Transcript Listening to Concert Hall Sound

Listening to Concert Halls
Leo Beranek
&
David Griesinger
www.theworld.com/~griesngr
Goals:
• To demonstrate how reflected sound changes the sound quality of music.
• To show how different time delays and levels of the reflected sound
changes the effect.
• To show how the audibility of reflected sound – and its benefits – depends
on the signal:
– Speech, solo instrument, symphony, opera.
• To show how the frequency dependence of the reflected energy strongly
affects our perception.
• To show how comparisons between different halls are strongly influenced
by our sub-conscious adaptation to the acoustics of a space.
• To hint at how recent research into acoustic perception might change the
way halls are designed.
Acoustics and Sound
• The acoustician working on the design of a hall is faced with
an almost impossible task:
– Thousands of details go into the architecture of a hall, and their
connection to the final result is still poorly understood.
– And yet the hall must be built.
– The job of the acoustician is by necessity intimately connected with
architecture.
• Our task is easier – we want to demonstrate and explain the
effects of reflections on the sound, leaving aside the problem
of how to achieve them
– and in particular how to achieve them uniformly throughout the hall.
– Our job is to understand perception: how the brain perceives and
interprets both direct and reflected sound.
Sonic Perceptions
(and how we might quantify them)
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Loudness: The perceived strength of the sound
– Frequency weighted total sound pressure
Definition (or Clarity)
– “ease of intelligibility” - (Sato)
– and/or “sonic distance” – (Griesinger)
Stopped reverberation: The decay of sound when the music stops
– Ideally should be enveloping (all around us), but often it is not.
– Reverberation Time (RT) (Sabine)
Running reverberation: The sound of the hall we perceive while the music is playing
– Reverberant Loudness (Griesinger – Gardner)
Envelopment (or spaciousness): The degree to which the running reverberation seems to
surround us with the sound of the hall.
– Listener Envelopment (Bradley – Soloudre)
– + Reveberant Loudness
For untrained listeners Envelopment is not easy to perceive or to remember, but once you
know what it is, it can become one of the most desirable properties of a good hall.
Loudness – total sound pressure
• To understand total sound pressure
we turn to the Bible of room
acousics – 50 years old this year,
and still going strong!
When the total absorption is uniformly distributed, sound
pressure depends on the total absorption (number of people) and
the number of musicians!
On page 312 of Beranek we clearly see that as we add people – thus increasing the
sound absorption – the mean square sound pressure goes down proportionally. We
also see we can compensate by increasing the number of musicians.
Conservation of Energy
• The analysis in Bernaek’s “Acoustics” assumes uniform absorption
throughout the hall.
– The assumption is OK when predicting the reverberation time.
– But in many halls the assumption does not accurately predict the reverberant
loudness.
• Audience areas are highly absorptive
– ~80% of the energy is absorbed.
– Usually the other surfaces in a hall are entirely reflective.
• Direct sound that hits an audience surface does not contribute to reflected
energy
– either early or late.
– If the view from the stage is almost entirely of people, the direct sound will
dominate for most of the seats.
• Surfaces around the orchestra that reflect sound into the audience provide
strong early reflections.
– But this energy will not be available for later reverberation.
– running reverberation and envelopment will be low.
• Thus we must choose between a strong early sound level, and the warmth
and envelopment of later reverberation.
Loudness of early sound vs late reverberation
• If we want running reverberation to be audible, we
need to provide many surfaces that direct sound can
hit without being absorbed.
– And the reflections from these surfaces should also not be
absorbed.
– This is the case in a classical “shoebox” hall.
• But design decisions can still influence loudness and
clarity.
Example: Two halls similar in size and capacity.
On the left – NYC Avery Fisher. On the right, Washington DC’s Kennedy Center. Overall, Kennedy is
not as loud. Is it because the listening position was more distant? Were there fewer musicians? Or is
there something else?
Loudness Comparison: Avery Fisher vs Kenendy
Center.
Two concerts are compared using the identical equipment,
and with a similarly loud segment of music.
Avery Fisher – Brahms Gm
String Quartet orchestrated by
Schoenberg – at recorded
level. The graph shows
500Hz to 2000Hz.
Kennedy Center – Brahms
Violin Concerto amplified
+6dB over recorded level.
500Hz to 2000Hz
We need about 6dB of amplification to match the Kennedy center recording to the Avery Fisher
recording. Only one dB of the difference can be accounted for by the greater distance to the
listening position. Perhaps another 2dB can be ascribed to the larger orchestra in New York.
3dB of loudness remain to be accounted for. Why is Avery Fisher louder than Kennedy?
(Hint – listen for the clarity of the tympani)
The secret may be the Avery Fisher stage house.
Avery Fisher stage house - plan
Avery Fisher stage – elevation
Note the stage is fully enclosed and entirely reflective. All the sound produced by
the orchestra that is not absorbed by the musicians eventually gets to the audience.
BUT!! Instruments in the back of the stage get lost in the muddle of sound.
Kennedy Center Stage
Normal
+1dB
+6dB
The Kennedy stage house is larger and
couples to the hall with more area.
The orchestra is surrounded by audience
boxes, reverberation chambers, and an organ
chamber, all of which act as absorbers of
early sound energy.
The audience hears a greater percentage of direct sound. The sound is clearer, but less loud.
