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Home | Technical | Science | The Physics of Sound

The Physics of Sound

While music has many spiritual and psychological aspects, to the more scientifically minded, music is sound and sound is vibration. All music can therefore be analysed in terms of a pattern of vibrations made in air (or another medium such as water). We could perhaps add to this definition the fact that music has some organisation underlying it as opposed to simply being random vibration or noise, but actually pinning down the point at which noise becomes music is not that easy. Naturally occurring noise and vibration also contains patterns and organisation and are perhaps the most fundamental and universal form of music. For the purposes of this section, therefore, we shall try to separate the term 'music' and the associations and preconceptions that it engenders from the factual science of sound.

There are two ways to look at the physics of sound and music. The first is from the more traditional scientific perspective, and the second is in terms of the emergent yet less accessible sciences of quantum physics and non-linear dynamics.

Traditional Physics:

Waveforms

In terms of physics then, music is simply sound and sound is vibration. The word 'simply' is perhaps not the best choice as the study of soundwaves can be extremely involved - the fact that sound waves contain patterns that we call music is a good measure of the complexity.

So what exactly do we mean by a sound wave?
A sound wave is the combined movement of a mass of particles. The particles themselves may be any material whatsoever - gas, solid or liquid and the 'wave' refers to the pattern in which they move. Analogies can be made with more visible waves such as tidal waves and Mexican waves. The movement of particles at one point influence those they are in contact with to produce a ripple or wave. The particles themselves do not actually move very far (i.e. a speaker does not project particles of sound across the room and into your ear). It is more akin to a series of pendulum like motions where each molecule returns to its original position but not before it has set its neighbour in motion. The process by which sound travels through a medium is known as sound propagation.

The vibration is picked up by the ear and converted first to hydraulic movement and then to electrical impulses which are sent on to the brain (via the aural nerve) for interpretation. Waveforms are common in nature (and to some quantum physicists everything is a waveform). Waveforms are characterised as having an amplitude and a frequency.

In terms of the musical nature of a sound wave the frequency corresponds to pitch and the amplitude to volume. This gives us the basis for the musical constructs of pitch relationships, tonality and dynamics. The simplest sound waves can be modelled on a computer - as you increase the waveform's frequency the waves get closer together and the pitch rises. If you increase the amplitude the waves get taller and the volume increases. Amplitude will tend to die away but frequency will usually remain more or less constant throughout a sound's duration.

Frequency is measured in cycles per second or Hertz (Hz) - human hearing extends from about 16-20Hz at the bottom end with an upper limit of about 25,000Hz. Animals can have very different hearing ranges. Bats and dogs can hear much higher pitches than we can and whales can hear lower ones.

Certain types of waveform have generic qualities and apply to waveforms in general, not just sound waves. Sine waves are the most basic and are used in synthesis as the basis for complex instruments (along with other waveforms and various other components to modulate the sound). A few of the more common waveforms are shown below:

SINE

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SAWTOOTH

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SQUARE

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Several things affect the formation and passage of sound waves:

1) The nature and magnitude of their source.
2) The medium through which they are travelling..
3) Objects in their path.
4) External forces (such as the intersection of other waveforms).

Noise
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Noise is the music of the universe - a constant source of vibration and sound waves. Although random in the sense that no obvious patterns are present, noise can come in a variety of colours - pink, brown and white. The study of the patterns within noise is perhaps best left to the quantum physics section or the fractal music pages. The noise we are referring to when we speak of brown, white and pink noise is that generated electronically for the purposes of synthesis etc.

White noise is harsh, while brown noise is a little more listenable, and even pleasantly ambient. Strictly speaking noise is defined as being sound composed of irregular frequencies whereas music consists of regular frequencies (notes). This distinction is not always easy to maintain though, and a good deal of research has been done into the mathematical properties of a piece of music as opposed to random noise. Subjects such as this are covered in greater detail in the fractal music section.

Acoustic Properties of Materials

The choice of material used in making an instrument is important in determining the harmonics. Different materials accentuate and suppress different harmonics and provide the richness and timbre that can characterise an instrument. In a wind instrument the basic pitch of a note is produced by the vibration of the column of air inside the body.

It is well known that wood is affected by heat and moisture which causes contraction or swelling. If not cared for properly, wood is also prone to cracking.

Quantum Physics:

In terms of quantum physics, everything in the universe is an interconnected and interrelated set of particles and much of the science of quantum physics revolves around the study of the frequency and vibration inherent in the most elementary particles. Essentially the whole shape and form of the universe can be seen as the consequence of the individual and associative vibration of these particles and the vibration of an individual particle will effect every other particle in the universe to some extent.

This kind of perspective on the universe opens up some interesting areas as far as the physics of sound is concerned. The idea of everything resonating in harmony and the notion of morphic resonance are extremely appealing from a musical point of view and could even go some way to explaining what it is about music that makes it affect our moods and behaviour. We could even go so far as to describe the universe as one enormous musical instrument that is continuously played by countless unseen hands.


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