Sound dampening with fibreglass

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Richard Bailey - 04 Dec 2004 20:56 GMT

I conducted an experiment to measure the sound dampening properties of
various materials, including fibreglass. Interestingly, the experiment
with fibreglass actually amplified the sound which I found strange.

Does anyone know what was likely to cause this? I was thinking that it
could be packed either too densely or too loosely.

Thanks,
Richard.

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Noral Stewart - 05 Dec 2004 02:23 GMT

What is even stranger is how the fiberglass or any material could make the
sound wet. Or perhaps are you trying to damp the sound rather than dampen
it. You really need to explain more clearly what you were trying to do.

>I conducted an experiment to measure the sound dampening properties of
> various materials, including fibreglass. Interestingly, the experiment
[quoted text clipped - 5 lines]
> Thanks,
> Richard.

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Richard Bailey - 05 Dec 2004 10:05 GMT

> What is even stranger is how the fiberglass or any material could make the
> sound wet. Or perhaps are you trying to damp the sound rather than dampen
[quoted text clipped - 9 lines]
> > Thanks,
> > Richard.

I was trying to absorb the sound (i.e. turn a portion of it into
thermal energy). I conducted an initial experiment with no influences
to compare with subsequent experiments. I measured the sound output
from a signal generator with a microphone. When I used fibreglass, the
voltage produced by the microphone was actually higher than when I had
used no absorbing materials at all, which surprised me. I'm not really
sure why this was.

Richard.

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Herb Singleton - 05 Dec 2004 14:56 GMT

> I was trying to absorb the sound (i.e. turn a portion of it into
> thermal energy). I conducted an initial experiment with no influences
[quoted text clipped - 3 lines]
> used no absorbing materials at all, which surprised me. I'm not really
> sure why this was.

Can you describe your experiment? Were you measuring transmission loss
or absorption? How was the fiberglass mounted? Was this experiment
performed indoors or outdoors? If indoors, what was the size of the
room, and where in the room were the microphone and fiberglass located?
What kind of microphone were you using (omni?)?

Signature

Herb Singleton
usenet3@ross-specrtrum.com
Sound & Vibration Measurements
http://www.cross-spectrum.com

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Bob Cain - 06 Dec 2004 00:19 GMT

> I conducted an experiment to measure the sound dampening properties of
> various materials, including fibreglass. Interestingly, the experiment
> with fibreglass actually amplified the sound which I found strange.
>
> Does anyone know what was likely to cause this? I was thinking that it
> could be packed either too densely or too loosely.

If the stimulus was of a single frequency you may have been
in an anti-node, due to interference with reflections, which
goes away with the absorber in place. If you tune your
generator to a frequency that shows a particularly high
output at your measurement point when the absorber isn't
there then you will likely see it go down with it in place.

Bob

Signature

"Things should be described as simply as possible, but no
simpler."

A. Einstein

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The Ghost - 06 Dec 2004 15:27 GMT

>> I conducted an experiment to measure the sound dampening properties of
>> various materials, including fibreglass. Interestingly, the experiment
[quoted text clipped - 10 lines]
> there then you will likely see it go down with it in place.
> Bob

A pressure node is the location of minimum pressure. A pressure anti-node
is the location of maximum pressure. The microphone would have to be in a
pressure node for the pressure to increase when the sound absorbing
material is introduced.
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.html

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Brian Ravnaas - 13 Dec 2004 22:40 GMT

we're doing some work in our humble lab on modeling the impact that
various absorption materials have on apparent cavity size and damping
of the mass-spring-mass resonance in a serious of test "panels".

that would be related to transmission loss, and from the description i
don't think that this relates to your inquiry, but it is possible for
the addition of absorbing material to worsen transmission loss at some
particular frequency... this can happen if a resonance shifts to a new
frequency. i.e., adding fiberglass moves the resonance from 85hz to
61hz, etc.

Brian

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Natural Science Forum / Physics / Acoustics / December 2004

Sound dampening with fibreglass