Multichannel solutions for sound enhancement and acoustic conditions in concert halls and operas

1
Multichannel solutions for sound enhancement and
acousticconditions in concert halls and operas

• David Griesinger
• Lexicon
• dgriesinger_at_lexicon.com
• www.world.std.com/griesngr

2
Major Goals

• To explain and demonstrate the degree to which
  the acoustics of halls and operas may not be
  the same as the sound in these spaces.
• The dependence of acoustics on visual aspects of
  architecture and on the expectations of the
  listeners may be underappreciated.
• To show how physics and psycho acoustics combine
  to produce absolute standards of acoustic quality
  for sound in opera houses and concert halls.
• To suggest that sonic distance the perceived
  audible distance between a performer and a
  listener is the major descriptor of this
  acoustic quality in an opera house.
• To explain and demonstrate how electronic
  acoustic enhancement can be used to achieve
  higher sonic quality in some halls.
• To play as many musical examples as possible
  using multichannel discrete surround and two
  channel to five channel conversion.

3
What constitutes good sound?
Leo Beranek JASA 107 pp368-383 Jan. 2000 rank
ordered houses by asking conductors to fill out a
questionnaire. Semperoper Dresden is ranked
nearly at the top, as is the Teatro alla
Scalla. But the SOUND of these two theaters is
extremely different. Semperoper is highly
reverberant, and La Scalla is highly damped. In
practice the remembered sound of an opera house
can depend strongly on non-sonic factors.
4
High-Definition Demo

• Brahms F minor Piano Quintet
• Performed by the faculty of the
  Point-Counter-Point Summer camp.
• Video is high-definition (with some artifacts.)
• Audio is two channel, single microphone pick-up.
• Played here (after post production) with
  two-channel to five-channel processing.

5
Why is there so much confusion?

• 1. Research methods based on questionnaires
  suffer from a fundamental properties of acoustic
  perception
• The supression of acoustic perception after a
  short time period.
• The inability to accurately remember the sound
  quality.
• 2. We might be asking the wrong questions to the
  wrong people
• The conductor is only one of the many people who
  work to present opera to the public
• For most of these people the music is secondary
  to the drama. Their job is to get the story and
  the emotion to the audience.
• To most people involved in opera production the
  Clarity of the singers and the balance between
  them and the orchestra is of the utmost
  importance.

6
Measurement methods for halls and operas are
inadequate and often misleading

• Sabines reverberation time is useful, but it is
  the combination of reverberation time and
  reverberation level that we perceive.
• Jordans EDT measure was intended to measure the
  direct/reverberant ratio.
• But EDT is based on the decay of very long
  sounds, and does not measure the hall response to
  short sounds.
• Schroeders method of measuring EDT (which is now
  an international standard) gives results that are
  independent of the direct/reverberant ratio.
• Schroeder misunderstood the purpose of the
  measure.
• His method yields results essentially identical
  to the reverberation time.
• C80 and related measures use 80ms as a division
  point between early and late.
• But in fact human perception utilizes THREE time
  regions 0-50ms, 50-150ms, and 150ms.
  Intelligibility correlates best with C50, not
  C80, and reverberance correlates best with the
  ratio between the energy from 0-50ms to the
  energy 150ms and greater.

7
It is difficult to remember the sound of acoustics

• Human physiology suppresses acoustic perception.
• After 5 to 10 minutes in a particular space we
  lose the ability to perceive its acoustic
  properties.
• Work by Shin-Cunningham suggests that the process
  of extracting speech information from acoustic
  interference is adaptive.
• We adapt to a particular situation in 5 to 10
  minutes, and the adaptation is unconscious.
• After the adaptation period the perception of
  muddiness (mulmig or glauque) becomes difficult
  to perceive and to remember.
• As a consequence, it is difficult to remember the
  properties of an acoustic space, particularly for
  speech.
• Unless intelligibility is seriously compromised.
• We need to compare acoustic sounds BEFORE our
  physiology adapts to them.
• We need relatively rapid A/B comparisons to
  accurately rank acoustic quality.

