Sound in a Vacuum Chamber Pt. 2: Experiment

In the previous part to this, I briefly discussed the predicted relationship between ambient air pressure and sound intensity. I have in fact actually measured this relationship, and at a few different frequencies for sound.

Before I discuss the results, I should describe my setup. I have a small glass vacuum chamber, into which I place a handheld sound meter and a small bluetooth speaker. There is a simple analog pressure gauge on the vacuum chamber, and the bluetooth speaker is paired with my tablet. On my tablet, I run the app FREQUENCY SOUND GENERATOR ver. 2.30, which allows me to control frequency and volume output for the speaker.

The basic procedure is to turn on the vacuum pump, evacuate the chamber to the desired pressure level (air may be allowed back into the chamber as necessary). I then allow the chamber to stabilize its pressure, and I record the backround pressure as well as the sound level with the frequency generator turned "off." I turn on the speaker and play the sound at the pre-selected frequency and volume, and then record the detected sound intensity level from the sound meter. This value fluctuates a bit, so I estimate a rough average, which shows one area with room for improvement, I suppose.

Anyway, I have the data plotted below, for a set of three frequencies. The differences between the frequencies may give an approximation for the error involved--I don't expect frequency to make much difference, but I would need to repeat the experiment to confirm this.

Note that the fit is actually parabolic (quadratic, I ~ P^2), rather than linear (I~P) as predicted. I think that this may be a result of the detector's having some response to background pressure which is itself linear. The detector is, after all, basically just a condensor microphone, which detects intensity via variations of pressure which manifest in changes of the capacitor plate separation, hence capacitance, hence stored charge.