When you’re on a plane, or just somewhere really high up, the air pressure is different to when you’re at ground level. I was wondering what would happen to the tuning of an instrument as a result of the change in pressure. There’s quite a bit of physics to think about for this, but the answer is pretty interesting!
When you’re on a plane, the air pressure is lower. This means that the speed of sound is quicker and therefore the wavelength is stretched. The fundamental frequency is lost and you get a different wavelength resonating in your instrument (or voice box). This has a higher frequency which equates to a higher pitch.
The tuning of musical instruments can vary dramatically under different ambient conditions. The most common factor affecting tuning is the temperature. There are, however, other things that can affect the pitch of the instruments and this includes the air pressure in your surroundings. The reason this affects the pitch is quite complex and I’ll discuss it in detail below.
How do sound waves work?
A sound wave has a frequency it will vibrate at and this frequency of vibration determines its pitch. The frequency is the speed of the wave divided by the wavelength. A higher frequency corresponds to a higher pitch and a longer wavelength. The speed of sound is given by the wavelength multiplied by the frequency.
A phrase you may hear often is “the speed of sound is constant in air”. This is a bit misleading. The speed of sound is constant in a constant medium. By that I mean that if the atmosphere the sound wave is passing through is the same throughout, then the speed of the sound wave will remain constant. However, if the properties of the medium change, then the speed of the sound wave might also change.
The speed of sound is dependent on the density of the medium it is travelling through. By this I mean how close together the particles that make up the medium are, and also the speed they are traveling at. If, like in air, the particles are spread widely, it takes sound a while to travel through them. This is because a sound wave is a vibration and needs to transfer itself between particles in the medium it travels in. Increase the density of the particles, and the vibrations will propagate more quickly through the medium.
How does the atmosphere affect sound waves?
The atmosphere the sound wave is in affects its speed. If the medium is at a constant temperature, then changing the pressure will not alter the speed of the wave, because the density will compensate for the change in pressure. However, we can think more easily about what happens, if we consider a different medium to air.
We all know that when you inhale helium, the pitch of your voice increases. But what is actually happening when we do this? Helium is lighter than air – this means the atoms it is made up of move around much faster than the atoms in air. Because of this, the sound wave will make its way through helium faster than it will through air (actually it turns out to be about three times faster).
What happens next is where it gets a bit more complicated. Say you sing a pitch at one frequency, and then inhale helium and sing a pitch at what you expect to be the same frequency. A higher pitch will sound. What’s important here is that when your vocal chords vibrate, they are vibrating at many frequencies at the same time. You are mostly hearing the fundamental (lowest) frequency, while the others are making the sound quality richer but you don’t hear them directly.
When you inhale helium, you speed up the sound waves. You haven’t changed the frequency you’re producing and we know that the speed is the product of the wavelength and the frequency, so if the speed increases and the frequency stays constant, the wavelength must increase.
Each person has their own unique voice box and this will resonate at particular wavelengths. The frequency of the pitch you originally chose will be at a particular wavelength that will resonate in your voice box (a note in your range). However, because the wavelength is stretched in helium, the new wavelength will not fit the shape of your voice box and therefore won’t sound. This means you have lost the fundamental frequency.
What now happens is that the other frequencies that were in the sound, adding to the timbre of the note, also get stretched in wavelength. The result is that these notes now have wavelengths that are a length that will resonate in your voice box and these sound instead. They’re a higher frequency, so you hear a higher pitch. This can now be related to what happens to an instrument on an aeroplane or other high up place.
What happens to pitch in low pressure environments?
When you are on a plane, or anywhere where the altitude is high, you have a low pressure environment. Air that is at a lower pressure has faster moving particles compared to high pressure air. This means, like with helium, the speed of sound will be greater in a low pressure environment.
The difference in speed of sound in differently pressurised air will be much less significant to when you substitute air for helium. This will result in a smaller difference in pitch, but the resulting note will still appear higher to the listener. Similarly to a voice box, the fundamental pitch will be stretched in wavelength, moved away from resonance, and a higher pitch will be heard, making the sound sharper than at normal pressure.
An important point to consider here is that the pitch increase is only there when there is somewhere for the sound waves to resonate. With your voice, the resonance is in the voice box; with a wind instrument, the resonance is in the tube through which the wave propagates (the body of the instrument). The original frequency remains unchanged, it just can’t resonate with its new wavelength and so is not detected by the ear, but it isn’t gone, it just isn’t resonating.
To sum up this post, if you were to play your instrument on a plane, it would sound sharp. Not because the frequency of the note got higher, but because the wavelength got longer, shifting the frequencies that resonate. This is the same effect as inhaling helium and then speaking with a higher pitch. You can try it out with a tuner and some helium to see how much the pitch itself actually alters.