Page 154 - Master Handbook of Acoustics

side, and the home or other sensitive area is considered to be 30 ft on the other side of the wall (the
shadow side). A 20-ft-high wall yields about 25 dB of attenuation from the highway noise at 1,000
Hz. However, the attenuation of the highway noise at 100 Hz is only about 15 dB. The wall is less
effective at lower frequencies than at higher frequencies. The shadow zone behind the wall tends to

be effective for the high-frequency components of the highway noise. The low-frequency components
penetrate the shadow zone by diffraction. To be effective, barriers must be sufficiently tall, and also
long enough to prevent flanking around the end of the barrier. The effectiveness of any barrier is
strictly frequency dependent.

FIGURE 7-4 An estimation of the effectiveness of a traffic barrier in terms of sound (or noise)
attenuation as a function of frequency and barrier height. (Rettinger.)

Diffraction by Apertures

Diffraction through apertures depends on the size of the opening and the wavelength of the sound. The

amount of diffracted sound relative to total sound passing through an aperture increases as the opening
size decreases. As with diffraction around obstacles, diffraction through apertures is wavelength
dependent. Diffraction increases as frequency decreases. Thus, an opening is more acoustically
transparent at low frequencies.
Figure 7-5A illustrates the diffraction of sound by an aperture that is many wavelengths wide. The
wavefronts of sound strike a solid obstacle; some sound passes through the wide aperture and

although not shown, some sound is reflected. By diffusion, some of the energy in the main beam is