Page 152 - Master Handbook of Acoustics

there is relatively little diffraction. (B) When a low-frequency sound impinges on the same obstacle,
there is considerable diffraction. The edges act as sources, radiating sound into the shadow zone.

As a final example, imagine an obstacle that is 0.1 ft across and an obstacle that is 1 ft across; if
the frequency of the sound impinging on the first obstacle is 1,000 Hz (wavelength 1.13 ft) and the
frequency impinging on the second is 100 Hz (wavelength 11.3 ft), both would exhibit the same

diffraction.
A highway noise barrier is a common sight along busy roadways, and an example of an obstacle
that is designed to isolate listeners (e.g., in homes) from impinging traffic sounds, as shown in Fig. 7-
3. The spacing of the sound wavefronts indicates relatively higher or lower frequency from the source
(such as traffic). At higher frequencies, as shown in Fig. 7-3A, the barrier becomes effectively large
and successfully shields a listener on the other side. Even if there is not complete isolation, at least

higher-frequency traffic noise will be attenuated. At lower frequencies, as shown in Fig. 7-3B, the
barrier becomes acoustically smaller. Low frequencies diffract over the barrier and are audible to the
listener. Because of the change in the frequency response of the noise, the listener essentially hears
bass-heavy traffic noise.