Pipe dreams
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.
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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 掳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鈥檚.
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 掳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
Answer: Yes, bagpipes do work with helium or helium mixtures鈥�100 per
cent 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
programme Local Heroes last year. 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.
Details can be found on the Local Heroes website at
www.bbc.co.uk/history/programmes/local_heroes/diy/heliracket.shtml. As an
alternative to helium, mixtures of gases heavier than air, such as oxygen and
neon, could be used to lower the pitch.
I can also report that changes of gas do nothing for tone quality.
Mark Williams
Winchester
Answer: 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鈥檛 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.
Laurie Griffiths
By e-mail, no address supplied
Answer: In response to the question asking whether bagpipes sound better if
played with a mixture of helium and oxygen instead of air: of course not,
bagpipes already sound perfect.
Joe Boswell
Aberdeen
This week鈥檚 questions
Text teaser: I was just reading p 31 of the 20 October 2001 issue of New
杏吧原创. The white text on the page seems to float above the black text.
Both are on a red background but the black text seems flush with the background
while the white text apparently floats above. I have noticed this effect with a
variety of coloured texts on different backgrounds. What causes it?
Bob Muirhead
By e-mail, no address supplied
Honey, I鈥檓 bendy: Why does a slice of bread spread with honey gradually
become concave?
Donal Trollope
Stonehouse, Gloucestershire