Early vs Late: Shoebox vs vinyard halls
• The “Vinyard” design is popular as a concert hall
design.
– The audience surrounds the orchestra
– Reflectors on the ceiling and side walls reflect
energy directly into the audience.
– The result is a strong early sound, and low late
reverberation and envelopment.
Shoebox vs Directed Halls
In Boston, the ceiling and side walls
are sound-diffusing, and not
absorptive.
A large percentage of the direct
sound to be trapped in the hall,
becoming late reverberation.
The sound is both clear and
reverberant!
In Los Angeles, the ceiling, vinyard walls, and
the side walls are arranged to reflect direct
sound back to the audience, where it is mostly
absorbed.
Early and middle reflected energy is increased,
and late reverberation is decreased.
The Ideal Reverberation above 1000Hz.
(Aside – this profile is a bit of a theoretical concept.
Measurement data in halls is sufficiently chaotic and
place dependent to prevent one from actually
observing a triple slope !)
The ideal profile has three
distinct slopes.
1. Reflections in the 20ms to
50ms time range with a total
energy of -4dB to -6dB
relative to the direct sound
combine with the direct
sound to produce a decay
rate under 1 second RT.
2. Reflections in the 50ms to
150ms time range decay
much more gradually – with
a slope greater than 2
seconds RT.
3. Reflections after 150ms
produce our perception of
reverberance, and should
decay at a rate appropriate to
the music.
Most real rooms (at all frequencies) have exponential decay
Exponential decay produces
a single-slope.
If the direct sound is strong
enough the effective early
decay can be short.
- But then there will be too
few early reflections and the
late reverberation will be
weak.
But – this type of decay may be ideal at 500Hz
and below.
If the direct sound above
1000Hz is weak, there will be
too much energy between 50
and 150ms, and the sound
will be MUDDY.
The ideal reverberation profile is frequency
dependent
• For frequencies above 1kHz (speech) the ideal profile has three distinct
slopes
– 1. The early slope – consisting of the direct sound and the 0-50ms
reflections. This slope is steeply down – less than 1 sec RT.
– 2. The middle slope – 50 to 150ms – is relatively flat – can have
an RT of 3s or more. This flat section of the profile maximizes the
late reverberant level while minimizing the muddiness.
– 3. The slope of the decay beyond 150ms can be around 1.3
seconds RT for opera and up to 2 seconds RT for orchestra (if the
early slope is short enough to maintain clarity.)
• Below 500Hz the decay probably should be single sloped, with RT of
1.7s or higher.
– This is because in our experience a single slope decay at low
frequencies produces the most pleasing sound on an orchestra.
• Thus for optimum acoustics the reverberation time and reverberation
level should increase below 500Hz.
Boston Symphony Hall, stage and hall occupied, mid
stage to front of balcony, 1000Hz
Boston Symphony Hall, occupied, mid stage to front
of balcony, 250Hz
Ideal reverberation profiles
The mystery of Definition (or Clarity)
A Major Surprise!
(and an enormous change in direction)
• The work of Dr. Barbara Shin-Cunninham at Boston
University has shown that human listeners sub-consciously
adapt to an acoustic envoironment over a period of 10-20
minutes.
– During that time period their accuracy on an intelligibility test improves
(sometimes dramatically).
– The improvement is fragile: they can be reset to the initial error rate
with an interruption of less than one minute.
– During the adaptation period the physiological algorithms change to
allow better extraction of speech sounds from acoustic interference.
• The adaptation is sub-conscious.
– Listeners are not able to remember how they did this trick, and they do
not easily remember the original acoustic properties of the space.
This work has enormous implications to
hall design!
• Our observations confirm Shin-Cunningham.
– Listeners adapt to the sound of a concert hall as they listen.
– After adaptation some of the acoustic qualities of the space
– particularly those relating to “Sonic Distance” and
intelligibility become difficult to perceive and to remember.
• When we compare halls through their remembered
qualities, just what are we comparing?
• Lets play some examples…..
Cantata Singers Rake’s Progress
Performance in
Jordan Hall, January
26, 2003.
Reverberation time
in Jordan ~1.4
seconds at 1000Hz.
This is similar to the
Semperoper
Dresden.
The typical audience
member is ~ 3
reverb radii from this
singer.
The dramatic
consequences are
highly audible.
It is amazing that in spite of the enormous acoustic distance, the performers still manage to
project emotion to the listener. The performance received fabulous reviews. But the situation is
not ideal. One reviewer commented on the regrettable lack of surtitles. The opera is in English.
Cantata Singers Rake’s Progress
Multimiked recording. Note the clarity of
vocal timbre (low sonic distance) and
good voice/orchestra balance.
Camera recording from under the first
balcony. Note the timbre coloration and
the poor balance. With the picture and
after adaptation the performance is
quite enjoyable.
Distance in Jordan Hall
• Reverberation time (occupied) measured as ~1.4 seconds at 1000Hz.
• Reverberation radius ~ 10 feet inside the stage house, ~14 feet in the
hall.
• Thus a typical listener will be ~ 3 reverberation radii away from a
singer who is fully upstage. This implies a direct/reflected ratio of
minus 10dB.
• Jordan Hall is not renowned as an opera venue – perhaps we are
hearing why.
Example: Boston Cantata Singers in Jordan Hall
Chorus and Orchestra