8
Boston Cantata Singers in Jordan Hall
9
Cantata Singers Rakes 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. (reverb 10dB
stronger than direct) 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.
10
Distance in Jordan Hall

• Reverberation time (full) 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 10dB.
• Jordan Hall is not renowned as an opera venue
  perhaps we are hearing why.
• But the size and reverberation time are almost
  identical to the Semperoper Dresden, which is
  currently regarded as one of the best!

11
Binaural Recordings

• Manfred Schröder suggested that Binaural
  recordings could be used to compare different
  concert halls in the laboratory.
• The method has many difficulties
• Matching of pinnae shape of the microphone to the
  listener.
• Matching of the playback equipment to the
  listener.
• These difficulties are particularly acute in
  studying concert hall acoustics.
• Schröder suggested the use of a cross-talk
  canceller to solve some of these problems.
• However, in our experience the differences
  between opera houses are so large that relatively
  simple recording and playback equipment can
  capture the essential aspects of the sound.
• And that these differences can easily be heard
  even with loudspeaker playback.

12
Glasses microphones
dual lavaliere microphones from Radio Shack
plug directly into a mini-disk recorder. The
result is free of diffraction from the pinnae of
the person making the recording, which is an
advantage.
When combined with a calibrated pair of
headphones, this system reproduces sonic
distance, intelligibility, and envelopment quite
well.
13
Binaural Examples in Opera Houses

• It is very difficult to study opera acoustics, as
  the sound changes drastically depending on
• the set design,
• the position of the singers (actors),
• the presence of the audience, and
• the presence of the orchestra.
• Binaural recordings made during performances can
  give us important clues.
• Here is a short example from the Semper Oper
  Dresden. This hall was rebuilt in 1983, and
  considerable effort was expended to increase the
  reverberation time. The RT is over 1.5 seconds
  at 1000Hz, which implies a reverberation radius
  of under 14.
• This hall is ranked nearly the best in Leos
  survey. Note the excessive distance of the
  singers, and the low intelligibility

14
Staatsoper unter den Linden Berlin
The Staatsoper Berlin is similar in size to the
Semperoper, and the acoustics in Berlin are
probably much closer to the original acoustics in
Dresden RT at 1000Hz 0.9s (without LARES). With
LARES the RT at 1000Hz is 1.1s, but the RT is
1.7s at 200Hz. Here is a recording made from the
parquet, about 2/3s of the way to the back wall.
Although this hall does not even appear in Leos
survey, it is currently by far the most vital of
the Berlin Opera houses.
15
Deutsche Oper, Berlin
In spite of the impressive wood paneling, the
sound in this hall is rated between pretty poor
and gastly by the people I interviewed during a
site visit.
It is perhaps significant that this hall is
moribund. They are searching for both a new
music director and a new general manager.
Concerning the acoustics, I was told that they
are just waiting for the architect to die, so
they can re-design it. But how should it be
redesigned? Just what is wrong with it as it is?
16
Bolshoi
The old Bolshoi in Moscow is similar in design to
the Staatsoper but larger. The recording was
made from the back of the second ring, and is
monaural. RT 1.1 seconds at 1000Hz, rising at
low frequencies.
In my opinion the sound in this hall is extremely
good. The dramatic impact of the singers is
phenomenal for such a large hall, and envelopment
in the parquet is high. This theater is
extremely popular nearly impossible to get into
without paying a scalper 100.
17
New Bolshoi
The New Bolshoi is very similar to the Semperoper
Dresden. The Semperoper was the primary model
for the design. RT 1.3 seconds at 1000Hz.
What is it about the SOUND of this theater that
makes communication with the singers so difficult?
The general manager views this theater as
unsuccessful acoustically. There have been many
complaints the singers are both too loud and
too hard to hear. This theater suffers greatly
from having the old Bolshoi next door!
18
The Sound of Opera the blind opera fan.

