Transmission of Sound across Medium Boundaries
Home Up Introduction Transmission of Sound in Open Space Derviation from Inverse Square Law Transmission of Sound across Medium Boundaries Sound Transmission Coefficient Sound Insulation


4. Transmission of Sound across Medium Boundaries

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.