AUDITORIA, CONCERT HALLS, and CLASSROOMS REFERENCES: Science of Sound, 3 rd ed., Chapter 23 Springer Handbook of Acoustics, 2007, Chapters 9, 10 Concert Halls and Opera Houses, 2 nd ed.,Leo Beranek, 2004

Free field Reflections p vs r log p vs log r SOUND FIELD OUTDOORS AND INDOORS

DIRECT AND EARLY SOUND SOUND TRAVELS AT 343 m/s. THE DIRECT SOUND REACHES THE LISTENER IN 20 to 200 ms, DEPENDING ON THE DISTANCE FROM THE SOURCE TO THE LISTENER. A SHORT TIME LATER THE SAME SOUND REACHES THE LISTENER FROM VARIOUS REFLECTING SURFACES, MAINLY THE WALLS AND THE CEILING. THE FIRST GROUP OF REFLECTIONS, REACHING THE LISTENER WITHIN ABOUT 50 to 80 ms, IS OFTEN CALLED THE EARLY SOUND. EARLY REFLECTIONS FROM SIDE WALLS ARE NOT EQUIVALENT TO EARLY REFLECTIONS FROM THE CEILING OR FROM OVERHHEAD REFLECTORS. IF THE TOTAL ENEERGY FROM LATERAL REFLECTIONS IS GREATER THAN THE ENERGY FROM OVERHEAD REFLECTIONS, THE HALL TAKES ON A DESIRABLE “SPATIAL IMPRESSION.”

PRECEDENCE EFFECT RATHER REMARKABLY, OUR AUDITORY PROCESSOR DEDUCES THE DIRECTION OF THE SOUND SOURCE FROM THE FIRST SOUND THAT REACHES OUR EARS, IGNORING REFLECTIONS. THIS IS CALLED THE PRECEDENCE EFFECT OR “LAW OF THE FIRST WAVEFRONT.” THE SOURCE IS PERCEIVED TO BE IN THE DIRECTION FROM WHICH THE FIRST SOUD ARRIVES PROVIDED THAT: 1. SUCCESSIVE SOUND ARRIVE WITHIN 35 ms 2. SUCCESSIVE SOUNDS HAVE SPECTRA AND ENVELOPES SIMILAR TO THE FIRST SOUND 3. SUCCESSIVE SOUNDS ARE NOT TOO MUCH LOUDER THAN THE FIRST

GROWTH AND DECAY OF REVERBERANT SOUND SOUND SOURCE SOUND AT LISTENER

GROWTH AND DECAY OF REVERBERANT SOUND SOUND SOURCE SOUND AT LISTENER RT = K (volume / area) RT = V/A (V in m 3 ; A in m 2 ) If room dimensions are given in feet, the formula may be written: RT= V/A (V in ft. 3 ; A in ft. 2 )

Sound decay Sound decay in a 400 m 3 classroom Sound pressure level as a function of time for that room SOUN D DECA Y

DECAY OF REVERBERANT SOUND

CALCULATING REVERBERATION TIME

CRITERIA FOR GOOD ACOUSTICS ADEQUATE LOUDNESS UNFORMITY CLARITY REVERBERANCE FREEDOM FROM ECHOES LOW LEVEL OF BACKGROUND NOISE

Desirable reverberation times for various sizes and functions Variation of reverberation time with frequency in good halls

Avery Fisher Hall (New York)

McDermott Concert Hall (Dallas)

Orchestra Hall (Chicago)

Meyerhof Symphony Hall (Baltimore)

Walt Disney Concert Hall

Disney

BING CONCERT HALL (Stanford) 844 seats, opening in January 2013 Named in honor of Helen and Peter Bing, major donors

Kimmel Center Auditorium (Philadelphia)

BACKGROUND NOISE CRITERIA

Spatial impression Intimacy Early decay time Clarity “Warmth” Important criteria for concert halls:

Concert halls throughout the World

CHURCHES CHURCHES AND SYNAGOGUES ARE NOT PRIMARILY CONCERT HALLS, BUT THEY SHARE MANY OF THE SAME REQUIREMENTS FOR GOOD ACOUSTICS OLD CATHEDRALS HAVE LONG REVERBERATION TIMES, AND THE SPOKEN WORD IS NOT AS IMPORTANT AS IN CONTEMPORARY WORSHIP. MUCH ORGAN MUSIC WAS COMPOSED FOR THESE SPACES BACKGROUND NOISE SHOULD BE VERY LOW ELECTRONIC REINFORCEMENT OF SOUND SHOULD BE USED ONLY WHEN NECESSARY!

CLASSROOMS NEED FOR GOOD ACOUSTICS: STUDENTS MUST BE ABLE TO UNDERSTAND THE TEACHER AND EACH OTHER MUST CONROL: REVERBERATION HEATING, VENTILATION, AND AIR CONDITIONING NOISE FROM OUTSIDE THE CLASSROOM ANSI STANDARDS: NC-25 to NC-30

WALLS AND NOISE BARRIERS The transmission coefficient is the ratio of transmitted to incident intensity: τ = I T /I 0 and the transmission loss is: TL = -10 log τ. At low frequency, the sound transmission loss follows a mass law, increasing with increasing frequency and mass density M of the wall: Transmission loss for a wall may fall below that predicted by the mass law, due to any of the following: 1. Wall resonances 2. Excitation of bending waves at the critical frequency where they travel at the same speed as certain sound waves in air 3. Leakage of sound through holes and cracks

