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TPBLeap - Creative Futures

by JJ Vernon

Chapter 2 - The Seven Characteristics of Sound

WAVEFORMS

We can not see sound waves, but in order to study them it is useful to be able to represent sounds graphically. Sound waves travel outwards from a vibrating object in a three dimensional spherical pattern. A waveform attempts to represent this on paper (or on screen) in a two dimensional form.

1. AMPLITUDE

The number of molecules displaced by a vibrating object creates the amplitude (volume or loudness) of a sound. In the diagram below the distance above & below the centreline represents the sound’s volume. Therefore looking at the waveform below we can say that Point A is probably silence & Point B is the loudest part of this particular sound. Time is travelling from left to right on the horizontal axis.

The greater the distance from the top to the bottom of the waveform; the more intense the pressure variation or electrical signal.

Compare the waveform above to this one below. The one above is the waveform of someone speaking. If you look carefully you should be able to identify the words & the silences between them. The one below shows the waveforms of a simple sine wave. You can see the individual cycles in this one because we have "zoomed in" much further. The two sound waves below will sound similar, but the upper one will sound about twice as loud as the lower one because, as you can see, there is 10 dBs volume difference between the two.

THE DECIBEL

We need a way to measure the volume or loudness of the sounds that we hear. Unfortunately this is no simple matter. Part of the problem is that the volume of a sound depends upon your closeness to it. The jet engine of an aircraft as you board it seems very loud but if you watch the same plane flying towards the horizon its engines seem relatively quiet. It is not that the sound has become quieter; it is just that the energy of that sound has been spread over a much, much larger area.

Loudness is also subjective thing. Take the teenager playing a stereo in their room. If a parent says “Turn that down, it is way too loud” & the teenager replies “No its not,” who is right? Well, they both are!

The sensible approach is to take a measurement of the sound pressure using a unit of pressure over area much like the old PSI (pound force per square inch) that was used to measure your tyre pressure.

We could use the metric unit of Pascals (symbol Pa). It is a measurement of force per unit area. (See http://en.wikipedia.org/wiki/Pascal_(unit) for more details). But the numbers this produces when dealing with loudness are far from friendly.This is because the human ear behaves in a peculiar way. In order for us to hear something at twice the volume, we have to multiply the sound’s intensity by 10.

So… a sound with an intensity of 1,000 is twice as loud as an intensity of 100, but half as loud as an intensity of 10,000.

Confusing isn’t it, but here is where the decibel (dB) rides to the rescue. The decibel is a logarithmic unit of measurement. This means it uses a mathematical formula to reduce a large range of values down to a more manageable range. So in decibels the threshold of hearing, or the quietest sound a human can hear, is simply 0 dBs. The loudest sound a human can stand before it starts to hurt, the threshold of pain, is 120 dBs. When the same sound's loudnesses are converted into decibels they become far more approachable. (dB spl = decibels of sound pressure level)

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