Page 150 - Master Handbook of Acoustics

Diffraction and Wavelength

For a given obstacle size, low-frequency sounds (long wavelengths) diffract more readily than high-
frequency sounds (short wavelengths). Conversely, for a given obstacle, high frequencies diffract less
readily than low frequencies. Diffraction is less noticeable for light than it is for sound because of the
comparatively short wavelengths of light. As a result, optical shadows are more distinct than
acoustical shadows. You might easily hear the diffracted music from a room down the hallway, but
you might not see the diffracted light from there.

Looked at in another way, the effectiveness of an obstacle in diffracting sound is determined by the
acoustic size of the obstacle. Acoustic size is measured in terms of the wavelength of the sound. An
obstacle is acoustically small if wavelengths are long, but the same object is acoustically large if
wavelengths are short. This relationship is described in more detail in the following text.

Diffraction by Obstacles

If an obstacle is acoustically small relative to wavelength, sound will easily diffract around it. The
sound will bend around a small obstacle with only slight disturbance, creating little or no acoustical
shadow. When an obstacle’s dimensions are smaller or equal to the wavelength, virtually all sound

will be diffracted. As noted, each wavefront passing the obstacle becomes a line of new point
sources radiating sound into the shadow zone by diffraction. On the other hand, if an obstacle is
acoustically large relative to wavelength, diffraction is less pronounced and a larger acoustical
shadow is created. In addition, sound will tend to be reflected by a large obstacle; thus we see that
obstructions act as frequency-dependent reflectors.

As noted, the effectiveness of an obstacle in diffracting sound is determined by the acoustic size of
the obstacle. Consider two objects in Fig. 7-1 and their behavior with the same wavelength of sound.
In Fig. 7-1A, the obstacle is small compared to the wavelength of the sound, thus sound strongly
diffracts around it; the obstacle has no appreciable effect on the passage of sound. In Fig. 7-1B,
however, if the obstacle is several wavelengths across, diffraction is less pronounced and the
obstacle casts an acoustical shadow. Although not shown in this figure, some sound waves are also
reflected from the face of the obstacle.