Second Exam revised exam date Thursday 4112002 instead

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 Ch 4 -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 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 New Tokyo Opera Excitation of steady tones

decay ~20 d. B in 0. 4 s reverberation time RT ~ 1. 2 s smooth decay of reverb: good

undesirable echo

Good room acoustics: Strong early lateral 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 45 m long, 24 m wide, 17 m high early lateral reflection: distance to side wall 12 m = 35 msec shallow balconies. reverb time (1000 Hz) 2. 05 sec example: calculate average absorption of walls 2. 05 sec = 0. 161 V/A A = Saave V = 18740 m 3 -> A = 1472 m 2 surface area of walls, ceiling, floor approx S=4500 m 2 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: 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 Philharmon

among the famous failures: NY Philharmonic 1962 16 M$ + 2 M$ 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:

Examples of noise suppression: Bridgewater concert hall in Manchester, England is immediately adjacent to a railroad track The entire building is supported on shock absorbers

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 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 consider problem that audience does not hear the same as the preformers examples: directionality of sound Bruckner 4 th Symphony asks for “raised trumpets” makes 15 d. B difference! Sound reflection off floor can make important difference Arrangement of instruments in orchestra