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Acoustics and Concert Halls Stephanie Hsu March 21, 2005.

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1 Acoustics and Concert Halls Stephanie Hsu March 21, 2005

2 Difficulties of Acoustic Design Purpose of hall Expense in achieving ideal acoustics Drawings  actual hall

3 Sound Waves waves: means of transmitting energy from point to point

4 Ground provides base for energy to be directly received or transmitted Flat and hard surfaces

5 If “ perfectly reflecting, ” sound will simply keep bouncing back and forth –Perfect reflectors = “ Eternal sound ” –Low absorption


7 Loudness vs. Intelligibility Surfaces around communicator increases efficiency of energy transfer Goal of acoustic design: achieve balance between –Need to deliver sufficient energy from source  listener –Need for good intelligibility

8 Loudness vs. Intelligibility (cont’d) Bathrooms –Low absorption  easy production of high level of sound –Multiple reflections (i.e. reverberation): sufficient overlap of successive sounds blurs out “ defects ” in sound

9 W. 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

10 Amount of absorption present X Time of reverberation = Constant Open window: most perfect absorber o.w.u.s still used as measures of absorption

11 Desirable 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)


13 Absorbents & Reflectors ½ dead ½ reverberant room Absorbent on back wall needed to prevent echoes or “ flutter effect ”

14 Methods 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


16 Adjustment 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

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