When an acoustic wave travelling
in one medium encounters the boundary of a second medium, reflected and transmitted waves
are generated. For example, when sound strikes upon a solid partition, part is reflected,
part absorbed within the material, and part transmitted to the other side or to elsewhere
in the building. (see Figure 5)
Figure 5 Distribution of Energy
from Air-borne Sound
Striking a Partition
The ratios of the
pressure amplitudes and intensities of the reflected and transmitted waves to those of the
incident waves depend on the following factors:
(a) In angle of incidence, q ,
(b) The densities of the two
media, and
(c) The speeds of sound in the
two media.
The sound transmission properties
of a single leaf solid partition can be divided into three distinct regions (Figure 6):
Figure 6 Distinct Regions Showing
the Way a Single Leaf Solid Partition will React to Different Frequency Sounds
Figure 7 Coincidence Effect
This is known as the coincidence
effect which has the following characteristics.
(i) The problem is not confined
to a single frequency.
(ii) The lowest frequency that
the problem occurs is when the velocity of the bending wave equals the velocity of sound
in air. This is called the critical frequency and the sound wave is at 0°
angle of incidence.
(iii) Above the critical
frequency, transmission is dominated by coincidence.