• What distinguishes the SOUND of the New Bolshoi
  from the Staatsoper Berlin, or the Royal Theater,
  Copenhagen?
• Reverberation time?
• Intelligibility?
• Envelopment?
• Balance?
• All might be involved
• An informal poll of acousticians gave the result
  that EVERY ONE thought 1.5 seconds was the ideal
  reverberation time.
• And yet the two Bolshoi theaters dramatically
  contradict this idea.
• Intelligibility in ALL the theaters I have
  visited is satisfactory. Here is dialog from the
  Semperoper
• Envelopment in the parquet of the old Bolshoi is
  high, even with a low reverberation time. Here
  is a segment from Gisielle
• Balance IS important but it is not sufficient
  to explain the differences we hear.

19
Balance between the orchestra and the soloists
Reverberation time affects balance, due to the
directional properties of the human voice. Note
that the loudness of the orchestra increases
about 1.5dB as RT rises from 1s to 1.5s. This
rise is not sufficient to explain the large
dramatic differences between Semperoper Dresden
and Staatsoper Berlin.
20
Sonic Distance

• Even casual listening to the examples in this
  paper reveals that the most obvious difference is
  how far away the voices seem.
• Loudness is a primary distance cue.
• This distance cue can be overcome by trained
  actors and singers, who know how to project their
  voices with sufficient energy.
• If you have the money you can hire singers with
  more vocal power.
• The main secondary cue for distance is the ratio
  between the loudness of the direct sound and
  reflected energy that arrives between 50 and
  150ms after the direct sound.
• When this energy is excessive the singers can
  sound loud, but muddled and far away.
• Dramatic connection between the actors and the
  audience suffers.

21
Human sound perception Separation of the sound
field into foreground streams.

• Acousticians are entranced with reflections
  rather arbitrarily divided into early and
  late.
• But human perception works differently.
• Human brains evolved to understand speech, and to
  ignore reflections.

Third-octave filtered speech. Blue 500Hz.
Red 800Hz Speech consists of a series of
foreground sound events separated by periods of
relative silence, in which the background sound
can be heard.
22
One of the most important preliminary functions
of human hearing is stream formation

• Foreground sound events (phones or notes) must be
  separated from a total sound field containing
  both foreground and background sounds
  (reverberation, noise).
• Foreground events are then assembled into streams
  of common direction and/or timbre.
• A set of events from a single source becomes a
  sound stream, or a sound object. A stream
  consists of many sound events.
• Meaning is assigned to the stream through higher
  level neural functions, including phoneme
  recognition and the combination of phonemes into
  words.
• Stream separation is essential for understanding
  speech
• When the separation of sound streams from noise
  is easy, intelligibility is high.
• Separation is degraded by noise and
  reverberation.
• This degradation can be measured by computer
  analysis of binaural speech recordings.
• Stream formation is entirely sub-conscious.
• We can consciously choose which stream listen to,
  but we can not influence the separation process.

23
Separation of binaural speech through analysis of
amplitude modulations
Reverb forward Reverb backward
Analysis into 1/3 octave bands, followed by
envelope detection. Green envelope Yellow
edge detection By counting edges above a certain
threshold we can reliably count syllables in
reverberant speech. This process yields a measure
of intelligibility not distance.
24
Analysis of binaural speech

• We can then plot the syllable onsets as a
  function of frequency and time, and count them.

Reverberation forward Reverberation
backwards
Note many syllables are detected (30)
Notice hardly ANY are detected (2)
RASTI will give an
identical value for both cases!!
25
How do we perceive distance and space?

• Reflected energy interferes with itself at the
  listeners ears, producing fluctuations in the
  sound pressure.
• We perceive fluctuations in level during a sound
  event and up to 150ms after the end of the sound
  as a sense of distance from the sound source.
• If the reflections are spatially diffuse (from
  all directions) the fluctuations will be
  different in each ear.
• Fluctuations that occur during the sound event
  and within 50ms after the end of the event
  produce both a sense of distance and the
  perception of a space around the source.
• This is Early Spatial Impression (ESI)
• The listener is outside the space and the sound
  is not enveloping
• But the sense of distance is natural and
  pleasant.
• Spatially diffuse reflections later than 50ms
  after the direct sound produce a sense of space
  around the listener.
• This can be perceived as envelopment. (Umgebung)

