Page 276 - The Master Handbook Of Acoustics

251
DIFFRACTION OF SOUND

30
h=20'
h=15'
h=10'
Attenuation - dB 20

10 h
30' 30'

0
50 100 300 500 1kHz 3kHz 5kHz 10kHz
Frequency - Hz
FIGURE 11-5

An estimation of the effectiveness of a sound barrier in terms of sound (or noise) attentuation as a function
4
of frequency and barrier height. (After Rettinger. )

the next longer path must be r + λ/2 where λ is the wavelength of the
sound falling on the plate from the source. Successive path lengths
are r + λ, r + 3/2λ, and r + 2λ. These path lengths differ by λ/2, which
means that the sound through all the slits will arrive at the focal
point in phase which, in turn, means that they add constructively,
2
intensifying the sound. See Fig. 11-7.

Diffraction around the Human Head

Figure 11-8 illustrates the diffraction caused by a sphere roughly the
size of the human head. This diffraction by the head as well as reflec-
tions and diffractions from the shoulders and the upper torso influ-
ences human perception of sound. In general, for sound of frequency
1–6 kHz arriving from the front, head diffraction tends to increase
the sound pressure in front and decrease it behind the head. For fre-
quencies in the lower range the directional pattern tends to become
circular. 2,3