TRANSMISSION LOSS THE EFFECT OF A HOLE ON TRANSMISSION LOSS

ELECTRONIC REINFORCEMENT OF SOUND IN A FREE FIELD (AWAY FROM REFLECTING SURFACES), THE SOUND PRESSURE LEVEL AT A DISTANCE r METERS FROM THE SOURCE IS:

SOUND FIELDS

POWER CONSIDERATIONS

LOUDSPEAKERS DYNAMIC LOUDSPEAKER MULTIPLE SPEAKERS IN A CABINET HORN LOUDSPEAKER HORN CLUSTERS

LOUDSPEAKER SYSTEMS SINGLE CLUSTER—MAINTAINS PROPER RELATIONSHIP BETWEEN THE SOUND SYSTEM AND THE APPARENT SOURCE MULTIPLE CLUSTERS—PROVIDES GOOD COVERAGE BUT SPREADS THE APPARENT SOURCE COLUMN MOUNTED---SUSCEPTIBLE TO INTERFERENCE EFFECTS DISTRIBUTED—SHOULD INCLUDE TIME DELAY TO MAINATAIN PROPER RELATIONSHIP WITH DIRECT SOUND PEWBACK SYSTEMS—PROVIDES GOOD COVERAGE IN CHURCHES

TIME DELAY SOUND THAT ARRIVES UP TO 50 ms AFTER THE DIRECT SOUND WILL REINFOCE THE DIRECT SOUND AND YET PRESERVE THE APPARENT DIRECTION OF THE SOUND SOURCE. TIME DELAY IS ESPECIALLY IMPORTANT IN THE CASE OF SUPPLEMENTARY SPEAKERS POSITIONED IN PROBLEM AREAS, SUCH AS UNDERNEATH A BALCONY OR FOR SPEAKERS MOUNTED ON SIDE WALLS.

LOUDSPEAKER PLACEMENT

LOUDSPEAKER DIRECTIVITY UNSATISFACTORY ARRANGEMENT OF LOUDSPEAKERS RADIATION PATTERN AND DIRECTIVITY FACTOR Q FOR A TYPICAL 8-INCH CONE LOUDSPEAKER

ACOUSTIC FEEDBACK

EQUALIZATION

ENHANCEMENT OF REVERBERATION ADJUSTMENT OF REVERBERATION TIME IS DESIRABLE IN MULTI-PURPOSE HALLS. MAXIMUM CLARITY OF SPEECH DEMANDS A SHORT REVERBERATION TIME, BUT A PIPE ORGAN SOUNDS BEST IN A REVERBERANT ROOM. ONE SOLUTION IS THE USE OF ELECTRONICALLY ENHANCED REVERBERATION OR “ASSISTED RESONANCE” ONE METHOD OF ENHANCEMENT PLACES A LOUDSPEAKER AND MICROPHONE IN A REVERBERATION CHAMBER ANOTHER USES A NUMBER OF TRANSDUCERS MOUNTED ON A THIN PLATE OR FOIL (KUHL PLATE) DIGITAL REVERBERATORS USE DIGITAL SIGNAL PROCESSING (DSP) TO SIMULATE REVERBERATION

ASSISTED RESONANCE SYSTEM REVERBERATION TIME IN THE ROYAL FESTIVAL HALL (LONDON) WITH AND WITHOUT ASSISTED RESONANCE (Parkin and Mogan, 2970).

REINFORCEMENT FOR THE HEARING IMPAIRED SPEECH INTELLIGIBILITY CAN BE IMCREASED BY PROVIDING A WAY TO ENHANCE THE SOUND AT THE LISTENER’S EAR. THIS CAN BE DONE BY ONE OF FOUR TYPES OF WIRELESS TRANSMISSION-RECEIVER SYSTEMS: MAGNETIC INDUCTION—EMPLOYS A LARGE LOOP OF WIRE TO SET UP A MAGNETIC FIELD THAT CAN BE PICKED UP BY HEARING AIDS FM BROADCASTING—(FCC HAS RESERVED A BAND OF HIGH FREQUENCY AM BROADCASTING—OPERATES IN THE BROADCAST BAND OR BELOW INFRARED LIGHT—DON’T OPERATE WELL IN BRIGHTLY-LIGHTED ROOMS

MICROPHONE PLACEMENT MICROPHONES ARE GENERALLY PLACED IN THE DIRECT FIELD OF THE SPEAKER OR PERFORMER SO THE MICROPHONE OUTPUT IS REDUCED BY 6dB FOR EACH DOUBLING OF THE DISTANCE. THIS REDUCES THE GAIN BEFORE FEEDBACK BY 6dB BUT IT ALSO MEANS THE PERFORMER CAN MOVE A LITTLE WITHOUT PRODUCING A LARGE CHANGE IN LEVEL WHEN A MICROPHONE IS A SMALL DISTANCE ABOVE THE FLOOR, CANCELLATION OF CERTAIN FREQUENCIES (‘COMB FILTERING’) CAN OCCUR. FOR EXAMPLE, IF THE MICROPHONE WERE 3 m FROM THE SOURCE AND BOTH WERE 1.5 m ABOVE THE FLOOR, THE PATH DIFFERENCE OF THE DIRECT AND ONCE-REFLECTED SOUND WOULD BE 1.23 m AND THE CANCELED FREQUENCY WOULD BE ABOUT 140 Hz.