View Full Version : explosions (observation while popping bubble wrap)

Astro Hog
04-06-2001, 09:15 AM
What is it that gives explosions their particular pitch? As I was sitting in my cube popping bubble wrap I got to thinking what it was about the compression and rapid expansion of explosions that gave them their particular sound. Why does bubble wrap have a loud sharp 'SNAP' sound where as puddle of gasoline makes a deep low sounding 'Woosh'. What about the different noises that various firecrackers make. How about the ordance explosions that you see in movies. Is the sound determined by the speed at which sound wave travels? IIRC from physics class the sound or pitch has to do with the frequency. How does the frequency of an explosion vary? Wouldn't an explosion be one quick compression wave?

Just something I was thinking about while I was waiting for my code to compile.

04-06-2001, 03:05 PM
I don't know the answer but I don't mind guessing.

First, one big difference between popping bubble wrap and lighting gasoline is the size of the source of the sound. Part of the reason the gasoline "whooshes" is because it is much more extended in space -- you are hearing the sound made by each part of the flamefront as it moves across the puddle. With the bubble wrap you hear a "pop" because it all happens in one place.

Second, the sound is governed by the frequency of the sound waves generated, but it's probably more useful to think of the frequency spectrum of the waves, rather than a single frequency. Musical sounds are composed of a set of harmonic frequencies with a fundamental frequency which dominates (that's the pitch). Explosive sounds are not harmonic. They consist of a jumbled collection of frequencies, usually a broad spectrum of closely spaced, or even continuous, frequencies.

Finally, as we rapidly exhaust my knowledge of the subject, another important parameter is how fast the sound is generated. Gunpowder is explosive because it burns so quickly. "High" explosives burn more rapidly yet. When all the energy is released at once that portion that is converted to sound waves is more compressed. These leads primarily to higher volume but obviously it has some effect on the spectrum of the noise as well. And at some point the pressure wave is generated so rapidly that it exceeds the speed of sound. This creates a shock wave which has its own sound characteristics.

Not much of a direct answer, Astro, but it may draw enough attention from knowledgeable dopers who now need to correct my explanation that we'll get the whole story.

04-06-2001, 06:00 PM
I don't know either, but like Pluto I don't mind guessing - and I want to elaborate on what he says.

The speed at which the explosion progresses must determine the apparent frequency of the sound. Two different but equally complete descriptions of the sound would be 1) the air pressure as a function of time, and, 2) the amplitudes and phases of all the perfect sine wave frequencies from zero to infinity (or from 20 Hz to 20 kHz). That is, these two things can be completely transformed into one another mathematically. Generally, very sudden changes in the pressure as a function of time are equivalent to a greater share of higher frequencies; and a brief duration for the entire event are equivalent to a lesser share of low frequencies. I think both of these are associated with an explosion that propagates quickly.

Sure, there is a dog's breakfast of frequencies jumbled together in this sound. But I buy that it has a "tone", a pitch apparent to the ear. I mean, you can snap your cheek with your finger and create a tune by changing the volume of your mouth, and that's a jumble of frequencies. Ears are fantastically good at picking out a frequency in signals that aren't much dominated by it.