Noise Pollution 425Figure 22-2. Sound waves. All parts of the figure show the spatial variation along
the wave at a particular instant of time. (A) Regions of compression and rarefaction
in the air. (B) Pressure wave; P = pressure amplitude (PmS = P/&) and A =
wavelength. (C) Intensity wave; the average intensity is denoted I.
EXAMPLE 22.1. In cast iron, sound waves travel at about 3440 ds. What would be
the wavelength of a sound from a train if it rumbles at 50 cycleds and one listens to it
placing an ear on the track?
c 3440
v 50A=-=--- - 69m.
In acoustics, the frequency as cycles per second is denoted by the name hertz,l
and written Hz. The range of frequency audible to the human ear is between 20 and
20,000 Hz. The middle A on the piano (concert pitch) is 440 Hz. The frequency is one
of the two basic parameters that describe sound. Amplitude, how loud the sound is, is
the other.
If the amplitude of a pressure wave of a sound with only one frequency is plotted
against time, the wave is seen to produce a sinusoidal trace, as shown in Fig. 22-2.
'In honor of the German physicist Heinrich Hertz (1857-1894).