What is sound?

Sound is produced by something in vibration - a guitar or piano string, a loudspeaker cone, a column of air (eg. inside a flute or trumpet). The vibrations cause variations in air pressure (called sound waves) which travel through the air, eventually reaching the listener's ear and creating the sensation we know as sound.

A sound wave is completed each time the air pressure comes back to rest at the point it started from.

Sound travels through the air at a speed of around 340 m (approx. 1120 ft) per second. Thus if you were standing 340 metres away from the source of the sound it would take a full second to reach you.

Frequency

The rate at which pressure waves are produced per second is called the frequency of a sound, and is measured in Hertz (Hz), eg.

50 per second = 50 Hz
1000 per second = 1000 Hz (1 kHz)
2000 per second = 2 kHz.

Frequency is directly related to the pitch of a sound. The greater the frequency the higher the pitch, the lesser the frequency the lower the pitch.

Wavelength

The length of a wave (the distance between exactly corresponding points on successive waves) can be calculated by dividing the speed of sound by the frequency, eg.

a sound of 100 Hz: Wavelength 340 m = 3.4 metres / 100
a sound of 1000 Hz: Wavelength 340 m = 0.34 metres (34 cm) / 1000

Thus lower sounds have longer wavelengths, higher sounds have shorter wavelengths.

Frequency range

We can hear sounds ranging from approximately 20 Hz - 20000 Hz (20 kHz). With age the upper limit falls - to as much as 8 kHz for people over 60.

We do not however hear all sounds equally well - sounds of mid-frequency are heard best (1 kHz - 4 kHz) while the ear is less sensitive to lower and higher sounds.

Harmonics and timbre

Most sounds are made up of more than a single frequency. The pitch of the sound is called the fundamental frequency but multiples of the fundamental frequency called 'harmonics' occur at the same time. It is the harmonics that help to give different instrumental sounds their special tone or 'timbre'.

Loudness and sound levels

The sensation of loudness depends upon various factors and is not an exact measurement. However sound levels, ie. the intensity of a sound, can be measured using the decibel unit. (dB)

Decibels are a Logarithmic Ratio:

10 dB is ten times 1 decibel 1OdB = 10xl=10
But 20 dB is 100 times 1 decibel 20 dB = 10 x 10 = 100
30 dB is 1000 times 1 decibel 30 dB =@ 10 x 10 x 10 = 1000.

Since decibels are a ratio, a sound measured in decibels is compared to another value. The 'threshold of hearing' (0 dB), when sound is just audible, is taken as the reference sound level. The intensity of sounds are measured as sound pressure levels (spl) expressed in dB. Below are some typical sound levels:

0dB Threshold of hearing
20dB Quiet living room
50dB Average office
70dB Busy street
80dB Average factory
110dB Thunder
120dB Airport runway
133dB Threshold of pain

We can physically experience pain at the level of the 'threshold of pain'. Constant exposure to high sound levels (particularly those with high frequency content) must be avoided or serious hearing damage will result. It has been claimed that some sounds even just above 70 dB can damage hearing.

Remember as a rough guide that a 6dB boost or gain when recording means a doubling of volume.

Phasing

When recording reflected soundwaves arrive slightly later at the microphone than direct sound. The reflected soundwaves are thus slightly behind the direct ones in time - when this happens soundwaves are said to be 'out-of-phase' with each other.

This can produce cancellations of sound (known as 'phasing') affecting sound quality in certain circumstances. Phasing can be particularly troublesome when widely spaced microphones are used.

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