Subjective quality:

The loudness of sound in a hall is obviously very important: the audience needs to be able to hear the performance without straining. Loudness can affect perception of other acoustic qualities as well, such as intimacy and spatial impression (for example, if the sound is too quiet, we do not feel that the space is intimate).

Objective measure:

Loudness in a concert hall may be objectively measured by the strength factor, G, in decibels. This is the measured sound pressure level at a position in the hall, relative to the sound level of the same source in a free field measured 10 m away:

G may be different for each frequency in a given space, so G(mid), which is the average of the measured values of G in the 500 and 1000 octave bands, is most commonly used to describe the loudness of a hall.

Substituting the RT equation into this one shows mathematically what we know logically: Loudness is inversely proportional to total surface area and absorption in a space. So in general, the smaller a room is, the louder sound will seem. Also, a room with more absorptive materials (producing a lower RT) will seem quieter than a room with more reflective ones.

Listen to halls with different Loudness.

Calculate Loudness, from RT and V.

Optimum values:

The best concert halls have a strength factor G between 4.0 and 5.5 dB.

Click for G(mid)’s of famous concert halls.

How to design:

Techniques to increase mid-frequency RT, such as reducing absorption in the space, generally help to strengthen G(mid); however, too much reverberation can cause muddled sounds, as discussed in the section on Clarity. An acoustician must balance the needs for strength, reverberation and clarity in a hall.

Side note: The strength of direct sound coming from the sound source will also affect perceived sound loudness. The direct sound level decreases the farther away an audience member is from the source, until you reach the reverberant level of the room. This distance (where the direct sound level equals the reverberant sound level) is called the critical distance, R, and may be calculated from the following formula:

where S is total surface area, Q is the directivity factor of your sound source (=1 if source radiates equally in all directions), and α is the average absorption coefficient in the room.

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