Basic Acoustics

Sound and its measurements elaborately explained by Alwin Susai.

What is Sound?
Sound is the quickly varying pressure wave within a medium that can travel widely in that medium. We usually mean audible sound, which is the sensation (as detected by the ear) of very small rapid changes in the air pressure above and below a static value. This “static” value is atmospheric pressure (about 100,000 Pascals) which does never-the-less vary slowly, as shown on a barometer. Associated with the sound pressure wave is a flow of energy. Sound is often represented diagrammatically as a sine wave, but physically sound (in air) is a longitudinal wave where the wave motion is in the direction of the movement of energy. The wave crests can be considered as the pressure maxima whilst the troughs represent the pressure minima. When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. at a frequency between 20Hz and 20kHz), sound is potentially audible even though the pressure variation can sometimes be as low as only a few tens of millionths of a Pascal. Movements of the ear drum as small as the diameter of a hydrogen atom, can be audible! Louder sounds are caused by greater variation in pressure. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1kHz - 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.

What makes Sound?
Sound is produced when the air is disturbed in some way, for example by a vibrating object. A speaker cone from a high fidelity system serves as a good illustration. It may be possible to see the movement of a bass speaker cone, if it is producing very low frequency sound. As the cone moves forward the air immediately in front is compressed causing a slight increase in air pressure, it then moves back past its rest position and causes a reduction in the air pressure (rarefaction). The process continues so that a wave of alternating high and low pressure is radiated away from the speaker cone at the speed of sound.

What is a Decibel (dB)?
The decibel is a logarithmic unit for ratios that is used in a number of scientific disciplines. Other examples are the Richter scale for earthquake event energy and pH for hydrogen ion concentration in liquids. In all cases, the logarithmic measure is used to compare the quantity of interest with a reference value, often the smallest likely value of the quantity. Sometimes, that reference can be an approximate or average value. Most often in common acoustics, the decibel is used to compare the sound pressure level (SPL) in air with a reference pressure. The reference level for sound intensity (I), sound power level (PWL) and sound pressure in water are amongst others that are in common use:

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