The reflection of sound is perhaps the most intuitively understood acoustical property. We know that when we stand outdoors in an open field and shout, the sound is immediately lost. We deduce that most of the energy travels outward into the air while some is absorbed by the ground. But when a sheer rock wall is nearby, we hear a distinct echo. Sound energy has traveled outward, struck the wall and bounced from it, and traveled back to us. This is sound reflection. It takes some time for the reflected sound to return to us, and we observe that the further away the wall, the longer the return time. With some experimentation, we might calculate the speed of sound: sound travels about 1130 ft in one second. Now consider a rock wall with thick moss growing on it. There is an echo, but it is softer. We observe that some surfaces are better sound reflectors than others. It appears that a good reflector returns almost all the sound energy to us while a poor reflector absorbs much of the energy and returns little.
Next we go indoors and repeat the experiments. The results are similar, but also more complicated. A hard surface such as a plaster wall efficiently reflects high-frequency sound while a soft surface such as a carpeted floor reflects almost none. However, we also observe that the room enclosing us provides not just one reflection, but many. A sound might reflect from one wall, then another and another. Each trip across the room takes some time (the larger the room, the longer the time), and the result is a multiplicity of reflections spread over time. Instead of a discrete echo, we hear densely spaced reflections, that is, reverberation. Logically, rooms with highly reflective surfaces provide long reverberation times while rooms with weakly reflective surfaces offer short reverberation times.
A room’s reverberation time is largely determined by the choice of surface materials. The question is thus presented: what reverberation time is best? The answer depends on how the room will be used. Any musician or music lover will tell you that reverberation is welcome. For example, concert halls tend to have luxuriously long reverberation times because it makes it easier for musicians to perform as an ensemble, and audiences like the reverberant sound. But a long reverberation time in a recording studio is undesirable because excess recorded ambience would decrease isolation between recorded tracks and make it difficult for a mixing engineer to tailor the sound of each instrument. Therefore, most recording studios have shorter reverberation times.
Also, while musicians might welcome reverberation, they will also be picky about the quality of the reverberation. For example, the frequency response of the reverberation is as important as the reverberation time. The room surface materials can dramatically affect the frequency response of reverberation. For example, a room with surfaces that reflect low frequencies and absorb high frequencies will have a relatively lower-frequency reverberation. A room with reverberation with this kind of frequency response might be considered “boomy” and thus undesirable. It is also important to note that while reverberation is highly prized in a concert hall, isolated and discrete echoes are not. Any audible echo in a concert hall or a recording studio would indicate a serious flaw in the acoustical design.
It is also important to consider the timing and directionality of room reflections. For example, in concert hall designs, early reflections (those arriving at the listener soonest) must be carefully timed to provide an adequate sense of spaciousness. Some control rooms are designed so that no early
reflections arrive from the front of the room, while many reflections arrive from the back of the room within a certain time period. Clearly, the reflection of sound is one of the most acutely judged qualities of any acoustical space.