Transcript Chapter 23: Auditorium Acoustics

AUDITORIA,
CONCERT
HALLS, and
CLASSROOMS
REFERENCES:
Science of Sound, 3rd ed., Chapter 23
Springer Handbook of Acoustics, 2007, Chapters 9, 10
Concert Halls and Opera Houses, 2nd ed.,Leo Beranek, 2004
SOUND FIELD OUTDOORS AND INDOORS
p vs r
Free field
Reflections
log p vs log r
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 = 0.161 V/A (V in m3; A in m2 )
If room dimensions are given in feet, the formula may be written:
RT= 0.049 V/A
(V in ft.3 ; A in ft.2 )
Sound decay
Sound decay in a
400 m3 classroom
Sound pressure level as a
function of time for that room
DECAY OF REVERBERANT
SOUND
CALCULATING REVERBERATION TIME
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
Important criteria for
concert halls:
•Spatial impression
•Intimacy
•Early decay time
•Clarity
•“Warmth”
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 = IT/I0 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
HORN
LOUDSPEAKER
MULTIPLE
SPEAKERS IN A
CABINET
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.