Auditorium acoustic (continued)

1. Sound sources

•Sound source can be characterized by power and directivity

Directivity factor Q – ratio of sound intensity in front of the source to the averaged sound intensity

Intensity drop as: 24)()(

r

PQ

rA

PrI

Intensity level:24

log10)(r

PQLrL w

For spherical source Q=1, for hemispherical Q=2, for source in a corner (which radiates into one-quarter of a sphere) Q=4

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2. Sound sources in a room

The distribution of sound level in a room can be considered as a sound field

Three regions in a sound field:•Near field - distance from the source is small compared to dimensions of the

source. Intensity varies with location and depends on the type of the source.

•Free field – intensity drops as 1/r2 (r is distance from the source). Intensity level drops 6dB when distance r doubles

•Reverberant field – region where reflected sound becomes important

rNear field Free field Reverberant field

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The shown temperatures are samples of a field

7782

83685566

8375 809091

7571

80

72

84

73

82

8892

77

8888

7364

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3. The Bass Loss ProblemWhen sound drops off according to the inverse square law, the sound further from the source is not only perceived as less loud but also as deficient in the bass frequencies. This is a natural result of the human hearing response as revealed by the equal loudness curves. There is a bass discrimination in the human hearing response for soft sounds.

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Phon is equal to the sound pressure level in decibels

at f = 1 kHz

Equal Loudness Curves

30 Hz: 103 dB ~ 80 phons; (103 -20) dB = 83 dB ~ 50 phons200Hz: 76 dB ~ 80 phons; (76 -20) dB = 56 dB ~ 60 phons

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4. Loudspeaker placemat

A. Single-source system •Preferred in most auditoriums.•Generally consists of a cluster of loudspeakers with selected directivity factors Q.

•Preferred location in the center of the room, over the speaker head. (Vertical displacement of the source is not distracting because of our inability to localize sound in a vertical plane.)

•Aimed toward the listeners at the rear of the auditorium.

B. Distributed-speaker system•Useful for long room with low ceiling.•Each unit mounted in the ceiling covers 60◦ - 90◦

•If room is long it is important to have electronic time delay for rear speakers to avoid echo.

• Speakers should not be placed along the side walls because some listeners will hear sound from several loudspeakers at the same time

•Speakers should not be placed on each side of the stage or front wall, because some listeners will hear sound from one of the speakers before they hear the direct sound. 6

5. Acoustic diffusers are used to maintain the acoustic energy in a room, but at the same time removing strong echoes.

•Acoustic diffusion occurs in a haphazard (irregular) way in many historical buildings through the use of ornate wall decorations and statues.

•Modern rooms rarely have such decorations but may include curved walls or other architectural features to achieve some of the same effect

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