Page 35 - The Master Handbook Of Acoustics

10 CHAPTER ONE

the initial intensity. The inverse square law states that the intensity of
sound in a free field is inversely proportional to the square of the dis-
tance from the source. This law provides the basis of estimating the
sound level in many practical circumstances and is discussed in a
later chapter.

Wavelength and Frequency

A simple sine wave is illustrated in Fig. 1-9. The wavelength is the dis-
tance a wave travels in the time it takes to complete one cycle. A wave-
length can be measured between successive peaks or between any two
corresponding points on the cycle. This holds for periodic waves other
than the sine wave as well. The frequency is the number of cycles per
second (or hertz). Frequency and wavelength are related as follows:
Speed of sound (ft/sec)
Wavelength (ft) (1-1)
Frequency (hertz)
which can be written as:
Speed of sound
Frequency (1-2)
Wavelength
The speed of sound in air is about 1,130 feet per second (770 miles per
hour) at normal temperature and pressure. For sound traveling in air,
Equation 1-1 becomes:

Wavelength

Peak
Amplitude

Time

FIGURE 1-9
Wavelength is the distance a wave travels in the time it takes to complete one cycle. It
can also be expressed as the distance from one point on a periodic wave to the corre-
sponding point on the next cycle of the wave.