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Fig. 4. Magellan synthetic aperture radar data is combined with radar altimetry to develop a computer generated three-dimensional perspective of the surface of Venus, showing the 5 km-tall volcano Maat Mons and the surround- ing terrain. The viewpoint is located 634 kilometers north of Maat Mons at an elevation of 3 kilometers above the ter- rain. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground, to the base of Maat Mons (simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft) (Credit: Jet Propulsion Laboratory).
al atmospheres on the timbre and pitch are markedly differ- ent. We will assume that the instrument has been construct- ed to withstand the environmental conditions (the high tem- peratures and pressures at ground level on Venus posing a particular challenge). Electrical instruments will be relatively unaffected. Care was taken in the above illustration to speci- fy that the organ was assumed to contain flue pipes only, so that the high sound speed on Venus would cause the pitch of the note to rise. However if the organ also contained reed pipes, fluid loading in Venus’ atmosphere would cause the
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If an organ tuned on Earth were to be played with both reed and flue pipes on Venus, the result would be very different from the simple pitch-shift dis- cussed above for the flue pipes, since the different types of pipes would experience a pitch shift in opposite directions. Furthermore the vibration frequency of the reed would no longer be matched to the resonances of its pipe. The effect of fluid loading is less on Titan and Mars (where the thin atmosphere might find it more difficult to make the reeds vibrate at all), such that there, the effect of sound speed would be the major consideration. However because on Venus a wealth of different physical phenomenon can affect the various mechanisms by which an instrument generates sound, shifting resonances in different directions on the same instrument, this planet is by far the most intriguing (especially when one considers that surface conditions will require the most ingenuity when it comes to the choice of
the predicted frequency.
The human voice responds to an alien atmosphere in a
pitch of those pipes to decrease.
materials).
The reason why Venus has such an effect is as follows.
The frequency of a flue organ pipe will, to a first approxima-
20 Acoustics Today, July 2009
tion, scale with the sound speed of the atmosphere (although there will be second order effects based on interactions with the pipe wall as the gas density increases). However the note of a reed organ pipe is primari- ly determined by the vibrational fre- quency of the lightly-damped reed (which is tuned to match the pipe frequency so that the pipe amplifies the note and provides timbre). In the dense atmosphere of Venus, the “added mass” associated with the displacement of atmosphere as the reed vibrates will reduce its natural
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dence.
The “added mass” associated
with an alien atmosphere will tend to decrease the natural vibration fre- quencies of a structure if the atmos-
phere is denser than Earth’s (as is the case on Venus, Titan, and the gas giants).53 However Mars’ thin atmosphere gives less “added mass” than Earth’s, tending to increase vibrational
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resonance frequency. Understanding such fluid/structure
interaction is vital for predicting the performance and safety of structures in dense environments, particularly as there is no opportunity to drop-test these on Earth. This is particu- larly so for the lightweight structures used on probes, as is illustrated by the following example, where neglect of the fluid/structure interaction leads to an error of nearly 100% in
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manner not dissimilar to the reed organ pipe. In contrast to the flue organ pipe note, the pitch of the human voice is largely unaffected by sound speed changes per se. Vowel pitch comes instead from the frequency of the mechanical vibra- tion of a solid (the vocal folds). Changes to the gas in the pipe (the vocal tract, including the larynx, the pharynx, and the mouth and nasal cavities) affect only the resonances by which the listener gains an impression of the physical size of the speaker (e.g., a small child or a large adult). A reduction in the sound speed of the gas within that tract makes the speak- er appear larger, while an increase in the sound speed makes the speaker appear smaller without altering the basic pitch of the voice (just as humans breathing helium sound smaller but, because the pitch is unaltered, they still sing in tune pro- viding they were able to do so in air). However the “added mass” effect will change the frequency at which the vocal folds vibrate, preliminary calculations suggesting that fluid loading on Venus will drop the pitch by around half an octave
However the resonances of the pipe to which the reed is attached will increase. Hence a reed pipe tuned on Earth will find its res- onances mismatched on Venus: for a given reed, the pipe length will need to increase if the resonances are to be brought back into correspon-
frequency.