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does acoustic foam only absorb high frequency sound?

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engineheat:
I have a sound source enclosed in a thick plexiglass chamber with a microphone. I used a chamber to isolate ambient noise. This caused quite a bit of echo. I found that the echo caused the signal amplitude picked up by the mic to rise dramatically. My first question is:

1. Does echo merely changes the amplitude of the sound (as picked up by the mic) or can it also alter the frequency? In another word, say the sound source is a pure 1000 Hz tune, would the presence of echo merely cause the amplitude to rise inside the chamber (as detected by the mic) or can it also introduce changes in frequency or other attributes of the sound? It's not obvious to me so far.

In an attempt to prevent echo, I placed acoustic foam inside the chamber. While this cut down on the echo, as indicated by the amplitudes of the signal that the mic picked up, I noticed that the sound signal looked different. Specifically, I noticed the low frequency component of the sound got more pronounced.

My hypothesis is that the foam was able to absorb most of the high frequency echo but not the low frequency, so the low frequency component of the sound is amplified in the chamber due to the echo. When I looked at the sound wave plotted against time, I see a noticeable low frequency sinusoidal component introduced, and this was also shown on the subsequent FFT plot.

Is that what's happening? If so, what can I do to absorb the low frequency component?

Thanks

duckduck:
Google "diaphragmatic absorbers"

CatalinaWOW:
You have dived in a very deep pool. 

1.  Simple absorbers cannot introduce new frequencies.  But any non-linear behavior in the absorber could potentially cause mixing and introduction of new frequencies.  So your solution might work at low volume, but as intensity rises things go south.

2.  All mechanical systems will move under the force of sound wave pressure changes.  Thus they will have resonant frequencies.  If stiff enough the resonances will be above your frequency range of interest.  But absorbers must move to absorb energy, thus tend not to be stiff, potentially lowering resonances to audio ranges.  The desirable low Q of absorbers broadens the resonance peaks allowing them to be excited by a broad range of frequencies.  The tuning of all of these effects to get good results is as much art as science.

3.  Echo is usually dissipated by complex surface shape (the pyramids seen in anechoic chambers).  The shapes need to be of dimension comparable or larger than the wavelength to be effective, so in general the echo will remain strongest at lower frequencies.  Chamber size should be large with respect to frequencies of interest.  This is a problem for most home projects.  Maybe a field trip to a remote quiet location would be the easy solution.

I have personally only worked these problems as a student and observer, but the real pros at this are rare and highly valued.

One of the better solutions for sound damping is problematic these days.  Uncovered lead wool.  Used to be widely available for packing cast iron plumbing joints but is now relatively expensive, and many people are uncomfortable using lead at all.

Cerebus:
Also note that the container  itself can have a huge effect, both from the build up of standing waves between parallel surfaces and from resonance of the whole volume (see Helmholtz resonances for the latter). It's no accident that recording studios often have sloping ceilings and internal walls that are not parallel to each other (often with resonant absorbers hidden behind internal walls in the spaces created between them and ordinary rectilinear external walls to deal with the inevitable 'left over' nodal resonances).

eti:
Save the thread some half-guesses and wasted speculation, I'll send you to someone who's doing lots of research and who knows his stuff:

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