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Previous: Auditorium Acoustics
Rooms that have unfavorable acoustic properties can often be improved through the use of well placed electronic audio systems. Likewise, even good auditoriums can often have their acoustic properties improved by the same means.
- Sound sources are typically characterized by their power and directivity, which vary with frequency.
- Sources usually become more directional at high frequencies, where the wavelength of sound is comparable to the dimensions of the source.
- The directivity factor is defined as the ratio of the sound intensity at a distance in front of a source to the sound intensity averaged over all directions.
- Sound pressure level depends on the power and directivity of the source, its distance, and the strength of the reflected sound. In a free field (with no reflections),
- The near field sound level of a source typically varies with location because some parts of the source may radiate more strongly than others.
- In the free field, the sound pressure varies as and sound level contributions from reflections are minimal.
- In the reverberant field, sound reflections become significant.
- The sound level due to both direct and reverberant sounds is
, where is the total absorption area.
- A large single source of loudspeakers is often preferred in an auditorium because it best preserves the spatial pattern of the sound field.
- A setup in which a listener may hear the sound from a loudspeaker before the direct sound should be avoided.
- Various loudspeaker shapes have different directivity patterns.
- A vertical column of speakers produces a radial beam which spreads out much more in the horizontal direction than the vertical direction. This increases the effective while maintaining a broad distribution in the horizontal direction.
- Horns usually have greater radiation efficiency than cone-type speakers, though their low-frequency response is usually weaker.
- Acoustic feedback usually limits the amount of gain that can be achieved with an amplfying system.
- Microphones of high directivity can often be used to reduce feedback.
- Since feedback usually occurs at the frequency of a prominent room resonance, room equilization can help to increase the useful gain of the amplification system.
- Equilization of a room sound system can result in a more natural sound by compensating for predominant room resonances, as well as suppressing acoustic feedback as mentioned above.
- A sound system with loudspeakers at several positions within a large auditorium will generally need some type of time delay to account for the natural time delay of sound propagation in the room.
- A frequency-dependent electronic (or mechanical) reverberation system can be used to provide ``resonance'' to an otherwise dull room. Also, such a system can be used to vary the reverberation characteristic to best match the particular use of the room.