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.
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.
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?
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)
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.
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.
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.
(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 !)
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.
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.
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.