26
The downside of Distance Perception

• Reflections during the sound event and up to
  150ms after it ends create the perception of
  distance
• But there is a price to pay
• Reflections from 10-50ms do not impair
  intelligibility.
• The fluctuations they produce are perceived as an
  acoustic halo or airaround the original sound
  stream. (ESI)
• Reflections from 50-150ms contribute to the
  perception of distance but they degrade both
  timbre and intelligibility, producing the
  perception of sonic MUD. (Mulmig,Glauque)
• The addition of mud to a speech or singing voice
  has serious dramatic consequences

27
Distance and Drama Copenhagen New Stage
We were asked to improve speech intelligibility
in this theater, specifically for drama. Using
some extraordinary technology we succeeded. But
we also increased the sense of sonic
distance. The theater directors decided to fix
the intelligibility problems by improving the
diction of the actors. We completely agreed!
28
Example of reflections in the 50-150ms range
Balloon burst in the New Bolshoi. Source was on
the forestage, and the receiver was in the
stalls at row 10. Note the HUGE burst of energy
about 50ms after the direct sound. The 1000Hz
0ctave band shows the combined reflections to be
6dB stronger than the direct sound. The sound
clip shows the result of this impulse response on
speech.
The result (in this case) is a decrease in
intelligibility and an increase in distance
29
Human Perception the background sound stream

• We perceive the background sound stream in the
  spaces between the individual sound.
• The background stream is perceived as continuous,
  even though it may be rapidly fluctuating.
• When masking by foreground sounds is low the
  background stream is perceived at an absolute
  level, not as a ratio to the foreground sound.
• This is why playing a recording at a higher level
  cause the perceived amount of reverberation to
  increase.
• Perception of the background stream is inhibited
  for 50ms after the end of a sound event, and
  reaches full sensitivity only after 150ms.

30
Example of foreground/background perception (as a
cooledit mix)
Series of tone bursts (with a slight vibrato)
increasing in level by 6dB Reverberation at
constant level Mix with direct increasing 6dB
Result backgound tone seems continuous and at
constant level
31
Example of background loudness as a function of
Reverberation Time
Tone bursts at constant level, mixed
with reverberation switching from 0.7s RT to 2.0s
RT, and reducing in level 8dB Output perceived
background is constant! (But the first half is
perceived as farther away!)
Note the reverb level in the mix is the same at
150ms and greater. One gets the same results
with speech.
32
Summary Perceptions relating to stream
separation

• First is the creation of the foreground stream
  itself. The major perception is intelligibility
• Second is the formation of the background sound
  stream from sounds which occur mostly 150ms after
  the direct sound ends. The perception is
  reverberance
• Third is the perception of Early Spatial
  Impression (ESI) from reflections arriving
  between 10-15ms and 50ms after the end of the
  direct sound. The perception is of distance and
  acoustic space around the source.
• Fourth is the timbre alteration and reduction of
  intelligibility due to reflections from 50 to
  150ms after the end of the direct sound event.
  The perception is MUD and distance.
• Human hearing has been designed to suppress the
  perception of ESI and of mud. As long as
  intelligibility is more or less satisfactory,
  after an adaptation period we no longer hear
  these properties of the room.
• And we usually can not remember them.
• This does NOT mean they are dramatically or
  artistically unimportant!

33
Synthetic Opera House Study
Dresden
Berlin

• We can use MC12 Logic 7 to separate the orchestra
  from the singers on commercial recordings, and
  test different theories of balance and
  reverberation.
• From Elektra Barenboim. Balanceand reverb in
  original is OK by Barenboim.

Original Mix Vocals Downmix with reverb on
the orchestra, but not on the singers Reverb from
orchestra Reverb from singers Downmix with
reverb on the singers. Note the result is MUDDY!
34
Localization

• Localization is related to stream formation. It
  depends strongly on the onset of sound events.
• IF the rise-time of the sound event is more rapid
  than the rise-time of the reverberation
• then during the rise time the IID (Interaural
  Intensity Difference) and the ITD (Interaural
  Time Difference) are unaffected by reflections.
• We can detect the direction of the sound source
  during this brief interval.
• Once detected, the brain HOLDS the detected
  direction during the reverberant part of the
  sound.
• And gives up the assigned direction very
  reluctantly.
• The conversion between IID and ITD and the
  perceived direction is simple in natural hearing,
  but complex (and unnatural) when sound is panned
  between two loudspeakers.
• Sound panning only works because localization
  detection is both robust and resistant to change.
• A sound panned between two loudspeakers is
  profoundly unnatural.

