ROOM ACOUSTICS THREE APPROACHES 1. GEOMETRIC RAY TRACING SOUND DISTRIBUTION

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1 ROOM ACOUSTICS THREE APPROACHES 1. GEOMETRIC RAY TRACING. RESONANCE (STANDING WAVES) 3. GROWTH AND DECAY OF SOUND 1. GEOMETRIC RAY TRACING SIMPLE CONSTRUCTION OF SOUND RAYS WHICH OBEY THE LAWS OF REFLECTION SOUND DISTRIBUTION θ θ THIS CAN BE USED TO INVESTIGATE. THE DISTRIBUTION OF SOUND EVENNESS/SHADOWS/FOCUSING IMPROVE REFELECTIONS TO THE REAR OF THE SPACE CHECK FOR THE POSSIBILITY OF ECHOES Weak reflections Focus Shadow SEATING ARRANGEMENTS USEFUL REFLECTIONS IS THERE DIRECT LINE OF SIGHT (AND HEARING!) Sound attenuated by absorption of audience RECTANGULAR AREA OF USEFUL REFLECTION SLOPING SEATS / RAISED STAGE IMPROVE DIRECT SOUND MODIFIED CEILING PROFILE AREA OF USEFUL REFLECTION INCREASED 1

2 PROBLEMATIC REFLECTIONS LONG (CUE BALL) REFLECTION WILL BE HEARD AS AN ECHO AT FRONT OF HALL (Delays of > 60ms) FLUTTER ECHOES Sound bounces back and forth between the parallel reflective walls Echo CURE --- ADD REFLECTOR OR DIFFUSER (OR ABSORBER) REFLECTOR DIFFUSER Flutter Echo in a Small Theatre View from stage View from rear with reflective cinema screen FIRST REFLECTION The time delay between hearing the direct sound and the first reflection gives the listener an acoustic impression of the space. Short delays make the space intimate (impression that there is a short distance between stage and audience) Long delays make the audience feel remote from the action action. View from rear with reflective cinema screen covered with curtain The acoustical measure of intimacy is called the initial time delay gap (ITDG) -- intimate if < 0 ms Vertical surfaces give early reflections short vertical walls provide early reflections to improve acoustic intimacy in a large space Concert Hall - Vineyard Terrace Design COMPUTER MODELS USE THE SAME TECHNIQUES TO BUILD UP A TIME HISTORY OF REFLECTED ENERGY

3 CONCERT HALL CEILING AND BALCONY REFLECTIONS WALL REFLECTIONS. RESONANCE (standing waves) Sound can reflect back and forth in rooms This can lead to standing waves at frequencies where the room dimension is a multiple of a wavelength Wave = plus Reflection Standing wave These waves are called ROOM MODES Node Anti-node ROOM MODES Axial Modes wall to wall Tangential lmodes edge to edge Oblique Modes Corner to corner 3

4 c f = EIGEN FREQUENCIES N L L N W + W The numbers N L, N W, N H are integers - 0,1, etc. - which denotes the mode. N H + H L, W & H are the room dimensions. c is the speed of sound (340ms -1 ) Modes 100, 010, 001, 00, etc are axial modes Modes 110, 101, 011, 10, etc are tangential modes Modes 111, 11, 11, etc are oblique modes Evenly spaced modes are necessary for good acoustics in small rooms. Example. Find the first axial mode frequency along the length of a room 10m x 4m x 3m high. Assume speed of sound in air = 340 ms -1 The length, L = 10m; width, W = 4m; height, H = 3m N L = 1, N W = 0, N H = 0, --- axial mode along L. c f = f = 17Hz N N N L W H + + L W H 340 f = If these are predicted from 0-00 Hz a plot of modal density (number of modes per freq. band) can be drawn and the distribution of modes determined EFFECTS OF ROOM RESONANCE ONLY A PROBLEM AT LOW FREQUENCIES AND SMALL ROOMS GROUPED MODES GIVE COLOURATION AT LOW FREQUENCIES. (BOOMY BASS) TRY SINGING IN THE BATHROOM (adds dynamic to voice) CURES USE GOLDEN RATIOS FOR ROOM DIMENSIONS USE NON-PARALLEL WALLS ADD ABSORPTION (DAMPING) Ideal Room Proportions Various proposals for ideal room proportions have been made. The enclosed area is the best-known, proposed by Bolt. Red dots are particular ideal room proportions proposed by other authors. Bolt, R. H. 1946, Note on normal frequency statistics for rectangular rooms, Journal of the Acoustical Society of America, 18, DECAY OF SOUND IN ROOMS Wallace Clement Sabine was a pioneer in architectural acoustics Over a century ago he started experiments in the Fogg lecture room at Harvard, to investigate the impact of absorption on the reverberation time. He filled the room with sound using an organ pipe then switched it off and timed how long it took for the sound to become inaudible. Later this was refined, the reverberation time was defined as the time taken for the sound to decay by 60 db (to one millionth of the original intensity) ROOM DECAY CURVE 4

5 Sabine Formula T = V/A where T is the reverberation time (s) V is the volume of the room (m 3 ) larger rooms have fewer reflection per second A is the total absorption in the room (m ) areas x absorption coefficients Other formulae (Eyring etc.) for dead rooms EFFECT OF REVERBERATION ON SPEECH CLEAR SEPARATE SYLLABLES No Reverberation Many factors influence speech intelligibility T = 0.8 seconds T = 1.5 seconds T =.0 seconds SYLLABLES MERGE TOGETHER MAKING SPEECH UNINTELLIGIBLE RECOMMENDED REVERBERATION TIMES T (Secs) 3.5 CHURCH MUSIC CONCERT HALLS 1.5 AUDITORIUM, SPEECH TELEVISION STUDIO REVERBERATON TIME INDICES EDT(T 10 ), T 0, T 30 are constructed from the slope over the first part of the decay given by the subscript. (db) Noise off 0 db 10 db 0 db 30 db Decay curve Room volume (m 3 ) Speech requires short RT --- to distinguish individual consonants Music requires longer RT --- blend, ensemble T 0 T (Seconds) The RT will vary with position and time in complicated rooms REVERBERATION TIME MEASUREMENT (Interrupted Noise Method) Three separate decays in a small church with balcony Microphone is in the same position for each decay The time counting starts at time (1) when the level (4) is 5dB down from (). The time counting goes on until (3), i.e. when the level has dropped to (5). It is important to specify a minimum distance to the noise floor which is (6). 5

6 DECAY CURVES ARE GENERALLY SMOOTHED Old method using a chart recorder where the decay is drawn by a pen. Selecting a slower pen speed smoothes out the decay Current instruments use curve smoothing algorithms REVERBERATION TIME MEASUREMENT (Impulse Excitation Method) Back integration Least squares fit The curve is averaged over a number of decays. A back integration technique developed by Schroeder smoothes the decay. A least squares line is fitted and the 60 db decay time found USE OF ORCHESTRA AS SOUND SOURCE Difficult to measure due to orchestra so use T 10 or T 15 I --- Early Decay of Instruments (perceived reverberation) R --- Later Decay of Room Objective measures for room acoustics Reverberation Clarity Intimacy Warmth and Brilliance Loudness Spaciousness Background Noise LINKS Room mode calculator Comparison of different hall shapes Designing Auditoria 0_EA/_web/Online_Course_on_Acoustics/ind ex-3.html 6