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does acoustic foam only absorb high frequency sound?
eti:
--- Quote from: engineheat on October 16, 2020, 03:24:30 pm ---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
--- End quote ---
Watch the video in my previous post, save yourself hours or days of debate and speculation here. The guy in the video has done vast amounts of exceptionally pedantic testing with acoustic enclosures and speakers.
fourfathom:
Look at "Bass Traps". These are designed to absorb lower frequencies and are often placed in the corners of a room. There are foam traps, and also some made with various types of fiber -- some people use rolls of fiberglass insulation. I got some pre-made bass trap panels that used a high-density fiber filling, and they did make a difference in my studio.
pwlps:
--- Quote from: fourfathom on October 18, 2020, 05:35:38 pm ---Look at "Bass Traps". These are designed to absorb lower frequencies and are often placed in the corners of a room. There are foam traps, and also some made with various types of fiber -- some people use rolls of fiberglass insulation. I got some pre-made bass trap panels that used a high-density fiber filling, and they did make a difference in my studio.
--- End quote ---
There are also active bass traps:
https://www.psiaudio.swiss/avaa-c20-active-bass-trap/
As I understand these use a sort of speaker driven in quadrature with respect to the sound pressure (at least this is how I understand "active velocity") to make its acoustic impedance real and with the gain adjusted to match the acoustic impedance of the air. Actually they say the acoustic impedance is even lower than that but it's not clear for me why they are doing so.
David Hess:
--- Quote from: engineheat on October 16, 2020, 03:24:30 pm ---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.
--- End quote ---
The echos alter amplitude and phase through constructive and destructive interference but do not alter frequency.
--- Quote ---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.
--- End quote ---
The acoustical foam dampened high frequencies more than low frequencies.
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