35
Detection of lateral direction through Interaural
Cross Correlation (IACC)
Start with binaurally recorded speech from an
opera house, approximately 10 meters from the
live source. We can decompose the waveform into
1/3 octave bands and look at level and IACC as a
function of frequency and time.
Level ( x time in ms y1/3 octave bands
640Hz to 4kHz) IACC Notice that there is NO
information in the IACC below 1000Hz!
36
Position determination by IACC
We can make a histogram of the time offset
between the ears during periods of high IACC. For
the segment of natural speech in the previous
slide, it is clear that localization is possible
but somewhat difficult.
37
Position determination by IACC (continued)
Level displayed in 1/3 octave bands (640Hz to
4kHz) IACC in 1/3 octave bands
We can duplicate the sound of the previous
example by adding reverberation to dry speech,
and giving it a 5 sample time offset to localize
it to the right. As can be seen in the picture,
the direct sound is stronger in the simulation
than in the original, and the IACCs - plotted as
10log10(1-(1/IACC)) - are stronger.
38
Position determination by IACC (continued)
Histogram of the time offset in samples for each
of the IACC peaks detected, using the
synthetically constructed speech signal in slide
2.
Not surprisingly, due to the higher direct sound
level and the artificially stable source the
lateral direction of the synthetic example is
extremely clear and sharply defined.
39
Summary so far

• Rank ordering opera houses or concert halls
  through the memory of conductors is probably not
  very useful.
• When the sounds of a house can be compared
  rapidly (through electronic enhancement or
  recording) there is almost unanimous agreement on
  the best sound, and this sound is highly
  articulate.
• The conductor will insist on some low-frequency
  envelopment on the orchestra, as long as vocal
  clarity is not compromised.
• Considerable experimentation has found that there
  is an ideal reverberation profile for opera
  performances.
• This profile is based on the physiological
  properties of human hearing
• And is thus the same profile as we need on a good
  recording.

40
The Ideal Reverberation above 1000Hz.

• The ideal profile has three distinct slopes.
• 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.

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 !
41
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. If the direct sound
is weak, there will be too much energy between 50
and 150ms, and the sound will be MUDDY.
42
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 in a hall with natural acoustics the
  reverberation time and reverberation level should
  increase below 500Hz.

43
Theatro Alla Scala, Milan
Echograms from LaScala. (From Beranek)
illustrate these profiles Top curve - 2kHz
octave band, 0-200ms At 2kHz note the high direct
sound and low level of reflections in the
50-150ms time range. Bottom curve - 500Hz octave
band 0-200ms Note the high reverberation level
and short critical distance.
44
Lets listen to Alla Scala!

• Matlab can be used to read these printed impulse
  respones and convert them into real impulse
  responses.
• 1. First we read the .bmp file from a scan, and
  convert the peaks in the file to delta functions
  with identical time delay, and an amplitude
  equivalent to the peak height.
• All the direct sound energy is combined into a
  single delta function, and the level of the
  direct sound is normalized (relative to the rest
  of the decay), so the 2kHz and 500kHz impulses
  can be accurately combined.
• 2. We then apply a random variable - 5ms to the
  delay time to correct for the quantization in the
  scan.
• 3. We then extend the echogram to higher times by
  tacking on an exponentially decaying segment of
  white noise, with a decay rate equal to the
  published data for the hall.
• 4. We then filter the result for the 2kHz
  echogram with a 1k high-pass filter, and combine
  it with the 500Hz echogram low-pass filtered at
  1kHz.
• 5. If desired we can create a right channel and
  a left channel reverberation by using a
  different set of random variables in steps 2 and
  3.
• 6. We convolve a segment of dry sound with the
  new
• The result is sonically quite convincing!

