The speed of light is about 1,000,000 times that of the speed of sound in air. So if the lowest resonance of our larger reverberation chamber is around 100 MHz, then the first sound wave resonance should be around 100 Hz. This is within the range of human hearing. So we wondered if we could operate the reverberation chamber in an AUDIO mode. The answer is YES.
We put a reference microphone in the chamber (this is a microphone designed to have a very flat frequency response). We also put a stereo speaker in to the chamber. The first test was to simply listen to see what "ambient noise" was normally present. The second test was to move the stirrer to see how much noise that made. The data from both of these tests was collected by a program called audacity. It was then Fourier analyzed to get a spectra. The results are below;
The two spikes are noise make by the stepper motors (a high pitched whine). Next we turned off the stirred and put white noise into the stereo speaker and listened to the reference microphone. This data was also collected by audacity and Fourier Analyzed. These results are added to the plot below;
We can hear the noise above the background and we see a lot of resonances. Finally we turned on the stirrer. This is a little more involved since the main stirrer took two minutes to rotate once and we could only Fourier analyze 23 seconds of data at a time. Therefore we took two minutes of data and performed six Fourier transforms of the data. Then we averages the Fourier components together. The results are shown below;
You can see that the averaging reduces the variation in the amplitude of the sound. To see this more clearly we have taken the region between 4,000 Hz and 5,000 Herz in the plot below;
You can see the effect that the mode stirring is having on the audio signal as well, reducing the variation of the amplitude in the chamber.
BUT CAN YOU HEAR IT? ABSOLUTELY.
We set up an audio oscillator at a fixed tone. Then we rotated the stirrer and listened. You can then hear the intensity of the audio signal go up and down as the stirrer rotates. Here are some files to listen to! Click on the links to hear the audio! You might want to set the "skin" of your player to oscilloscope so you can actually see what is happening!
1678 Hz (this is a wave file and is about 12 MB large)
You can also hear the resonances. We attached an oscillator to the speaker and slowly changed the frequency while listening to the microphone. You can hear the level of the audio changes as you pass through a resonance.
Resonance sweep. (This is a wave file and is about 12 MB large)
(I can put up MP3 files if someone needs them....)