Reflection and Refraction
Sound can be reflected by hitting an object larger than one-quarter wavelength of the sound. When the object is one-quarter wavelength or slightly smaller, it also causes diffraction of the sound (bending around the object). Refraction occurs when the sound passes from one medium to another (from air to glass to air, for example, or when it passes through layers of air having different temperatures). The velocity of sound increases with increasing temperature. Therefore sound emitted from a source located on the frozen surface of a large lake on a sunny day will encounter warmer temperatures as the wave diverges upward, causing the upper part of the wave to travel faster than the part of the wave near the surface. This causes a lens-like action to occur, which bends the sound back down toward the surface of the lake (Figure 3.4).
Sound will travel great distances over frozen surfaces on a quiet day. Wind blowing against a sound source causes temperature gradients near the ground surface that result in the sound being refracted upward. Wind blowing in the same direction as the sound produces temperature gradients along the ground surface that tend to refract the sound downward. We hear it said, “The wind blew the sound away.” That is not so; it refracted away. Even a 50- mph wind (and that’s a strong wind) cannot blow away something traveling 1130 ft/s:
770.45 mi/h is the velocity of sound at sea level at 72.5°F.
Wind velocities that vary with elevation can also cause “bending” of the sound velocity plus or minus the wind velocity at each elevation.
Reflections from large boundaries, when delayed in time relative to the direct sound, can be highly destructive of speech intelligibility. It is important to remember, however, that a reflection within a nondestructive time interval can be extremely useful. Reflections that are at or near (within 10 dB) equal amplitude and that are delayed more than 50 ms require careful attention on the part of a sound system designer. Figure 3.5 shows how to calculate probable levels from a reflection. Figure 3.6 shows other influences. Calculation of the time interval is found by:
where c is the velocity of sound in ft/s or m/s, DR is the distance in feet or meters traveled by the reflection, and DD is the distance the direct sound traveled in feet or meters.
A large motor speedway used to make very effective use of ground reflections on the coverage of the grandstands behind the pit area. The very high temperature gradients encountered warp the sound upward during the hot part of the day and in the cool of the morning, the ground reflection helps with the coverage of the near seating area. The directional devices are aimed straight ahead along the ground rather than up at an angle, and when the temperature gradient “bends” the sound upward, it’s still covering the audience area effectively (Figure 3.4).
One caution about using ground reflections in northern climes is that a heavy snowfall can provide unbelievable attenuation, as the authors can attest after trying to demonstrate, years ago, a high-level sound system the day after a blizzard in Minnesota.