Speech Clarity in a Reverberant Space

Sound systems make it possible to understand speech in situations where it would otherwise be impossible. On the other hand, when it is possible to understand speech without a sound system, a sound system can be a serious impediment to communication.

Sound systems are particularly useful in highly reverberant spaces like churches. Perhaps you have been in an old, high stone church where the sound bounces around forever. Organists and choir directors love these churches. They are wonderful to sing in. Without a sound system, however, it is almost impossible to understand speech at a distance.

I should say without a properly designed sound system. A poorly designed system---there are more of these than properly designed systems---can make it even more difficult to understand speech. This is because a poorly designed sound system merely amplifies the speaking voice. The voice is louder, and so is the reverberation that covers it up. There is really no need for additional volume in a highly reverberant space; the reverberation itself amplifies sound quite enough.

The key is to boost the voice over the reverberation, without simultaneously increasing the reverberation. To illustrate how this is done, let's do a mental experiment. Imagine that you are with a friend in a highly reverberant cathedral. If you stand close enough to your friend, you can easily converse. If you want to increase the distance at which you can understand each other, cup your hands to create a megaphone. This will increase the directivity of your voice, increasing the distance at which you can be clearly understood. Note that if you simply shout it doesn't help at all.

Similarly, there are two ways that a sound system provides speech intelligibility in a reverberant space:

1. bring loudspeakers close to the listeners, or
2. "beam" sound at the listeners with directional loudspeakers.

Bringing loudspeakers close to the listeners requires more loudspeakers; otherwise bringing a loudspeaker closer to one listener takes it farther from another. This approach is called a distributed system.

Directional loudspeakers need to be large, and have a clear shot at the congregation. Highly directional loudspeakers can be enormous, and they are not exactly inspiring to look at. A new development in directional loudspeakers, called a line array, is large only in one direction. They are large thin columns. This posses less of an aesthetic problem, but they are still extremely expensive and tricky to design and install properly.

A good sound system comes as close as one gets in life to having your cake and eating it too: clarity within a reverberant space is the holy grail of acoustics. It is impossible, however, to completely avoid difficult choices.

Here are those choices:

• A distributed system is easier on the eyes. But because of the large number of loudspeakers, this approach is usually more expensive. A distributed system requires a signal delay system to give the impression that the talker is located in the front, rather than in the nearest loudspeaker.

• An extreme example of the distributed system is to install the loudspeakers in the pews. This is an excellent approach in an enormous hard volume. However, installation is very labor intensive.

• A central cluster is usually the least expensive, but a large loudspeaker cluster directly over the chancel can be a real eyesore. Furthermore, some people do not like the sound quality of highly directional loudspeakers.

• The line array has provided excellent speech intelligibility in highly reverberant spaces such as Philadelphia's 30th Street Station (with a 9-second reverberation time). However, it is very expensive, tricky to implement properly, and sound quality is less than ideal. Furthermore, they are not appropriate for all spaces.

The sound system is rarely considered until late in the design. However, early consideration can open up approaches that might not be possible later. For instance, a large central cluster can be integrated into the visual aesthetics of the room. This may open up the possibility of a cost- effective approach to providing speech clarity in your reverberant sanctuary.

Amplifyed music requires a more expensive system than a pure speech system because it requires a full-frequency system.

Considering acoustics from the very beginning of design, including the approach to amplification will allow you to make much more effective use of your tight budget.

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