Virgil C. Summer September 2002 Exam 50-395/2002-301 FINAL ...
Second Exam: revised exam date: Thursday 4/11/2002 (instead of 4/9/2002)
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Transcript of Second Exam: revised exam date: Thursday 4/11/2002 (instead of 4/9/2002)
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
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)
“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
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
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.