[Part 1 of this article examines optimal speaker placement, IEC rooms, room energy evolution, LEDE rooms, non-environment rooms, and diffuse reflection rooms. Part 2 looks at acoustic and psychoacoustic principles applied to the clinical measurement of hearing ability, as well as procedures used in psychoacoustic testing. Part 3 looks at filter shapes and time domain effects, as well as equalization and tone controls.]
7.5 PUBLIC ADDRESS SYSTEMS
Sound reinforcement of speech is often taken for granted. However, as anyone who has tried to understand an announcement in a reverberant and noisy railway station knows, obtaining clear and intelligible speech reinforcement in a real acoustic environment is often difficult. The purpose of this section is to review the nature of the speech reinforcement problem from its fundamentals in order to clarify the true nature of the problem.
We will examine the problem from the perspective of the sound source, the listener, and the acoustics. At the end you should have a clear appreciation of the difficulties inherent in reinforcing one of our most important, and sensitive, methods of communication.
There are several aspects of an acoustic space that affect the intelligibility of speech within it.
As discussed in Chapter 6 (see Section 6.1.12), bigger spaces tend to have longer reverberation times and well-furnished spaces tend to have shorter reverberation times. Reverberation time can vary from about 0.2 of a second for a small well-furnished living room to about 10 seconds for a large glass and stone cathedral.
There are two main aspects of the sound to consider:
- The direct sound: This is the sound that carries information and articulation. For speech it is important that the listener receive a large amount of direct sound if they are to comprehend the words easily. Unfortunately, as discussed in Chapter 1, the direct sound gets weaker as it spreads out from the source. Every time you double your distance from a sound source the level of the direct sound goes down by a factor of four, that is, an inverse square law. Thus the further away you are from a sound source, the weaker the direct sound component.
- The reverberant sound: The second main aspect of the sound is the reverberant part. This behaves differently to the direct sound, as discussed in Chapter 6. The reverberant sound is the same in all parts of the space.
The effect of these two aspects is shown in Figure 7.28. As one moves away from a source of sound in a space, the level of direct sound reduces but the reverberant sound stays constant. This means that ratio of direct sound to reverberant sound becomes less and so the reverberant sound becomes more dominant.
FIGURE 7.28 Regions of dominance for direct and reverberant sound.
The critical distance, where the reverberant sound dominates, is dependent on both the absorption of the space and the directivity of the source. As the absorption and directivity increase so does the critical distance, but only proportionally to the square root of these factors. As discussed in Chapter 6 the critical distance is:
Dcrit = 0.141√RQ (7.3)
where R = Sα / (1 - α) the room constant (in m2)
and Q = the directivity of the source (compared to a sphere)