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Fig. 1. Composite, with planet size to scale, of Venus (top left), Earth (top right), Mars (bottom left) and Titan (bot- tom right). An atmospheric halo is visible around Titan. Images making up this composite are courtesy of NASA/nasaimages.org, Lunar and Planetary Institute and Jet Propulsion Laboratory.
anemometry, or key gas properties
through the measurement of atmos-
pheric sound speed and dissipa-
27–34
tion. Related studies include the
active seismic experiments35 of
Apollo 14, 16 and 17. Second, pertur-
bations in other signals (EM) can be
interpreted by appealing to acoustic
models of fluctuations in their source
or the propagation medium.
Examples include modal acoustic
waves in planets36 and stars,37,38 and
acoustic perturbations in planetary
rings, comets and noctilucent
39–44
an audio record for human listeners. The fourth aspect, however, of detecting the audio-frequency soundscape of another world, remains elusive. We have been tanta- lizingly close to providing this, with the wind noise data from Venus and Titan mentioned above, and the pas- sive seismic geophone data from the
46–51
measure the soundscapes of other worlds are not discouraged by lack of success in early missions. In addition to the physi- cal hostility of the environment, instrumentation must be designed to match the acoustical issues of the alien world. Some of these we will have solved for terrestrial instrumenta- tion (such as the wind noise effect discussed above), but some will be particular to a given world (such as the high absorption on Mars52 or the fluid loading effects on Venus53). However it is vital that those missions are equipped with sen- sors that are designed with knowledge not only of the envi- ronment they will encounter, but also of the likely sound- scapes they will be expected to measure. To design sensors, and interpret soundscapes, we need tools to predict how sounds will be generated, and how they will travel significant distances from source to receiver, on alien worlds. The remainder of this article uses the examples of music and speech to illustrate how we might begin to provide such tools.
Bach and planetary acoustics
Predictive modeling is key to the effective planning, design and interpretation of extraterrestrial acoustic mis- sions. Models of the generation and propagation of sound on other worlds are used in the sound files accompanying this paper that demonstrate how organ music and speech would sound on Venus, Mars and Titan. These two sounds are cho- sen because the instruments involved provide extreme exam- ples of the different ways in which extraterrestrial worlds affect the range of terrestrial instruments (and other sound
Third, signals that were never acoustic (such as radio waves generated by Jovian lightning) can, for the purposes of providing a sub- jective impression, be converted into
45
Optimized
instrument design is required to ensure that attempts to
clouds.
Apollo missions.
18 Acoustics Today, July 2009
sources). Although this exercise may seem fanciful, in that it will probably be many decades before an astronaut on Mars, waiting for the return trip, constructs an instrument outside the living area, the great complexity of musical sound sources and the discernment with which we assess their performance means that they provide an ideal demonstration of the factors (material, geometrical and dynamic) which need to be con- sidered when any stiff, light, structure vibrates on another world. Such structures are not restricted to musical instru- ments—they might range from atmospheric dirigible-like vehicles to sensors on planetary probes, such as those that respond with high sensitivity to changes in the inertia or stiffness associated with vibrating surfaces as, for example,
54
The reason for studying speech and music is that they are familiar sound sources which display a wealth of effects from an alien world’s atmosphere, in comparison to Earth’s. On Venus, for example, the pitch of a flue organ pipe will increase because it is susceptible to changes in sound speed but not fluid load- ing, while the note of a harmonica reed will fall since it has exactly the opposite sensitivities. The voice is susceptible to both, giving perceived changes in both the pitch and size of the speaker. Understanding of how such familiar structures can give such different responses to an alien world may help us identify the sources from the soundscape of another world, and design sensors appropriate for the expected
species accumulate upon an oscillating plate.
sounds.
Three extraterrestrial worlds—Mars, Venus, and Titan—
are studied and compared with Earth (Fig. 1). Its low tem-