Acoustics and Concert Halls

Sound Field

Imagine an acoustic impulse is generated in a hall. The first sound to arrive at the listener will be the sound that travels in a straight line from the source, namely the direct sound. This is followed by a series of sounds that have traveled by paths including one or more reflections from room surfaces. The reflection could be specular, namely the incidence angle equaling the reflection angle, if a surface is large and acoustically smooth (i.e., surface roughness significantly smaller than the wavelengths), or diffuse if the surface is irregular. Compared with the direct sound, the amplitude of a reflected sound is always less because part of the sound energy is absorbed by the reflected surfaces and it travels farther. Reflections that arrive at the listener immediately after the direct sound are called early reflections. The late reflections are called the reverberant sound.

Reverberance

Reverberance is arguably the most important attribute for the acoustic perception of concert halls. It can be measured by the reverberation time (RT), which is defined as the time taken for a sound to decay 60 decibels after a continuous sound source is stopped, although actually it is obtained from the 5- to 35-decibel decay and then extrapolated to correspond to a 60-decibel decay because a signal to background noise ratio of 60 decibels is difficult to achieve in practice. It is a function of the volume of the room and of the amount of sound absorption within it. The perception of reverberance is well correlated to the early slope of a sound decay, so the early decay time (EDT) is often used. EDT is obtained from the initial 10 decibels of the decay slope, and then multiplied by a factor of 6. In an ideal diffuse sound field, the decay curves are perfectly linear, and thus, EDT equals RT.

Reverberation is related to liveliness. With a longer reverberation, especially at middle (e.g., 500 Hz-1 kHz) and high frequencies, people feel a concert hall is more lively, and a hall with a short reverberation is perceived as deadly or dry. If a hall is reverberant at low frequencies, say below 350 Hz, it will sound “warm.” At middle frequencies, the recommended occupied reverberation time is 1.5 seconds for chamber music, 2 to 2.4 seconds for symphony music, and more than 2.5 seconds for organ music.

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