A direct sound from a source travels to the receiver without striking any surface. Within an enclosed environment, the direct sound also travels toward the surfaces and boundaries of the room. When the sound hits these surfaces it is sometimes reflected back into the space. These reflections can travel back toward the source or to the receiver as an indirect sound. If the direct sound is followed by a series of indirect reflected sounds, the space can become noisy and speech can become hard to understand. In such a case, reflections can have unwanted consequences.

Reflections can also be used to benefit sound distribution within a space. Direct sound diminishes in intensity as it travels. In large spaces, the sound can diminish before it reaches all of the receivers, particularly those furthest away from the source. Reflections can be used to help distribute and amplify the presentation to all of the listeners. Of course, this requires specific characteristics and location of reflective surfaces.

The characteristics of a surface greatly influence the way it reflects sound. Materials that reflect sound are those that are rigid, smooth and non-porous. Rigid materials cannot bend with a sound wave in order to decrease the intensity of the reflected sound. Smooth materials do not have surface undulations that could scatter the sound wave and diffuse it. Non-porous materials have very little air space where sound could be absorbed.

Sound is reflected in different ways depending on the shape of the reflecting surface:

Flat Surfaces
A flat surface is effective in distributing sound. If the surface is large enough and positioned correctly, a flat surface can project sound toward the listeners. Flat surfaces can also cause problems if placed incorrectly. For example, a flat, reflective rear wall in an auditorium will reflect sound back toward the speaker, this is called "slap-back". Parallel reflective walls can create a reflection between the two surfaces, this is referred to as "flutter echo" or "standing wave". Two flat surfaces coming together to form a peak can act as a megaphone and amplify the reflected sound.

Concave surfaces
Concave surfaces cause reflections to be concentrated rather than dispersed. This causes an abundance of reflection to be heard by the listeners in the focal point, or the point at which all of the reflections are focused. Reflections can also travel along a concave surface bringing delayed reflections around the room.

Convex surfaces
Convex surfaces are the best surfaces for distributing sound. They provide a wide spread of reflected sound.

Reflections can be controlled through room shaping as described above, irregularities within the room such as columns, trusses, surface undulations, etc., or through the use of absorptive materials (those with a high NRC.)

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