Two faces of sound
If a tree falls in a forest without anyone to hear it, does it make a sound? The dual meaning of ‘sound’, as physical phenomenon and sensation, provides a clear answer: yes and no. The relationships between the physical and sensual aspects of sound are complex, in that many of the impressions sound makes on us are related to its physical parameters but not reducible to them. So: high-frequency sounds sound higher pitched—usually. And more powerful sounds sound louder—on the whole. Furthermore, many sounds, from sirens to skirls and from lullabies to lions’ roars, make emotional impacts on us which have only the vaguest relationships to their physical parameters.
The physical aspects of sound are far better understood than the emotional ones, so it is with physics that we should begin.
Pressure waves
Sounds are usually made by something moving in a cyclic manner: the diaphragm of a loudspeaker pulsing in and out, the gap between the vocal folds narrowing and widening, or a guitar string vibrating back and forth. It is the transmission of these motions to the surrounding medium (solid, liquid, or gas), and their progression through that medium, that constitute sound. In some cases the motion begins in the medium itself, such as the air in the neck of a bottle when one blows across it. Non-moving sources include sudden releases of heat energy, such as by explosions or sparks, and rapidly oscillating heat sources.
When the motion is that of a loudspeaker’s diaphragm, the cause is a varying electrical signal with the same pattern as the sound wave that the diaphragm will produce. Each time the diaphragm moves out, it squeezes the air molecules immediately in front of it closer together, forming a region of high pressure. These molecules press on their neighbours, moving them closer together in their turn, and so a pulse of close-together molecules (a compression) moves through the medium, followed by a low-pressure area (rarefaction), which is produced as the diaphragm moves inwards.