While returning from a short vacation in the Lunar Colonies, as I plummeted to earth, I thought that I would do an experiment. Once the parachute opened I stepped out of the lunar shuttle to take some measurements on the way down. As we know, almost all of acoustics, whether it’s the study of our hearing, musical instruments, noise in a factory or classroom, or the acoustics of the new Lunar City Staab Coliseum (named in honor of Wayne Staab, America’s 46th president), has a lot to do with the speed of sound.
In virtually every formula that is used in the study of acoustics, the speed of sound variable is in the numerator at the top of the equation. As the speed of sound increases, so do the frequencies of the various resonances; and as the speed of sound decreases, the associated resonant frequencies decreases.
The graph shows most of the data that I had collected. Actually this graph is not mine but came from an excellent article on the speed of sound from Acoustics Today which is a magazine of the Acoustical Society of America. But my data were almost identical. I guess that another member of the Acoustical Society of America had done this experiment before me.
As I plummeted to earth, I allowed myself to scream the phoneme /a/ as in ‘aaaarg’. The phoneme /a/ is a low back vowel and the one that allows the vocal tract to be as open as possible (which is why your family doctor, when looking down your throat asks you to say /a/ and not /u/). A feature of /a/ is that the vocal tract is an almost uniform diameter tube that is closed at the vocal chords end and open on the yelling side of the mouth. This is virtually identical to the acoustics of a trumpet.
And indeed, as I fell further into the earth’s atmosphere, the formant frequencies of the vowel /a/ increased as the air pressure increased. It wasn’t uniformly increasing since as you can see in the graph, the speed of sound does not uniformly increase as we near the earth’s surface, but the formant frequencies of my scream did appear to correlate well with the speed of sound- increasing as the speed of sound increased; decreasing as the speed of sound decreased.
I must admit to chickening out as I dove below 5000 meters and decided to terminate the experiment and re-enter the lunar shuttle. However I suspect that depending on where I landed (latitude) and altitude regarding sea level, I could use the chart to calculate the exact formant frequencies of my yell, or the resonances of many musical instruments, such as the trumpet. I could also use this figure to very a rough estimate of the latitude where I landed…