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shift experienced by those resonances which depend on the atmospheric sound speed. A qualitative impression of the effect could be obtained by placing the mouthpiece from a larger member of an instrument family, into the body of a smaller member, since in general, the dense gas reduces the frequency of vibrating structure (by an amount depending on their geomtry and mass53), but increases the resonances of gas columns. So for example, given that the flue organ pipe was transposed up a minor third between Earth and Venus, one might suppose that the effect of Venus on a C-clarinet (a barely viable instrument even on Earth) can be understood (though not quantitatively reproduced) by placing its reed on the body of an E-flat clarinet (scaling the gas column reso- nance approximately correctly for the fact that Venus’ ground level atmospheric sound speed is 120% that of Earth). However such sophistry is barely needed, since we know that clarinets are relatively robust to reed changes (the A and B- flat soprano instruments use the same mouthpiece). Indeed the authors were delighted to find that the A and B-flat soprano instruments still made recognizable tunes when played with the mouthpeice from the B-flat bass clarinet. Of course when we refer here to robustness to reed changes, we are not writing as connoisseurs of sound quality, since from that perspective many clarinetists would consider the fact that A and B-flat clarinets share mouthpieces to be a com- promise. The bass B-flat reed on an A-clarinet plays a tune at the pitch of the A-clarinet, as expected, but the sound quali- ty has been compromised. The dynamics of the mouthpiece are complicated,56 and Venusian fluid loading should be mimicked by swapping the reed only, not the mouthpiece, since we simply wish to add inertia (and not volume) to the source, but this is not practical without significant instru- ment adjustment. However the context here of swapping mouthpieces is to provide a quick terrestrial illustration of the possibilities of subjective changes, rather than attempt an accurate simulation of the performance on Venus. Swapping mouthpieces does not properly explore how an alien world truly affects the interactions between the source of vibration and the many resonances that determine sound quality and power.
In other families, the mismatch between the mouthpiece and body may compromise the balance to a greater degree, even to the extent that it becomes unplayable. However the fact that the human voice is recognizable in a helium atmos- phere suggests that some combinations may provide interest- ing results, and the importance of, say, fluid loading and the coupling of waves between gas and solid on Venus, opens up the opportunity to design instruments specifically for that world, instruments that make use of interactions that are less strong on Earth.
Combining the vibrating structure from one instrument with the body of a smaller one provides only a qualitative illustration of the effect of Venus. Quantitative calculation of the musical “colors” requires an understanding of the mech- anisms by which the sound is generated. While a clarinet might appear to be very similar to a reed organ pipe, the pre- dicted effect of an alien atmosphere is very different to that of the reed organ pipe, because in the clarinet the reed vibrates
at a frequency much greater than that of the note being played, the pitch of the note being determined by the maxi-
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22 Acoustics Today, July 2009
In similar vein, per- cussion instruments of similar size such as a church bell and a kettledrum (or tympani) might seem similar at first glance, but the effect of an alien world on both would be very differ- ent (assuming they are constructed to survive). The vibra- tions from the massive bell depend primarily on the wave speeds in the metal, and so will be primarily affected by, say, temperature while being relatively insensitive to the density and sound speed in the gas (the gas will of course influence the sound detected at distance from all instruments). However the vibration of the kettledrum is the result of com- plex interactions between the membrane, the kettle and the gas, which affect the time responses and decays of the com- ponents, and shifts the frequencies of the dominant modes from the set of inharmonic modes that would be predicted for an ideal circular membrane in free space, to generate har- monic partials in the kettledrum. These frequency shifts are caused by the fluid loading of the gas in the drum (strongly dependent on gas density), the resonances of the gas within the drum (that will depend on the atmospheric sound speed),
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mum acoustic impedance of the pipe.
and the motion of the drum and membrane.
Audio clips that show the effects of atmospheric filtering
of the organ piece (assuming use of only flue pipes) can be heard in the online media files (throughout this article, the effect of alien worlds on source level has not been included). These files show how the atmosphere of Venus can raise the pitch of the organ pipe by a minor third but drops the pitch of a child’s voice by nearly an octave, while making the speak- er appear smaller (as though coming from a very short bass). The various classes of instruments will therefore be affected in different ways by alien environments. An orchestra on another planet could only in part correct for these changes by retuning or changing strings, or transposing music to anoth- er key, lengthening pipes and lightening reeds, etc. While such measures can, to a limited extent, compensate for pitch changes, the alien world will impart changes in timbre, such that the process of writing music becomes an activity that is specific for a given world.
The sound files
In September 2007, recordings were made of one of the authors (TGL, with verbal introductions by his children) playing the organ at St. Margaret’s Church, East Wellow, Hampshire, UK (Fig. 6). These were processed using Adobe® Audition® to shift the pitch accordingly. Then filters were made to attenuate the sounds assuming absorption and an inverse square law for the acoustic intensity (spherical spreading). The specification of the organ (Fig. 7) is as fol- lows (numbers conventionally indicate the length in feet of the lowest pipe of the stop): Great Organ: 8 ft Open Diapason; 8 ft Stopped Diapason; 8 ft Salicional; 4 ft Principal; 4 ft Harmonic Flute. Swell Organ: 8 ft Open Diapason; 8ft Lieblich Gedeckt; 8 ft Viol di Gamba; 8 ft Vox Angelica T.C.; 4 ft Gemshorn; 2 ft Flageolet; 8 ft Cornopean. Pedal Organ: 16 ft Bourdon; 8 ft Bass Flute extn.
The voices also were modified using STRAIGHT,59 to