From "The New Scientist" of current date:

Pipe dream
 

Question
During a conversation about playing the bagpipes at high altitude, I wondered what would happen to the sound of the bagpipes if they were played in the helium/oxygen mix used by deep-sea divers, which distorts speech. Would the double-reed chanter (the output part of the bagpipe that consists of a tube with holes and is played with the fingers) be affected in the same way as the single-reed drones (the output pipes confined to single notes)?

Answer
The construction of a bagpipe allows a
continuous supply of air to be maintained. A flexible bag is filled with air and acts as a reservoir. By squeezing the bag while a breath is taken, the flow of air can be kept up in both drone pipes and chanter.

The fundamental frequency of a resonating cavity, whether it is the voice or a
resonating tube like a bagpipe chanter, is directly proportional to the speed of sound of the gas occupying the cavity. The speed of sound is proportional to the square root of T/M (where T is the absolute temperature of the gas and M is its molecular weight).
Therefore the speed of sound is higher in gases with smaller molecular weights. For example, the speed of sound in air (where M = 28.964) at 0 °C is 331.3 metres per second. And in helium (where M = 4.003) the speed is 891.2 metres per second. The resonance
frequencies of the vocal tract are therefore almost 2.7 times higher for helium than for air and the pitch will be much higher than usual, rather like Donald Duck's.

The original question is, of course, the wrong one. It is difficult to imagine playing the Scottish bagpipe in the confines of a diving bell filled with the helium-oxygen mix. The question is more relevant to the Irish whistle which is easily portable and still satisfies a deep-seated human need for Celtic music.

I carried out an experiment by inhaling from a toy helium balloon with my brass Sindt D whistle 41 metres above sea-level, where the ambient temperature was 22 °C. Once a stable note had been reached, the pitch jumped up almost exactly three semitones from D to F and remained in tune from then on. Although I had to blow harder to keep the notes constant I could play the first 12 bars of Down by the Sally Gardens without taking a breath, albeit slightly faster than usual. The air/helium mix I exhaled after taking the first breath of air returned the pitch to D sharp. However a pure D did not return for some time as residual helium was slowly cleared from my lungs. Residual lung volume accounts for about 25 per cent of total lung volume, therefore the first breath of helium was probably about 75 per cent mix, and the second approximately 18 per cent, assuming that the gas inhaled from the balloon was pure.

Tony Lamont, Brisbane, Queensland
 
 

Answer
The pitch of both types of pipe in the bagpipes is determined by the effective length of the pipe (which is varied by opening holes in the chanter) and not by the reed. The reed adapts its frequency to the resonance set up in the pipe in which it sits. The frequencies of the modes of any pipe are proportional to the velocity of sound in the gas and, because this is much higher in helium than in air, the pitch of the bagpipes must rise.

I used to teach the physics of music to opera singers at a major music college, and they were always impressed when I took along a helium cylinder and had them fill their vocal cavities and lungs with it. When you do this you need to be careful to retain some carbon dioxide in your lungs because this stimulates the automatic breathing reflex. In the case of singers, the pitch does not in fact change, because it is determined by the vocal cords, not the pipe. The resonances are not strong enough to dominate the heavy vocal cords and their di-muscular control. What does change is the frequency of every resonance of the vocal tract, and hence the tone colour (actually, the formant) of the voice changes
dramatically. The voice sounds higher because the colour shifts to higher frequencies, not the actual pitch.

In practice, very few singers managed to hear much of their new voice, because they
invariably laughed at the unfamiliar sound they produced and quickly expelled the helium.

John Elliot, UMIST, Manchester