45
Alla Scala at 500Hz reading the plot
Top curve 500Hz measured impulse response as
given by Beranek. JASA Vol. 107 1, Jan 2000, pp
356-367 Bottom curve impulse response as
regenerated from delta functions, passed through
a 2kHz 6th order 1 octave filter. Note the
correspondence is more than plausable.
46
Alla Scala 500Hz randomizing and extending
Top graph Alla Scala published data Bottom
graph regenerated impulse response after
randomization and extention.
47
Listen to Alla Scala, NNT Tokyo, Semperoper
2kHz
500Hz
2kHz and 500Hz Impulse responses from Scala
Milan NNT Theater Tokyo Semper Oper
Dresden (All data from Beranek)
Original Sound
48
How can we make a room ideal for opera?

• A conventional opera house can be made to
  approach the sonic ideal by MAXIMIZING the reverb
  radius for the soloists, for frequencies above
  700Hz.
• This involves arranging the audience and
  reflectors around the stage to direct the sound
  of the singers directly into the audience.
• These architectural features increase the very
  early energy while decreasing the sound power
  available to the middle and late reverberation.
• At the same time, we should try to maximize the
  reverberation time below 500Hz.
• To some degree, the success of a design can be
  seen immediately in a picture taken from the
  stage.
• We need only notice how much absorption we see in
  front of us. The more absorption and less bare
  wall we see, the higher the clarity.

49
Pictures from the stage
Deutsche Oper might as well tear it down.
New Bolshoi just add curtains on the back wall.
Deutsche Staatsoper vital, exciting, and alive
with or without the LARES.
50
Compromises

• The fight between those who like clarity and
  those who like reverberance is relatively recent.
• Reveberance currently has the upper hand.
• One of the purposes of this talk is to suggest
  that the emphasis on reverberance is misguided.
• In every case where the author has worked closely
  with a music director, the director has wanted a
  more reverberant sound. like the Semperoper
• However, when given the opportunity to hear what
  Semperoper reverberation actually sounds like,
  the director invariably prefers a much less
  reverberant sound.
• In fact, it is my observation that the difference
  between the reverberance the conductor wants, and
  the natural reveberance of a dry opera house is
  extremely subtle.
• In a controlled test at the Royal Theater in
  Copenhagen (set up by Anders Gade) 80 of the
  test subjects could hear no difference at all.
• In every case where we have had the opportunity
  to increase clarity, or improve the balance
  between the singers and the orchestra, the
  improvement has been noticed immediately, and
  appreciated, by everyone, including the conductor.

51
Ideal sound through electronics

• Electronic enhancement has the potential to
  create ideal opera acoustics
• But only if the system is capable of creating a
  triple-slope decay at high frequencies, and a
  single-slope decay at low frequencies.
• This combination is not common with currently
  available systems!

52
Acoustic Feedback bane or boon?

• All enhancement systems have significant feedback
  between the loudspeakers and the microphones.
• A single slope decay with an RT of 1.7 seconds
  MUST create a reverberation radius which is
  relatively small usually under four meters in a
  typical opera house.
• If the pickup microphones are separated from each
  sound source by more than this distance, they
  MUST pick up more reverberation than direct
  sound.
• Current enhancement systems divide into two
  types
• Those that utilize the acoustic feedback to
  increase the reverberation time directly.
• Philips MCR
• Carmen
• And those that include a reverberation device in
  the electronics, and couple this device
  electronically to the hall.
• Lares
• Paoletti (Stagetec)
• ACS, SIAP
• Only the second type are capable of creating a
  dual or triple-slope decay

