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, one can keep up the flow of air 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 32°F is 1,087 feet per second. And in helium (where M = 4.003) the speed is 2,924 feet 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.
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.
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.
Many folks are impressed when they fill their vocal cavities and lungs with helium.
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 color, actually, the formant, of the voice changes dramatically. The voice sounds higher because the color 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.
Bagpipes do work with helium or helium mixtures-100 percent helium in the bag raises the pitch by about an octave and some retuning is required between chanter and drone.
Trials were done with bagpipes as a precursor to designing the heli racket, an instrument entered in the new musical instrument challenge run by BBC2 TV program Local Heroes. Helium and air were blown through a bagpipe chanter, and notes changed by varying the ratio of gases using a mixing valve, in this case, a bathroom tap, rather than finger holes. The instrument gave a passable televised rendition of “Twinkle Twinkle Little Star.”
As an alternative to helium, mixtures of gases heavier than air, such as oxygen and neon, could be used to lower the pitch.
Changes of gas do nothing for tone quality.
The resonant frequencies of all pipes and air chambers are directly proportional to the speed of sound. A helium/oxygen mixture will increase all the frequencies but carbon dioxide will have the reverse effect, so musicians beware.
Woodwind players know that they must avoid drinking fizzy drinks before performances. If you belch into the instrument as you play it, you fill it with carbon dioxide which has a lower sound velocity.
The instrument goes horribly flat and doesn’t recover its pitch until all the carbon dioxide has been blown through. Changing from pure air to pure carbon dioxide would send the instrument about seven semitones flat.
Bagpipes already sound perfect.