Acoustics and Acoustics and Concert HallsConcert Halls
Stephanie Hsu
March 21, 2005
Difficulties of Acoustic DesignDifficulties of Acoustic Design
Purpose of hall Expense in achieving ideal acoustics Drawingsactual hall
Sound WavesSound Waves waves: means of transmitting energy
from point to point
Ground provides base for energy to be directly received or transmitted
Flat and hard surfaces
If “perfectly reflecting,” sound will simply keep bouncing back and forth– Perfect reflectors = “Eternal sound”– Low absorption
Loudness vs. IntelligibilityLoudness vs. Intelligibility
Surfaces around communicator increases efficiency of energy transfer
Goal of acoustic design: achieve balance between– Need to deliver sufficient energy from
sourcelistener– Need for good intelligibility
Loudness vs. Intelligibility (cont’d)
Bathrooms– Low absorptioneasy production of high
level of sound– Multiple reflections (i.e. reverberation):
sufficient overlap of successive sounds blurs out “defects” in sound
W. B. SabineW. B. Sabine Harvard lecture room—too reverberant Calibrate absorption of cushions,
curtains, people, etc.– Measured length of cushion/amount of
absorption needed to bring time of reverberation back to original value
Amount of absorption present
X
Time of reverberation
=
Constant
Open window: most perfect absorber o.w.u.s still used as measures of
absorption
Desirable Reverberation RatesDesirable Reverberation Rates Time of reverberation: time taken for
energy density of a sound to fall to the level of the threshold of hearing from a given level of sound
(a + x)T = kV– a=amount of absorption in empty room– x=amount of sound added– T=time of reverberation– V=volume of room – k=0.171 (works for rooms of many different
shapes and sizes)
Absorbents & ReflectorsAbsorbents & Reflectors ½ dead ½
reverberant room
Absorbent on back wall needed to prevent echoes or “flutter effect”
Methods of Acoustical DesignMethods of Acoustical Design
Small scale model of hall– 1/8 actual size
Recording test sounds (dead enclosure)– Played at 8X normal speed inside model
Resulting sound recorded– Played back at 1/8 speed
Polystyrene foam blocks
Adjustment of AcousticsAdjustment of Acoustics
Changes in distribution of absorbing and reflecting surfaces
Movable parts of walls or reflectors can be changed to suit different purposes
Electronic modification of acoustic characteristics– Microphones in front, loudspeakers in
sides and back– Microphones in space between suspended
and structural ceiling