Figure 5-1: Sine waveform
In electronic sound recording and output devices, the attributes of sounds are represented
by the parameters of amplitude and frequency. Frequency is the number of cycles per
second, and the most common unit of frequency is the Hertz (Hz), which is 1 cycle per
second. Large values, or high frequencies, are measured in kilohertz (KHz) or megahertz
(MHz).
Frequency is strongly related to the perceived pitch of a sound. When frequency
increases, pitch rises. This relationship is exponential. An increase from 100 Hz to 200 Hz
results in a large rise in pitch, but an increase from 1,000 Hz to 1,100 Hz is hardly
noticeable. Musical pitch is represented in octaves. A tone that is one octave higher than
another has a frequency twice as high as that of the first tone, and its perceived pitch is
twice as high.
The second parameter that defines a waveform is its amplitude. In an electronic circuit,
amplitude relates to the voltage or current in the circuit. When a signal is going to a
speaker, the amplitude is expressed in watts. Perceived sound intensity is measured in
decibels (db). Human hearing has a range of about 120 db; 1 db is the faintest audible
sound. Roughly every 10 db corresponds to a doubling of sound, and 1 db is the smallest
change in amplitude that is noticeable in a moderately loud sound. Volume, which is the
amplitude of the sound signal which is output, corresponds logarithmically to decibel level.
The frequency and amplitude parameters of a sine wave are completely independent.
When sound is heard, however, there is interaction between loudness and pitch. Lower-
frequency sounds decrease in loudness much faster than high-frequency sounds.
- Audio Hardware 131 -
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