Sound refraction (cont.)
However, if the temperature gradient increases with height then instead of being bent up the sound waves are bent down, as shown in Figure 1.17. This effect can often happen on summer evenings and results in a greater sound level at a given distance than predicted by the inverse square law. This behaviour is often responsible for the pop concert effect where people some distance away from the concert experience noise disturbance whereas people living nearer the concert do not experience the same level of noise.
Figure 1.17 Refraction of a sound wave due to an inverted temperature gradient.
Figure 1.18 Refraction of a sound wave due to a wind velocity gradient.
Refraction can also occur at the boundaries between liquids at different temperatures, such as water, and in some cases the level of refraction can result in total internal reflection. This effect is sometimes used by submarines to hide from the sonar of other ships; it can also cause the sound to be ducted between two boundaries and in these cases sound can cover large distances. It is thought that these mechanisms allow whales and dolphins to communicate over long distances in the ocean.
Wind can also cause refraction effects because the velocity of sound within the medium is unaffected by the velocity of the medium. The velocity of a sound wave in a moving medium, when viewed from a fixed point, is the sum of the two velocities, so that it is increased when the sound is moving with the wind and is reduced when it is moving against the wind.
As the velocity of air is generally less at ground level compared with the velocity higher up (due to the effect of the friction of the ground), sound waves are bent upwards or downwards depending on their direction relative to the wind. The degree of direction change depends on the rate of change in wind velocity as a function of height; a faster rate of change results in a greater direction change. Figure 1.18 shows the effect of wind on sound propagation.
1.5.3 Sound absorption
Sound is absorbed when it interacts with any physical object. One reason is the fact that when a sound wave hits an object then that object will vibrate, unless it is infinitely rigid. This means that vibrational energy is transferred from the sound wave to the object that has been hit.
Some of this energy will be absorbed because of the internal frictional losses in the material that the object is made of. Another form of energy loss occurs when the sound wave hits, or travels through, some porous material. In this case there is a very large surface area of interaction in the material, due to all the fibres and holes. There are frictional losses at the surface of any material due to the interaction of the velocity component of the sound wave with the surface. A larger surface area will have a higher loss which is why porous materials such as cloth or rockwool absorb sound waves strongly.
1.5.4 Sound reflection from hard boundaries
Sound is also reflected when it strikes objects and we have all experienced the effect as an echo when we are near a large hard object such as a cliff or large building. There are two main situations in which reflection can occur.
In the first case the sound wave strikes an immovable object, or hard boundary, as shown in Figure 1.19. At the boundary between the object and the air the sound wave must have zero velocity, because it can't move the wall. This means that at that point all the energy in the sound is in the compression of the air, or pressure. As the energy stored in the pressure cannot transfer in the direction of the propagating wave, it bounces back in the reverse direction, which results in a change of phase in the velocity component of the wave.
Figure 1.19 shows this effect using our golf ball and spring model. One interesting effect occurs due to the fact that the wave has to change direction and that the spring connected to the immovable boundary is compressed twice as much compared to a spring well away from the boundary. This occurs because the velocity components associated with the reflected (bounced back) wave are moving in contrary motion to the velocity components of the incoming wave, due to the
Figure 1.19 Reflection of a sound wave due to a rigid barrier.
change of phase in the reflected velocity components. In acoustic terms this means that while the velocity component at the reflecting boundary is zero, the pressure component is twice as large.