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