Sound proofing

[Marshall Dudley]

I am working on a device which uses a sensor that is very sensitive to
vibration (an unintended consequence of it's construction). In fact it
is so sensitive, it makes an excellent microphone! The sensor is flat
and thin as a CD or DVD, and about 1.5X2.5 inches oval. Anyway, I need
to find a material to sound proof it so it is insensitive to sounds. I
tried putting 1/4 inch of dense foam rubber on each side of it (the kind
that is used for insulation which is sticky on one side), but it did
nothing as far as I could tell. I put it into an aluminum box, and that
increased the sensitivity by 10 or so db, even wrapped in foam rubbber!

Does anyone have any suggestions. Potting in epoxy or polyester resin
might work, but the device cost $50 and I really don't want to do that
unless I am sure it will work.

Thanks,

Marshall

[GregS[_3_]]

In article >, Marshall Dudley > wrote:
>I am working on a device which uses a sensor that is very sensitive to
>vibration (an unintended consequence of it's construction). In fact it
>is so sensitive, it makes an excellent microphone! The sensor is flat
>and thin as a CD or DVD, and about 1.5X2.5 inches oval. Anyway, I need
>to find a material to sound proof it so it is insensitive to sounds. I
>tried putting 1/4 inch of dense foam rubber on each side of it (the kind
>that is used for insulation which is sticky on one side), but it did
>nothing as far as I could tell. I put it into an aluminum box, and that
>increased the sensitivity by 10 or so db, even wrapped in foam rubbber!
>
>Does anyone have any suggestions. Potting in epoxy or polyester resin
>might work, but the device cost $50 and I really don't want to do that
>unless I am sure it will work.
>
>Thanks,
>
>Marshall

Was the rubber touching the device ? what is the mounting scheme ?
Any vibration sitting on a table, for example, will transfer into it. It needs
mass, and suspension for the overall box, and that box needs insulation
on the outside and inside. A lead box would be better,
or steel.

greg

[Arny Krueger]

"Marshall Dudley" > wrote in message

> I am working on a device which uses a sensor that is very
> sensitive to vibration (an unintended consequence of it's
> construction). In fact it is so sensitive, it makes an
> excellent microphone! The sensor is flat and thin as a
> CD or DVD, and about 1.5X2.5 inches oval. Anyway, I need
> to find a material to sound proof it so it is insensitive
> to sounds. I tried putting 1/4 inch of dense foam rubber
> on each side of it (the kind that is used for insulation
> which is sticky on one side), but it did nothing as far
> as I could tell. I put it into an aluminum box, and that
> increased the sensitivity by 10 or so db, even wrapped in
> foam rubbber!
> Does anyone have any suggestions.

Put the sensor into an evacuated chamber, suspended by springs that give it
a very low resonant frequency, but keep it from touching the sides of the
chamber.

[Richard Crowley]

"Marshall Dudley" wrote ...
>I am working on a device which uses a sensor that is very sensitive to
>vibration (an unintended consequence of it's construction). In fact it is
>so sensitive, it makes an excellent microphone! The sensor is flat and
>thin as a CD or DVD, and about 1.5X2.5 inches oval. Anyway, I need to find
>a material to sound proof it so it is insensitive to sounds. I tried
>putting 1/4 inch of dense foam rubber on each side of it (the kind that is
>used for insulation which is sticky on one side), but it did nothing as far
>as I could tell. I put it into an aluminum box, and that increased the
>sensitivity by 10 or so db, even wrapped in foam rubbber!
>
> Does anyone have any suggestions. Potting in epoxy or polyester resin
> might work, but the device cost $50 and I really don't want to do that
> unless I am sure it will work.

How is it attached? By the edge? Is it the flexing of the center part of
the oval that is microphonic? Can you touch it (top or bottom) to keep
it from vibrating without impeding the main function (undisclosed) of the
device?

If you can pot it, why can't you clamp the flat surface (top and bottom)
in a foam/steel sandwich?

If you really want to suspend it then you have to deal with the "sprung
mass" and the strength of the "springs". Suspending something rather
lightweight with stiff foam would predictably do almost nothing to
isolate it.

This is really more a "mechanical" question than an "audio" question.

[Eeyore]

Marshall Dudley wrote:

> I am working on a device which uses a sensor that is very sensitive to
> vibration (an unintended consequence of it's construction). In fact it
> is so sensitive, it makes an excellent microphone! The sensor is flat
> and thin as a CD or DVD, and about 1.5X2.5 inches oval. Anyway, I need
> to find a material to sound proof it so it is insensitive to sounds. I
> tried putting 1/4 inch of dense foam rubber on each side of it (the kind
> that is used for insulation which is sticky on one side), but it did
> nothing as far as I could tell. I put it into an aluminum box, and that
> increased the sensitivity by 10 or so db, even wrapped in foam rubbber!
>
> Does anyone have any suggestions. Potting in epoxy or polyester resin
> might work, but the device cost $50 and I really don't want to do that
> unless I am sure it will work.

Without knowing what the device is and the mechanism by which it is affected
by sound, any reply would be pure guesswork and therefore likely worthless.

Graham

[Martin Drautzburg]

Marshall Dudley wrote:

> I tried putting 1/4 inch of dense foam rubber on each side of it (the
> kind that is used for insulation which is sticky on one side), but it
> did nothing as far as I could tell. I put it into an aluminum box, and
> that increased the sensitivity by 10 or so db, even wrapped in foam
> rubbber!

When you want to prevent sound from reaching your sensor, you need to
create an impedance mismatch between the sound source and the sensor.
This means you have to put something very heavy (or something very
stiff) in between. Neither foam rubber nor aluminum satisfy these
criteria.

You can also build a sandwich of mechanical mass/spring lowpasses. This
could e.g. be a heavy box inside another heavy box with something
elastic (e.g. air) in between. But you'll need to adjust the weights
and elasticity properly, which can be next to impossible.

Then you need to prevent any direct sound paths, i.e. the box must be
air tight so sound cannot travel through air and the device must not
touch the box so sound cannot travel though solid matter.

If its a small device and you cannot e.g. place it inside a concrete
block, all this will be very difficult.