Among the typical phenomena of sound waves, sound refraction can certainly be included, in which waves bend or expand depending on changes in the wave’s speed. Refraction is not a phenomenon that only concerns acoustics, but can also be identified in other situations. One example involves ocean waves approaching the coast parallel to the beach. Another is provided by the reason why glass lenses can be used to channel weak light waves into a single point. Under normal circumstances, the sun heats the Earth which, in turn, transfers heat to the atmosphere. Often, the phenomenon of sound refraction is due to the presence of the atmospheric temperature gradient. The air cools the higher one rises, following the values determined by the vertical thermal gradient. The vertical thermal gradient is a rate indicating the variation of air temperature in relation to altitude. Sound waves propagate faster in warm air, thus being quicker when close to the ground. To analyze this phenomenon, Huygens’ Principle comes to our aid. Simplifying the definition outlined by the Dutch physicist, the sound wave generated by a source propagates spherically. Every point of the various successive wavefronts becomes a secondary source that in turn generates other waves with the same characteristics as the original wave (the wavefront is the set of points vibrating in unison as a specific wave passes). Huygens’ Principle can be explained with a simple example. Sometimes we see lightning descending from the sky but are unable to hear the sound of the thunder that follows it. This occurs due to sound refraction and Huygens’ Principle: the sound waves of the thunder refract heavily in a vertical direction, creating a “shadow zone” where the noise is not heard. Typically, this particular phenomenon can be noticed at a distance of about 22.5 kilometers from the strike point of lightning that originated at an altitude of 4,000 meters.
The phenomenon of sound refraction can be controlled by placing certain elements within a specific environment. Concert halls or those with a large capacity require careful acoustic design to achieve the correct level of reverberation and wave propagation. In these vast environments, it can indeed seem difficult to make any unamplified sound produced by musical instruments or voices clear and intelligible. The solution is provided by mirrors and baffles, wooden panels of various shapes that possess smooth or rough surfaces, depending on the needs. These elements are arranged to direct and diffuse sound waves to allow all listeners to hear the sounds emitted by the source perfectly and uniformly, regardless of their position within the room. The opposite effect, however, is provided by sound-absorbing panels which, instead of reflecting sound, absorb it. These porous materials are often used in cinemas where it is necessary to reduce reverberation and unwanted sounds given the power of the acoustic speakers inside. To optimize sound refraction, curved acoustic mirrors are used primarily in theaters which, thanks to their lens-effect, propagate acoustic waves toward the audience. These acoustic lenses consist of various layers of shaped materials; each is responsible for modifying the sound in relation to the wave’s propagation speed, much like what happens with respective optical lenses, where the lens processing determines their power. Acoustic lenses are widely used to capture acoustic signals within sonar due to their unique property of amplifying and directing the signal. A further application of these useful devices occurs within the medical sector, for example in ultrasounds for sonography.
The phenomenon of sound refraction can also be observed in various circumstances of our daily lives. For example, during the night or on days when the sky is quite overcast, a temperature inversion occurs. The air, in fact, is warmer the higher one rises. In this case, sound refraction occurs toward the ground. The phenomenon of temperature inversion is the reason why sound waves can be heard more clearly from greater distances at night. The effect is even better if the sound is propagated over water, allowing it to be audible extremely clearly at great distances.
Sound refraction is also more prevalent during windy days. The wind, moving faster at higher altitudes, causes a change in the effective speed of sound in relation to the distance from the ground. A further example of sound refraction is provided by the oceans. Under normal circumstances, ocean temperature decreases as depth increases. This results in the downward refraction of the sound wave generated underwater (exactly the opposite of the phenomenon described previously regarding the “shadow zone” created by the vertical refraction of the sound wave from thunder). According to marine biologists, the phenomenon of sound refraction in the oceans increases the propagation of sound waves produced by marine mammals such as whales and dolphins, helping them communicate with each other over long distances.