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Second Exam: revised exam date: Thursday 4/11/2002 (instead of 4/9/2002) study guide handed out today (answers on Thursday 4/4/2002) covers Ch4-9 and Homework 4-8 (homework 8 will be handed out on Thursday 4/4/2002) back copies of old homework and answers available.

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  • Second Exam:

  • revised exam date: Thursday 4/11/2002 (instead of 4/9/2002)

  • study guide handed out today (answers on Thursday 4/4/2002)

  • covers Ch4-9 and Homework 4-8

  • (homework 8 will be handed out on Thursday 4/4/2002)

  • back copies of old homework and answers available

  • Topics for paper:

  • guide to select a topic was handed out 3/18/2002 -

  • Old homework and exams:

  • if not picked up will be discarded at end of week


Physics 371

April 2, 2002

Acoustics for Musicians

  • Room Acoustics (cont)

  • smoothness of decay, diffusers

  • early reflection, canopies

  • bad and good concert halls:

  • criteria and shape of hall

  • (“shoe box”, “vineyard terrace”)

  • noise reduction

  • Excitation of steady tones

New Tokyo Opera


decay ~20dB in 0.4 s

reverberation time RT ~ 1.2 s

smooth decay of reverb: good



Good room acoustics: Strong earlylateral reflection

direct sound:

same signal in both ears

lateral reflection:

difference in intensity

in L and R ear and

difference arrival time!


Birmingham, U.K: large adjustable canopy over orchestra

increases early reflection + adds to performer satisfaction


Shape of concert halls: the classic “shoe box” hall

Vienna Musikverein Saal (1870) - RT 2.05 sec.

used by J. Brahms, A. Bruckner and G.Mahler


Symphony Hall, Boston (1900)- good “show box”:

dimensions 45m long, 24m wide, 17m high

early lateral reflection: distance to side wall 12m = 35msec

shallow balconies.

reverb time (1000Hz) 2.05 sec

example: calculate average absorption of walls

2.05 sec = 0.161V/A A = Saave

V = 18740 m3 -> A = 1472 m2

surface area of walls, ceiling, floor approx S=4500 m2

thus ave absorption of walls a= A/S = 1472/4500 = 0.32

example of application: what is effect of carpeting the aisles?

a increases from 0.03 (concrete) to 0.37 (heavy carpet

on concrete) over about 15% of floor area (homework)


Another “shoe box”: Royal Festival Hall London (1951) 3000 seats

elevated stage, shallow balconies


“Fan-shaped” halls are rareley very good: 3000 seats

lack of intense lateral reflections (Liverpool Philharmonic)


New Ideas: Vineyard Terrace 3000 seats

(e.g. Berlin Philharmonic - Hans Sharoun, Architect)

advantage: more seats close to stage - more direct sound



among the famous failures: NY Philharmonic 1962 3000 seats

16M$ + 2M$ in attempted improvements -

problems related to:

bulging concave side walls to provide more

seating

elimination of sound-diffusing elements ($)

sound absorption by plywood panels on balconies

led to understanding of need for early lateral

reflection

(rebuilt as Avery Fisher Hall 1974)


“Electronic Enhancement” of Concert Halls 3000 seats

  • compensate lack of loudness at rear of hall and , improve uniformity

  • loudspeakers permit fine tuning of room acoustics

  • provides some flexibility for different performances

  • difficult: audience should be unaware of it

  • essential: direct sound must come first! (delay speaker signal)

  • for speech: directional speakers toward audience improves intelligibility



Examples of noise suppression: 3000 seats

Bridgewater concert

hall in Manchester,

England is immediately

adjacent to a railroad

track

The entire building is

supported on shock

absorbers


  • large ducts (low air velocity)

  • flexible coupling

  • new: electronic noise cancellation

  • electronic noise compensation

  • useful for periodic noise


  • known elements of successful concert hall design: 3000 seats

  • loud sound, early lateral reflection, smooth reverberation

  • solid construction: walls of concrete and plaster on wire lath

  • yields good bass reflection (“warmth)

  • strong lateral reflection yields “envelopment”

  • short initial time delay gap yields “intimacy”

  • direct path from source to listener gives “presence”

  • raised musician platform, raked seating, raked shallow balconies,

  • careful placement of diffuser improves uniformity

  • attention to construction details (including noise reduction)

  • use electronic enhancement sparingly

  • models are of benefit

  • advantages of computer design(“rational design”) still disputed

  • ….but anotherproblem:

  • how objective is the quality judgment of concert halls?


Importance of concert hall acoustics 3000 seats

for composers, conductors and musicians

different composers (and conductors) prefer different acoustics

examples: Mozart, Beethoven preferred to conduct in

Wiener Hofburg with 1.4 sec reverberation time

Brahms, Bruckner Mahler preferred Musikvereinsaal

Vienna with 2.1 sec reverberation time.

Wagner assisted in acoustic design of Bayreuth Theater

Insertion of galleries in Thomaskirche, Leipzig reduced

reverb time. This permitted articulation of fast passages

-> resulted in Bach Mass in B-minor and St.Mathew Passion.


Performers and Conductors 3000 seats

consider problem that audience does not hear the same

as the preformers

examples: directionality of sound

Bruckner 4th Symphony asks for “raised trumpets”

makes 15 dB difference!

Sound reflection off floor can make important difference

Arrangement of instruments in orchestra


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