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/ 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 Excitation of steady tones New Tokyo Opera

decay ~20dB in 0.4 s reverberation 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 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 = Sa ave V = 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 Philharmonic

among the famous failures: NY Philharmonic M$ + 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 concert hall in Manchester, England is immediately adjacent to a railroad track The entire building is supported on shock absorbers 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 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 4th Symphony asks for “raised trumpets” makes 15 dB difference! Sound reflection off floor can make important difference Arrangement of instruments in orchestra