Page 29 - Winter 2008

Table 1 Direct-to-Reverberant Sound Levels for Speech Intelligibility in Rooms
ues. This calculation confirmed the viability of the loud- speaker layout.
Intelligibility—Arrival times
Many have argued that not all the direct field sound energy is good and not all the reverberant field energy is bad. This has led to intelligibility metrics that take the signal arrival times into account. In these metrics all sound arriving before a given time, after the first sound, is good, and all sound arriving after that time is bad. Not infrequently the first arrival is not the loudest sound especially when there are multiple sources: a talker on stage, a point source in the face of the stage, a central cluster, and perhaps a delayed distrib- uted system plus reflections. Consequently some judgment must be exercised in selecting the cutoff time.
As we can see, there are at least two ways of parsing the signal-to-noise: (1) by level, with the lower level contribu- tions having less influence, and (2) by time, with later arrivals counted as noise. The second approach, using metrics that integrate (add up) the energy arriving at a receiver over time, requires a great deal more calculation than a static model. It is not clear that the added information is worth the extra cal- culation. Thus I prefer the first method.
To summarize we have a choice of ways of controlling intelligibility. We can raise the direct signal level at a receiver by using high directivity loudspeakers or we can use many distributed loudspeakers placed close to the audience. We can also decrease the reverberant field level by lowering the level produced by individual (distributed) loudspeakers or by using directional loudspeakers that contribute lower levels of sound power to the reverberant field. Finally we can add absorption to the receiving space to decrease the reverberant field as well as the reverberation time.
Perception of direction
The perception of direction is another important design consideration. It is important to maintain the illusion that the amplified sound is coming from the original source. This is accomplished by using the human reaction to the first arrival sound, which determines the perceived direction, even when the later arriving sound is louder. The phenomenon is dependent on the relative level and time delay. Normal
design practice is to control the time of the second arrival so that it occurs about 5 milliseconds after the first. This pre- serves the directional cue obtained from the first arrival.
In a cluster design we do this by locating the loudspeak- ers above the source, taking advantage of the fact that our perception of direction is less sensitive in the vertical plane than in the horizontal plane. We can also add localizing loud- speakers near the sound source, and delay the signals fed to the other loudspeakers. The illusion can be preserved even though the delayed loudspeakers may be contributing more to the overall level at a given receiver.
In small rooms such as the chapel, the talker can be its own localizing source. In larger rooms point sources must be added in the face of the stage in auditoria or in the face of steps in worship spaces. They are relatively easy to disguise in these locations. Localizing loudspeakers do not have to reproduce the full bandwidth of sound. Step mounted trans- ducers are limited to 6-inch diameter cones in a 7-inch-high step face. Delays can be fixed by calculating the propagation time and adding 5 milliseconds. After installation they can be adjusted by measurement or by ear. It is not difficult to set delay times by ear simply by listening for changes in per- ceived source direction. In fact, differences of one or two mil- liseconds are quite perceptible.
Another important consideration in the design of delays is to provide a smooth transition as the listener moves away from the source. The loudspeakers covering a given area must not only provide even level coverage but also consistent source imaging. Often distributed overhead loudspeakers are
Fig. 4. Elevation showing loudspeakers and wall panels.
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