Out-of-this-World Sound

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Summer is the season for outdoor music festivals. Here in Regina, for example, the Folk Festival will fill Victoria Park with music this weekend.

But as you sit on the grass at your favorite festival listening to your favorite group play your favorite songs, no doubt the thought will cross your mind, as it so often crosses mine: “What would this sound like on another planet?”

Fortunately, more knowledgeable minds than yours or mine had the same thought, and the magazine Science News reported on their findings in its June 30 edition.

But first, some general background on sound.

Sound is a pressure wave: a vibrating object bunches up the molecules in the atmosphere each time it bangs into them, and they in turn bump into molecules further from the object. The result is a series of bands of bunched-up molecules, separated by bands where the molecules are slightly spread out, hurtling away from the vibrating object at the speed of sound, roughly 1,200 kilometres per hour at sea level.

(It’s worth pointing out that the air molecules themselves aren’t travelling at the speed of sound: if they were, you’d blow your house apart every time you slammed a door. The wave travels at that speed, but the molecules stay in place, just like “The Wave” at a football game can travel around the stadium much faster than any of the players could run, even though no one moves except to stand up.)

When these speeding bands of compression reach our ears, they push our eardrums in slightly. Our eardrums spring back as the pressure band passes, then get pushed in again by the next band. Thus, our eardrums vibrate in sympathy with the original vibrating object. This activates nerve signals that our brain then interprets as sound.

How well sound waves propagate depends on atmospheric pressure (which changes with both temperature and altitude), and the medium through they are travelling. Sound travels very well through water, which is much denser than air, but it sounds different. Sound is also affected by thick fog: the water droplets absorb energy, so that sounds are muffled; but they affect some frequencies more than other, so that some sounds carry better than others.

Wind can disrupt or, in some cases, help carry the expanding bands of pressure. High in the atmosphere, sound is hard to hear, because there are fewer air molecules to bump into each other. And as the makers of the movie Alien noted (though the makers of Star Trek and Star Wars have yet to admit that it’s true), in space, no one can hear you scream.

So, how do Venus, Mars and Titan compare to Earth as concert venues? 

Andi Petculescu of the University of Louisiana at Lafayatte and Richard Lueptow of Northwestern University recently undertook extensive computer simulations to find out, and reported to the Acoustical Society of America in June that Mars would be a lousy choice, because its atmosphere is so thin sound fades away after travelling just a few meters.

Venus has a very dense atmosphere, but it’s mostly carbon dioxide, which muffles high-pitched sounds.

The best place for a concert? Titan. Its very cold, nitrogen-based atmosphere offers the best sound transmission across all frequencies.

(You can download an audio file comparing the sounds of concerts on Earth, Venus and Titan here.)

Petculescu and Leuptow’s paper isn’t really about finding the best place in the solar system for a conert, of course: it’s intended to demonstrate how analysis of acoustical properties on other planets and moons can provide valuable information about those celestial bodies’ atmospheres.

Still, if Titan is where the music sounds the best, you can bet some impresario will figure out a way to hold a concert there eventually.

Sure, there are a few logistical problems: you can’t breathe the air, the temperature is -178 C., it rains liquid methane, transportation to and from the venue will be take months or years both ways, and parking...well, parking will be a nightmare.

All the same, maybe it’s time to book your tickets now for the opening show of the Rolling Stones’ 2050 comeback tour.

Yeah, they’ll just be cryogenically preserved disembodied heads by then. But so what?

Die-hard Stones fans probably won’t even notice.

These weekly columns on science appear in the Regina (Saskatchewan) Leader Post and Red Deer (Alberta) Advocate.  They are available for one-time publication or regular syndication to any interested newspapers, magazines or on-line publications.  E-mail me for details.