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