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) 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. 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 !)

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 above 1000Hz is weak, there will be too much energy between 50 and 150ms, and the sound will be MUDDY. But – this type of decay may be ideal at 500Hz and below.

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