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James F. Lynch
Postal:
Woods Hole Oceanographic Institution MS #11 Woods Hole Massachusetts 02543 USA
Email:
jlynch@whoi.edu
Acoustics and Astronomy
The universe has a sound foundation!
Introduction
Two famous science fiction movies have left the general public a little bit conflicted about whether the words “sound” and “space” (as in “outer space”) belong together. The promotion for the movie Alien touted the phrase, “In space, no one can hear you scream,” whereas in the popular Star Wars movies, the whine of the Empire’s “TIE fighters” is an iconic sound. Of course, in the rarefied interstellar medium in which both Alien and Star Wars occur, Alien wins on the question of “correct physics” hands down. The TIE fighters’ sound (which is a wonderful mixing of the sound of a car on wet pavement and a screeching elephant, including Doppler ef- fects as the fighters pass by; see http://acousticstoday.org/fsounds) simply does not exist in the near vacuum of interstellar space.
The basic reason for this is simple: sound requires a reasonably substantial mate- rial medium for its propagation. A typical density of atoms (mostly hydrogen) in interstellar space is about 10 per cubic centimeter, with a temperature of about 100 K, and thus a root-mean-square (RMS) velocity of about 1 km/s. Under such conditions, an atom of hydrogen collides with another roughly once every billion seconds! Assuming that a sound wave has to be as less than or equal to the fre- quency of the atomic/molecular collisions, we see that we are looking at a billionth of a hertz sound. Not exactly in the range of possible human hearing, for which 20-20,000 Hz is a generous bandwidth estimate.
However, the universe is both vast and old (by human standards), and sound has actually been produced in the universe at many different places and at various times. Moreover, sound is also an incredibly useful tool for studying the universe.
In doing a grand tour through space and time (because the farther out in space we look, the farther back in time we go), it makes sense to organize the tour in some orderly fashion. Our chosen ordering will be to start at home in our present solar system and then work our way outward.
Sound on Earth
Because you are reading Acoustics Today, you are likely familiar with the proper- ties and many uses of sound on Earth. Sound provides one of the most common bases of communication between animals (with light and chemical communica- tion being the other prominent means). More important to this article, sound also provides an excellent tool for remote sensing of solids, liquids, gases, and even plasmas (ionized gases). By first looking briefly at how we study sound on Earth and use it for environmental sensing, we can set the stage for how to expand this knowledge outward.
Let us start with our atmosphere. A very standard acoustic atmospheric probe is sonic detection and ranging (SODAR), the acoustic backscattering version of radar in air. In addition to ranging, SODAR can give very good vertical profiles of tem- perature, wind speed, and turbulence. Another (hybrid) acoustic technique used
©2017 Acoustical Society of America. All rights reserved. volume 13, issue 4 | Winter 2017 | Acoustics Today | 27