53
Feedback and coloration

• Any time there is significant acoustic feedback
  there will be coloration.
• Acoustic feedback paths have complex frequency
  response, and this response is audible.
• This coloration must be minimized in a successful
  design.
• There are no easy solutions. Almost all systems
  start with a multichannel design.
• With many channels the individual response
  variations in each channel tend to average out.
• But each channel must have its own microphone and
  speaker, and all devices must be separated
  physically by the reverberation radius.
• This physical separation is tricky to realize in
  practice.
• Alas, most available systems minimize the amount
  of coloration by minimizing the system gain.
• Most available systems are not capable of doing
  very much at all.
• This is sometimes an advantage, as Eckhard will
  tell.
• Some available systems minimize the coloration by
  denying that feedback exists (ACS, and to some
  degree SIAP)

54
Lares System

• Lares uses a multichannel concept
• But it uses an electronic trick to allow a single
  pair of microphones to drive a large number of
  output channels (typically four or eight)
• As a result it becomes practical to place the
  microphones close to the performers.
• The result is a cleaner pickup. The pickup
  microphones contain less coloration and
  reverberation.
• The energy content in the 50 to 150ms time range
  can be minimized this way (and only this way).

55
Lares Block Diagram
A typical Lares installation includes two pickup
microphones and eight separate output
channels. Each microphone is connected to each
output channel through a separate, independently
time varying reverberation device. The frequency
dependence of the reverberant level, and the
frequency dependence of the reverberation time
can be separately adjusted. Lares also includes a
noise generator and 1/3 octave analyzer for
setting and verifying the overall system gain.
56
Lares is highly resistant to coloration

• This is achieved through the multichannel design,
  and the independent time variance.
• The type of time variance used minimizes the
  pitch-shift, which is not audible when the system
  is correctly adjusted.
• As a result a high reverberant level can be
  achieved, even when the pickup microphones are
  far from the sound sources.
• And this is sometimes a problem. Customers turn
  the system up too high, or insist on placing the
  microphones too far away.
• The result can be both muddiness and excessive
  coloration (at least to my ears.)
• There are way too many existing Lares
  installations that have these problems!

57
Demonstrations of Lares
58
Exponential Decay

• Sabines breakthrough
• Extensively studied by Morse, Beranek, Eyring,
  etc.
• In rooms where the absorption is relatively
  uniformly distributed the decay of sound follows
  a straight line when plotted logarithmically.
• When the decay is exponential we can precisely
  predict the ratio between the direct sound and
  the reflected sound in the 50-150ms time range.
• For computing sonic distance the direct sound may
  be augmented by reflections that arrive before
  50ms.
• At very short reverberation times the reflected
  energy is concentrated into times less than 50ms
  after the direct sound, and perceived distance is
  low, regardless of the direct/reflected ratio.
• Moderate reverberation times (1.2 1.6 seconds)
  concentrate the energy between 50 and 150ms.
  Halls with these reverberation times can easily
  sound muddy. (mulmig or glauque)

59
Acoustic research through synthesis

• We do not need to use reflections to generate the
  perception of acoustics!
• It is the total reflected energy in different
  time bands that matters, along with the spatial
  and frequency distribution of that energy.
• We can synthesize reverberation by convolving an
  input signal with an impulse response sculpted
  from noise.
• This technique allows to investigate the effects
  of different energy profiles.
• I decided to convolve four identically shaped
  noise bursts, each 46ms long, with a segment of
  the Rakes Progress.
• These segments can be then strung together with
  different delays and amplitudes to form an
  arbitrary reverberation.
• For example, lets synthesize an exponential
  decay of 1.4 seconds RT, with a variable
  direct/reverberant ratio

60
Synthetic impulse response
linear amplitude scale
log amplitude scale Synthetic impulse
response from noise 1.4s exponential decay This
is the sound of a one sample click at 22050
samples/sec. This is NOT music or speech.
61
Window averaging, direct/reverb 0dB
25ms averaging window
100ms averaging
window We can average the impulse response over a
selected time period. Mathematically this is the
same as the average response of the system to an
input signal (phone or note) with a duration of
the averaging period. The first window
represents the response of the room to a 25ms
sound, and the second to a 100ms sound. Note the
EDT we perceive is HIGHLY dependent on the length
of the note!
62
Schroeder Integration, direct/reverb 0dB
Schroeder Integration reverse integration
represents the response of the room to a note of
infinite duration. Jordans method of determining
EDT takes some account of the strength of the
direct sound. Schroeders method for EDT
completely ignores the strength of the direct
sound. Neither method is likely to predict the
response of the room to speech or normal music.
63
Window Averaging, direct/reverb -3dB
25ms Averaging Window 100ms Averaging
Window For a 25ms sound the effective
reverberation time is 0.9 seconds, so at least
these sounds are heard with high articulation.
100ms sounds on the other hand, are smoothed to
nearly the same slope as the late reverberation
time
64
Schroeder Integration, direct/reverb -3dB
Very long notes still show some dual-slope decay.
Jordans method for EDT is sensitive to this
difference, Schroeders is not.
65
Examples

