl.noaa.gov/et1/infrasound/

Be sure to get the pdf doc:
http://www.etl.noaa.gov/et1/infrasou...infrasound.pdf


-Greg