User Guide

Sound System Design Reference Manual
Wavelength, Frequency, and Speed of
Sound
Sound waves travel approximately 344 m/sec
(1130 ft/sec) in air. There is a relatively small velocity
dependence on temperature, and under normal
indoor conditions we can ignore it. Audible sound
covers the frequency range from about 20 Hz to 20
kHz. The wavelength of sound of a given frequency
is the distance between successive repetitions of the
waveform as the sound travels through air. It is given
by the following equation:
wavelength = speed/frequency
or, using the common abbreviations of c for speed ,
f for frequency, and λ for wavelength:
λ = c/f
Period (T) is defined as the time required for
one cycle of the waveform. T = 1/f.
For f = 1 kHz, T = 1/1000, or 0.001 sec, and
λ = 344/1000, or .344 m (1.13 ft.)
The lowest audible sounds have wavelengths
on the order of 10 m (30 ft), and the highest sounds
have wavelengths as short as 20 mm (0.8 in). The
range is quite large, and, as we will see, it has great
bearing on the behavior of sound.
The waves we have been discussing are of
course sine waves , those basic building blocks of all
speech and music signals. Figure 1-1 shows some of
the basic aspects of sine waves. Note that waves of
the same frequency can differ in both amplitude and
in phase angle. The amplitude and phase angle
relationships between sine waves determine how
they combine, either acoustically or electrically.
Chapter 1: Wave Propagation
Figure 1-1. Properties of sine waves
1-1