Reference Guide

779
Digital audio fundamentals
Editing audio
Since humans can hear frequencies well above 10 kHz, most sound cards and digital recording
systems are capable of sampling at much higher rates than that. Typical sampling rates used by
modern musicians and audio engineers are 22 kHz, 44.1 kHz, and 48 kHz. The 44.1 kHz rate is
called CD-quality, since it is the rate used by audio compact discs.
The other important aspect of the measuring process is the sampling resolution. The sampling
resolution determines how accurately the amplitude of each sample is measured. At present, the
music industry has settled on a system that provides 65,536 different values to assign to the
amplitude of a waveform at any given instant. Thus, each sample saved by your computer requires 2
bytes (16 bits) to store, since it takes 2 bytes to store a number from –32,768 to 32,767. The scaling
of the electrical input signal level to amplitude value is determined by your audio hardware and by
the position of your input level control.
What if the amplitude of the sampled signal gets too high, such that a 16-bit number is not large
enough to represent it? What typically happens is that the signal is clipped, cut off at the maximum
value.
Here is what a clipped waveform might look like:
Clipping is not usually desirable and may have unpleasant audible effects. Sudden irregularities in
the waveform of any type can cause clicks, pops, and distortion of the original sound.
See:
“Digital audio fundamentals” on page 774
The decibel scale
In acoustics, the decibel (dB) scale is a scale for measuring the relative loudness of two sounds. For
example, environmental noise is often measured as follows:
L = 20 log (p/p0)
where L is the sound pressure level (in dB), p is the sound pressure amplitude, and p0 is a reference
amplitude of 20 micropascals (less than one billionth of atmospheric pressure). On this scale, a
barely audible sound (p = p0) has a sound pressure level of 0 dB, normal conversation (p =
1,000*p0) is at a level of around 60 dB, and a jet engine at close range (p = 1,000,000*p0) is
at a level of around 120 dB.
Similar decibel scales are used in other branches of science and engineering to measure electrical
power levels and other signal levels, always with respect to some reference level.
In SONAR, decibels are used in several places: