Chapter One: An Acoustics Primer
4. What are the characteristics of sound waves?
Sound waves can be characterized by four basic qualities (with many others related):
frequency | amplitude | waveshape | phase*
Some sound waves are periodic, in that the change from equilibrium (average atmospheric pressure) to maximum compression, to maximum rarefaction, and back to equilibrium is repetitive. The round trip back to the starting point just described is called a cycle. Periodic wave motion, regardless of its complexity, is characterized as being continuously repetitive.
Periodic motion depends on two primary factors: (1) elasticity, in that the medium being distorted returns to its original state (equilibrium), and (2) a source of energy to initiate and sustain motion. In the case of sound waves, the atmospheric pressure will return to the ambient pressure without an energy source to disturb it, and any vibrating surface will constitute an energy or excitation source.
CYCLE
The round trip back to the starting point just described is called a cycle. A cycle is traditionally graphed as a function of time or distance.
MEASUREMENT
The cycle measurement need not be taken from the zero crossings but can be made from any point on the wave that eventually returns to its origin.
It is possible to measure frequency in seconds per cycle or period, but it is far more common for sound measurements to use cycles per second or simply cps. A "seconds per cycle" or period measurement is the inverse of a "cycles per second" measurement; two seconds per cycle equates to 0.5 cycles per second, and two cycles per second equals 0.5 seconds per cycle. Additionally, one hertz (abbreviated Hz—note the capitalization) equals 1 cycle per second; they are synonymous and interchangeable values used to convey frequency.
Hertz (Hz)
The official international (SI) measurement for cycles per second as of 1960 is the hertz, abbreviated Hz. It is named in honor of Heinrich Rudolf Hertz, a 19th-century German physicist credited with definitively proving the existence of electromagnetic waves, earlier predicted by James Clerk Maxwell, and their propagation through space as radio waves. Hertz was a student of Hermann von Helmholtz, mentioned earlier in this text.
*A single waveform's phase is usually not discernible at audio rates, but becomes a factor when two or more waves of differing phase are played simultaneously, or when a single waveform is used at a very low rate as a control signal in synthesis to create, for example, vibrato.
