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What would happen to sound if I traveled at Mach 1?

Dear Cecil:

I have observed that traveling high speeds causes strange things to occur (e.g., the Charger's hubcaps in Bullitt regenerated twice during the high speed chase scene). If I traveled down Austin Avenue at a speed exceeding Mach 1 (the speed of sound) would the noises made by the people on the streets be distorted or totally inaudible to me? While cruising at this speed, if I blasted my car radio, would the people on the streets hear it as I would, or would I be traveling faster than radio waves, causing my radio to die? What phenomenon would occur if I shifted into second around North Avenue and my Buick exceeded the speed of light? This information will be helpful when I rush my wife to the hospital sometime in January to deliver our baby.

Life in the Fast Lane, Chicago

Cecil replies:

Dear Life:

Nice to hear from a guy who takes a practical view of things. Dealing with the easy parts of your question first: At the speed of sound you won’t hear sounds happening behind you, because the sound waves won’t be able to catch up with you. You’ll hear sounds occurring in front of you, but they’ll be increased in pitch about an octave due to the Doppler effect–i.e., as you move toward a sound source, you crowd up on the oncoming sound waves, which increases their frequency relative to you. By the same token, people on the street won’t hear you coming, but they’ll hear you (and a sonic boom) after you’ve passed–with the pitch decreased about an octave.

What was more interesting to the Straight Dope Science Advisory Board was what you would hear if you turned on your car radio at the speed of sound. If you keep the windows closed there’s no problem, because you, the radio, and the air will all be stationary relative to one another. With the windows open, however, you wouldn’t hear the rear deck speakers at all–the air carrying the sound waves would be blown backward too quickly. However, you’d hear the dashboard speaker normally (or as normally as you’d hear anything at 700 MPH)–the twin Doppler effects of source to air and air to observer would cancel out.

Finally, regarding your last question, don’t you know the velocity of light is the speed limit of the universe? Any attempt to exceed it would defy one of the fundamental principles of creation. Besides, it’s hell on the tires.

Cecil Adams

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