Page 24 - Winter 2011

Fig. 2. The relative rate of nerve firings from the direct sound and the build-up of reverberation. in the frequency range of 1000Hz to 4000Hz in unoccupied Boston Symphony Hall, row R, seat 11, with a source at the podium.
almost every seat in a shoebox hall the opposite is true, and for seats more than half-way back the early reflections from any direction disturb our ability to hear the music. Vienna’s renowned Grosser Musikvereinssaal nearly falls into this cate- gory. If you luckily get a ticket in the front half the sound is fan- tastic, but more than half-way back the sound is just loud. You are better off in the standing room. Shielded by the balcony from most of the reflections the sound is clear, well balanced, and beautiful. (But get there early. You need to be in the front of the crowd.) Unfortunately most modern hall designs direct first order reflections down into the audience, particularly from the side walls and balcony fronts. The result is disastrous. In far too many seats the direct sound is not detectable. These first-order reflections are then absorbed—and their energy cannot contribute to desirable late reverberation.
If we measure the impulse response of BSH when the stage and hall are fully occupied we find that although the reverberation time varies very little with frequency, the strength of the early reflections above 1000Hz in the rear half of the hall is weaker than it is at lower frequencies. The front of the first balcony is especially blessed by this lower level of early reflections. The reason the first balcony succeeds is clear from the geometry of the hall. This position does not receive strong early reflections from any direction. The underside of the first balcony along the side walls directs all the specular lateral reflections that would otherwise travel to the first balcony down into the seats on the floor. This explains why listeners beyond row Z have difficulty localiz- ing. The balcony fronts are transparent to sound, and have absorbing legs of audience behind them. The strong early reflection that normally comes from the ceiling is deflected back to the orchestra by the many coffers that decorate the ceiling. The coffers in BSH have just the dimensions needed to act as retro-reflectors for frequencies above 1000Hz, while allowing lower frequencies to reflect specularly. The absence of strong early reflections in the front of the first balcony gives the brain stem time to separate the direct sound. The result is high clarity and rich reverberation.
Most shoebox halls fail to provide the clarity and rever- beration of BSH because the early reflections in the rear of the hall are too strong, and come too soon. Seats in the front of such halls are not problematic, as the direct sound is strong, the early reflections are relatively weak, and they have a longer delay relative to the direct sound. Knowledgeable people and critics sit there. As you move back in the hall the direct sound is weaker, the first-order reflections are stronger, and they come sooner. At some critical distance localization becomes impossible, and the instruments blend into a circular blob of sound. Our experiments and binaural recordings show that the boundary between the two types of sound is often only one or two meters wide. Seats in front of this critical distance give wonderful, and nearly identical, sound. Beyond this critical distance for localization the sound is muddy and blended. In countless halls most of the seats have this kind of sound.
There is a simple graphic that lets you see how the ear is hearing the beginning of a sound event. Let’s assume we have a sound source that suddenly turns on and then holds a con-
stant level. Initially only the direct sound stimulates the basi- lar membrane. Soon the first reflection joins it, and then the next, etc. The nerve firing rate from the combination of sounds is approximately proportional to the logarithm of the total sound pressure. But instead of plotting the total rate of nerve firings we plot the rate of nerve firings from the direct sound and the reflections separately. In the following graphs the vertical axis is labeled “rate of nerve firings”, normalized such that the rate is 20 units for the sum of both rates once the reverberation is fully built-up. The scale is chosen such that the value of the rate can be interpreted as proportional to the decibels of sound pressure. Thus in Fig. 2, the rate for the direct sound is about 13, implying that the total sound pres- sure will eventually be 7dB stronger than the direct sound. Figure 2 shows the relative rate of nerve firings from the direct sound and the build-up of reverberation in the frequency range of 1000Hz to 4000Hz in unoccupied Boston Symphony Hall (BSH) row R, seat 11, with a source at the podium. The dashed line shows the rate of nerve firings for a sound of con- stant level that begins at time zero. The solid line shows the firing rate due to the reverberation as it builds up with time. The dotted line marks the combined final firing rate for a con- tinuous excitation, and the 100 ms length of the time window the brain stem uses to detect the direct sound. In this seat the direct sound is strong enough that the ratio of the area in the window under the direct sound (the total number of nerve fir- ings from the direct sound in this window) to the area in the window under the build-up of the reflections is 5.5dB. This implies excellent localization and clarity. Shown in Fig. 3 are the nerve firing rates for the direct sound and the build-up of reflections in unoccupied BSH, row DD, seat 11. Notice the direct sound is weaker than in row R, and there is a strong high-level reflection at about 17ms that causes the reflected energy to build up quickly. The ratio of the areas (the total number of nerve firings) for the direct sound in the first 100ms to the area under the line showing the build-up of the reflections is 1.5dB. Localization is poor in this seat.
Figure 4 shows the graph for the front of the first balcony.
20 Acoustics Today, January 2011