Page 18 - Jul2009

THE SOUND OF MUSIC AND VOICES IN SPACE
PART 1: THEORY
Timothy G. Leighton
Institute of Sound and Vibration Research, University of Southampton Highfield, Southampton SO17 1BJ, United Kingdom
and
Andi Petculescu
Department of Physics, University of Louisiana at Lafayette Lafayette, Louisiana 70504
“While probes to other planets have carried an impressive array of sensors for imaging and chemical analysis, no probe has ever listened to the soundscape of an alien world. With a small number of exceptions, planetary science missions have been deaf.”
Introduction
While probes to other planets
have carried an impressive
array of sensors for imaging
and chemical analysis, no probe has
ever listened to the soundscape of an
alien world.1–3 With a small number of
exceptions, planetary science missions
have been deaf. The most successful
acoustic measurements were made by
the European Space Agency’s 2005
Huygens probe to Titan, but although
this probe was spectacularly successful
in measuring the atmospheric sound
speed and estimating the range to the
ground using an acoustic signal that
the probe itself emitted,4–7 we still have
no measurements of sounds generated
by alien worlds. Although microphones
have been built for Mars,8 the Mars Polar Lander was lost during descent on 3 December 1999, and the Phoenix probe microphone was not activated (because the Mars Descent Imager system to which it belonged was deactivated for fear of tripping a critical landing system).9 Instead of measuring acoustic signals that had propagated to the microphone from a distance, aerodynamic pressure fluctuations on the microphone (caused by wind on the surface of Venus in the case of the 1982 Russian Venera 13 and 14 probes,10,11 and turbulence during the parachute descent in the case of Huygens) masked the soundscape on these Venus and Titan missions. Given the lack of such data from these earlier mis- sions, some early enthusiasts for acoustics in the space com- munity are now skeptical as to whether it will ever have a useful role. However basing such an assessment on past per- formance presupposes that the sensor systems have been optimized for the environment in question.
Space programs work within challenging mission con- straints (e.g., in terms of sensor weight, power consumption, bandwidth, ruggedness). Acoustic systems can match these constraints well.1–3 Cutting edge acoustical capability goes far beyond what is commercially available, yet even the latter holds potential solutions to problems that limited past mis- sions. For example, the aerodynamic pressure fluctuations which prevented measurement of the soundscape by Venera and Huygens might have been mitigated by the deployment of appropriate microphone windshields (extraterrestrial ver-
sions of those used by journalists to report from stormy locations) or the use of two or more synchronized microphones to separate the real acoustical signals from aerodynamic pressure fluctuations12–15 At the cutting edge, appropriate models of the gener- ation and propagation of sounds are today being inverted to estimate key environmental parameters (such as rainfall at sea, tornado detection, ani- mal location, icecap erosion, crack formation in aircraft wings, erosion in hydroelectric turbines,16–20 in addition to the established techniques for seis- mic and global test ban monitoring). Given the vast expense involved in sending an acoustic sensor to another world, it is vital that that sensor be
properly designed for the alien environment, and that the data it detects be sufficiently free of artifacts so that it can be interpreted correctly. Detailed modeling of acoustic charac- teristics of alien worlds is therefore vital to the design of instrumentation, the planning of the acoustical components of the missions, and the correct interpretation of the data. If the astronaut from the future is walking down a Martian hill- side, looking downwards, can we design microphones to warn him of the fall of a rock dislodged behind him? How well can sound be used to confirm the opening of vanes out of camera sight on unmanned probes, or undertake diagnos- tics of motors, pumps and drills? What gain, bandwidth, sen- sitivity and self-noise are appropriate for microphones in the atmospheres of Mars, Titan, Venus, and the planets, or hydrophones in the lakes and oceans of Titan, Europa and Enceladus? Would we be able to recognize sounds as coming from “dust devils” on Mars,21 “waterfalls” on Titan,22 ice cracking on Europa,23–26 or lightning on Venus? Could not novelists, film, and documentary makers attempt to portray the soundscape with the same integrity they apply to the visual depiction of other worlds?
For acousticians, the measurement of the soundscape is
probably the most interesting role for extraterrestrial
acoustics, as the sources of sound are themselves part of the
alien world. However acoustics has three other roles in space
1
exploration. First, measurement of the propagation of signals
generated by the probe itself can be used for range-finding,
Sounds in Space 17