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 371April 2, 2002

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 sreverberation time RT ~ 1.2 s

smooth decay of reverb: good

undesirable echo

direct sound: same signal in both ears

lateral reflection: difference in intensity in L and R ear and difference arrival time!

Good room acoustics: Strong early lateral reflection

Birmingham, U.K: large adjustable canopy over orchestraincreases 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 highearly lateral reflection: distance to side wall 12m = 35msecshallow balconies.

reverb time (1000Hz) 2.05 secexample: 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 carpeton concrete) over about 15% of floor area (homework)

Another “shoe box”: Royal Festival Hall London (1951) 3000 seatselevated stage, shallow balconies

“Fan-shaped” halls are rareley very good: lack of intense lateral reflections (Liverpool Philharmonic)

New Ideas: Vineyard Terrace (e.g. Berlin Philharmonic - Hans Sharoun, Architect)advantage: more seats close to stage - more direct sound

Berlin Philharmonic

among the famous failures: NY Philharmonic 1962

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

• 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

Unwanted background sounds and vibration:

Bridgewater concerthall in Manchester,England is immediatelyadjacent to a railroadtrack

The entire building issupported on shockabsorbers

Examples of noise suppression:

Suppression of ventilation system noise:

• 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: 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 for composers, conductors and musicians

different composers (and conductors) prefer different acousticsexamples: Mozart, Beethoven preferred to conduct in Wiener Hofburg with 1.4 sec reverberation timeBrahms, Bruckner Mahler preferred MusikvereinsaalVienna with 2.1 sec reverberation time.

Wagner assisted in acoustic design of Bayreuth Theater

Insertion of galleries in Thomaskirche, Leipzig reducedreverb time. This permitted articulation of fast passages-> resulted in Bach Mass in B-minor and St.Mathew Passion.

Performers and Conductors

consider problem that audience does not hear the sameas the preformers

examples: directionality of soundBruckner 4th Symphony asks for “raised trumpets” makes 15 dB difference!Sound reflection off floor can make important difference

Arrangement of instruments in orchestra