Transcript AUDITORIUM ACOUSTICS

AUDITORIA,
CONCERT
HALLS, and
CLASSROOMS
ELECTRONIC
REINFORCEMENT
OF SOUND
REFERENCES:
Science of Sound, 3rd ed., Chapters 23, 24
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 overhead
reflectors. If the total energy from lateral reflections is
greater than the energy from overhead reflection, the
hall takes 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 sound arrives provided that:
1. Successive sound arrives with 35 ms;
2. Successive sound have spectra and envelopes
similar to the first sound;
3. Successive sounds are not too much louder than the
first sound.
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
●Uniformity
●Clarity
●Right amount of 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)
(2742 seats)
McDermott Concert
Hall (Dallas)
(2065 seats)
Orchestra Hall
(Chicago)
(2582 seats)
Meyerhof
Symphony Hall
(Baltimore)
(2467 seats)
Walt Disney Concert Hall
Disney
(2265 seats, opened 2003)
BING CONCERT HALL (Stanford)
844 seats, opened in January 2013
Named in honor of Helen and Peter
Bing, major donors
Kimmel
Center
Auditorium
(Philadelphia)
Verizon Hall: 2500
seats
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 was 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 control:
• Reverberation
• Heating, ventilation, and air conditioning noise
• 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
Ref: Science of Sound, Chapter 24
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 covderage but spreads the
apparent source
COLUMN MOUNTED---Susceptable to interference effects
DISTRIBUTED—Should include time delay to maintain 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 reinforce 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 undereneath a balcony, or for speakers
mounted on the 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 multipurpose halls. Maximum clarity of speech demands a short
reverberation time, but a pipe organ sounds best in a
reverberant room. One solution the use of electronicallyenhanced 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 increased 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—Doesn’t work too well in brightlylighted rooms.
MICROPHONE PLACEMENT
Microphones are generally placed in the direct field of
the speaker or performer, so the microphone output is
reduced by 6 dB for each doubling of the distance. This
reduces the gain before feedback but it also means the
performer can move only 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 (see Fig. 24.14 in Science of Sound).
Questions and Exercises, Chapter 23
No homework required in Chapter 23.
However, answering the Review Questions may
provide a good review of the material.
You may submit them by Wednesday, March 19 for
extra credit.
Questions for Thought and Discussion:
1. Why does the use of cushioned seats help to make the
reverberation time of an auditorium independent of audience
size?
2. Which is easier to correct, a reverberation time that is too long or
one that is too short?
3. What are desirable reverberation times for speech and for
orchestral music in an auditorium with V=1000 m3 ?
4. An auditorium is thought to have excessive reverberation,
especially at low frequency. It is proposed that the ceiling be
covered with acoustic tile to reduce this. What do you think of
this solution? Is there a better one?
5. What is your opinion of the acoustics of the Bing Hall? How
many concerts have you heard? Where did you sit? ( All
answers will be considered confidential).