• See surround encoded DTS exponential decay

66
Non-exponential decay direct/reverb -3dB
It is interesting to ask what happens when there
is a high burst of very early reflections,
followed by a relatively level energy curve out
to beyond 160ms. This type of decay minimizes
sonic distance, while maintaining reverberance
and envelopment
67
Non-exponential decay direct/reverb -3dB
amplitudes of the different time periods in
dB all dB values correspond to the energy
content of the mix d1 -1.7 direct sound l1
-1.7 20ms-60ms l2 -8.5 60ms-100ms l3
-8.5 100ms-140ms l4 -8.5 140ms-180ms l5
-8.5 180ms-220ms l6 -10.2 220ms-260ms l7
-11.9 260ms-300ms l8 -13.6 300ms-340ms
This is the MATLAB code that sets up the
non-linear reverberation. Note that for this
example, the early reflections have equal energy
to the direct sound. Sonically, it is much better
if the early energy is 4dB to 6dB relative to
direct.
68
Non-exponential decay direct/reverb -3dB
25 ms averaging window 100ms averaging
window With this non-linear decay both 25ms
sounds and 100ms sounds are perceived with high
articulation. Longer notes and sounds also have
high reverberance. Once again, it would be
sonically more pleasant if the early reflections
were reduced.
69
Examples

• See surround encoded DTS non-linear decay

70
Frequency Dependence

• We have so far been studying broadband
  reverberation.
• However human perception is highly frequency
  dependent.
• As a consequence, our perceptions of
  intelligibility, articulation, loudness, and
  sonic distance are primarily influenced by
  frequencies above 700Hz.
• However the perception of reverberance, warmth,
  and envelopment primarily arise from frequencies
  below 500Hz.
• It is possible to have both high clarity and high
  envelopment at the same time by carefully
  controlling the frequency dependence of the
  reflected energy.

71
The frequency transmission of the pinnae and
middle ear
From B. C. J. Moore, B. R. Glasberg and T.
Baer, A model for the prediction of thresholds,
loudness and partial loudness, J. Audio Eng.
Soc., vol. 45, pp. 224-240 (1997).
The intensity of nerve firings is concentrated in
the frequency range of human speech signals,
about 700Hz to 4kHz. With a broad-band source,
the ITD and IID at these frequencies will
dominate the apparent direction.
72
Boston Symphony Hall, occupied, stage to front of
balcony, 1000Hz
73
Boston Symphony Hall, occupied, stage to front of
balcony, 250Hz
74
Adelade - Festival Center Theater
75
Conclusions

• There is an ideal acoustic profile for opera
  performance.
• This profile may or may not be achievable through
  conventional acoustics.
• Our goal is not ideal acoustics, it is ideal
  SOUND.
• When restricting the design to conventional
  acoustics, the optimal sound as determined by a
  rapid A/B test is less reverberant than most
  conductors think they want in the absence of an
  A/B test, at least above 700Hz.
• An optimal design will maximize the reverb radius
  above 700Hz, aiming for a strongly dual-slope
  decay as measured by the decay time to 6dB of a
  50ms to 100ms sound.
• This goal is best achieved by directing the
  direct sound (and first reflections) from the
  soloists into the audience.
• The optimal design will maximize the
  reverberation time and the reverberant level
  below 500Hz.
• Given the choice between high clarity and a
  compromise that reduces clarity somewhat in favor
  of more reverberance for the orchestra, CHOOSE
  CLARITY!