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How do musical instruments produce sound?

Updated Wednesday 2nd January 2019

Alexander Kolassa looks at how we hear music and discusses the categories which musical instruments belong to.

Here’s a strange question: how many different kinds of instruments are there?

Well, the western orchestra has up to thirty different types of instruments. But that does not typically include instruments we recognise every day in popular music, or instruments found in other cultures around the world. It does not include ancient music, nor those waiting to be unearthed. And what about instruments yet to be invented, or other objects not thought to be musical instruments but regularly used as such?

A precise answer to this question, then, is probably impossible. It is, at least, limiting. But, the title of this post perhaps holds one solution: how do musical instruments produce sound?

A crash course in sound

The answer to that question is at once highly complex and entirely simple. Instruments, one way or another, make sounds, and all sounds are vibrations which travel across particles that make up the air around us (for humans anyway). These vibrations are then transformed in our eardrum - itself a thin vibrating membrane, like you might find on a drum - to be translated, eventually, into something our brains ‘hear’. Phew, simple!

A musical instrument, in short, might itself vibrate, or have a part that vibrates, or amplifies and/or modifies another vibration. Those vibrations bump into neighbouring air molecules as per my description above, and you can think of those forming a wave like those in the figure below. A faster vibration creates more waves; the rate at which they move is called the ‘frequency’, which we measure in hertz (Hz), or cycles per second. The higher the frequency (or the more ‘bumps’ in the waveform) the higher the pitch.

Copyrighted image Icon Copyright: By LucasVB Sine waves across different frequencies.

So where do the different instruments come into this?

Why, I am glad you asked! One way of addressing my initial question is to consider precisely how it is that musical instruments make the vibrations that go on to become sound in the first place. The issue, then, is a matter of ‘technology’; that is, the ideas, the processes, and ultimately the materials which distinguish different musics and musical instruments. I might not be able to tell you how many instruments there are, but I can take a stab at the number of different types of instruments. And to this we turn to the field of ‘organology’ and, specifically, the Hornbostel-Sachs classification system.

Organology is the science of musical instrument classification; Hornbostel-Sachs is, perhaps, its most famous product. Made by, you guessed it, (Curt) Sachs and (Erich von) Hornbostel over 100 years ago, the Hornbostel-Sachs system of classification has proven to be a resilient way of categorising instruments. And to once again cut a very long story short, it does this through reference to how different instruments produce sound.

Hornbostel-Sachs proposes four categories for all musical instruments:

  • Idiophones: whose own bodies vibrate, often, though not exclusively, by being struck (this includes many percussion instruments).
  • Membranophones: in which a membrane or thinly stretched material is vibrated. Consider a drum, but also a kazoo (in which the human voice excites a membrane).
  • Chordophones: where sounds are produced by vibrating strings (perhaps amplified through a body) as in violins, guitars, or pianos.
  • Aerophones: the instrument does not vibrate, nor do any strings; sound is produced by vibrating air i.e. a flute or brass instrument.

The system is, of course, more complex than it appears here and each of these categories will reveal additional subcategories. Instruments might also be composite and combine different categories (consider, for example, the kazoo, which is both a membranopone and an aerophone). Hornbostel-Sachs, nevertheless, is an elegant solution to the problem of classification which also highlights the technological means by which music and instruments make sound; it also celebrates commonalities - or, what is shared - in music around the world.

It is also being consistently updated. Even Sachs had the foresight, in 1940, to introduce a fifth category to the list: ‘electrophones’. This last category of instruments produce sound through electrical means, and this includes everything from the Moog synthesizer to your home studio computer. Whilst the ‘electrophone’ category is itself contested, and a fast rate of technological change proposes a constant challenge to people in the ‘instrument-categorisation game’, remember that what we do today with music is still part of a tradition as old as humanity itself.

A note about timbre

We have limited space here, but if I can recommend one subject for further study it would be that of ‘timbre’. Above, is an image of uniform ‘curvy’ waves to demonstrate instrumental sound. This is a massive simplification. Actual sound waves can come in many shapes which have different sonic effects (the waves in figure above represent sine waves which have a very pure and artificial sound). Any single sound we hear also contains many additional waves and pitches.

Sound, then, is a truly interesting and complex phenomenon. An instrument’s unique timbre is the product of a set of different pitches competing for your attention. Think about that next time you play or listen to some music.


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