Reflection of Sound and Refraction of Sound

 Reflection of Sound

We call the reflection of sound an echo. The fraction of energy carried by the reflected sound wave is large if the surface is rigid and smooth and less if the surface is soft and irregular. Sound energy not carried by the reflected sound wave is carried by the "transmitted" (absorbed) wave.

Sound reflects from a smooth surface the same way that light does-the angle of incidence is equal to the angle of reflection (Figure 20.6). Sometimes, when sound reflects from the walls, ceiling, and floor of a room, the reflecting surfaces are too reflective and the sound becomes garbled. This is due to multiple reflections called reverberations. On the other hand, if the reflective surfaces are too absorbent, the sound level will be low and the hall will sound dull and lifeless. Reflection of sound in a room makes it sound lively and full, as you have probably discovered while singing in the shower. In the design of an auditorium or concert hall, a balance must be achieved between reverberation and absorption. The study of sound properties is called acoustics. 

It is often advantageous to place highly reflective surfaces behind the stage to direct sound out to an audience. Reflecting surfaces are suspended above the stage in some concert halls. The ones in Davies Hall in San Francisco are large shiny plastic surfaces that also reflect light (Figure 20.7). A listener can look up at these reflectors and see the reflected images of the members of the orchestra. The plastic reflectors are somewhat curved, which increases the field of view, Both sound and light obey the same law of reflection, so, if a reflector is oriented so that you can see a paricular musical instrument, rest assured that you will hear it also. Sound from the instrument will follow the line of sight to the reflector and than to you.

Refraction of Sound

Sound waves bend when parts of the wave fronts travel at different speeds. This happens in uneven winds or when sound is traveling through air of uneven temperatures. This bending of sound is called refraction. On a warm day, the air near the ground may be appreciably warmer than the rest of the air, so the speed of sound near the ground increases. Sound waves therefore tend to bend away from the ground, resulting in sound that does not seem to travel well. Different speeds of sound produce refraction.

We hear thunder when the lightning is reasonably close, but we often fail to hear the thunder for distant lightning because of refraction. The sound travels slower at higher altitudes and bends away from the ground. The opposite often occurs on a cold day or at night when the layer of air near the ground is colder than the air above. Then the speed of sound near the ground is reduced. The higher speed of the wave fronts above causes a bending of the sound toward the ground, resulting in sound that can be heard over considerably longer distances (Figure 20.8)