ABSTRACT
Before answering the question posed by the title of this chapter, perhaps we had better begin with the question “What is sound?” According to Fahy 1
“sound may be defined as a time-varying disturbance of the density of a fluid from its equilibrium value, which is accompanied by a proportional local pressure, and is associated with small oscillatory movements of the fluid particles”. The difference between the equilibrium (static) pressure and the local, oscillating pressure is known as the sound pressure. Normally, for human beings, the fluid in which sound propagates is air,
which is heavier than most people think – it has a mass of about 1.2 kg per cubic metre at a temperature of 20 degrees C at sea level. It is also interesting to note that sound propagation in air is by no means typical of its propagation in all substances, especially in that the speed that sound propagates in air is relatively slow, and is constant for all frequencies. For music lovers, this latter fact is quite fortunate, because it would be hard to enjoy a musical performance at the back of a concert hall if the notes arrived jumbled-up, with the harmonics arriving before the fundamentals, or vice versa. Conversely, as we shall see later, most of the materials from which loudspeakers are made do not pass all frequencies at the same speed of sound, a fact which can, at times, make design work rather complicated. The speed of sound in air is about 343 metres per second (m/s or ms−1)
at 20 degrees C and varies proportionally with temperature at the rate of about 0.6 m/s for every degree Kelvin. (In fact, the speed of sound in air is only dependent upon temperature, because the changes that would occur due to changes in atmospheric pressure are equal and opposite to the accompanying changes in density, and the two serve to cancel each other out). Air therefore has some clearly defined characteristic properties, and our perception of sound in general, and music in particular, has developed around these characteristic properties. The job of a loudspeaker is to set up vibrations in the air which are
acoustic representations of the waveforms of the electrical signals that are being supplied to the input terminals. A loudspeaker is therefore an electro-mechanico-acoustic transducer. Loudspeakers transform the electrical drive signals into mechanical movements which, normally via a vibrating diaphragm, couple those vibrations to the air and thus propagate acoustic waves. Once these acoustic waves are perceived by the ear, we experience a sensation of sound.
