So far in this book we have considered acoustics and psychoacoustics as separate topics. However, real applications often require the combination of the two because although the psychoacoustics tells us how we might perceive the sound, we need the acoustic description of sound to help create physical, or electronic, solutions to the problem.
The purpose of this chapter is to give the reader a flavor of the many applications that make use of acoustics and psychoacoustics in combination. Of necessity these vignettes are brief and do not cover all the possible applications. However, we have tried to cover areas that we feel are important, and of interest.
The level of detail also varies but, in all cases, we have tried to provide enough detail for the reader to be able to read, and understand, the more advanced texts and references that we provide, and any that the reader may discover themselves, for further reading. The rest of this chapter will cover listening room design, audiometry, psychoacoustic testing, filtering and equalization, public address systems, noise reducing headphones, acoustical social control devices, and last, but by no means least, audio coding systems.
7.1 CRITICAL LISTENING ROOM DESIGN
Although designing rooms for music performance is important, we often listen to recorded sound in small spaces. We listen to music, and watch television and movies, in both stereo and surround, in rooms that are much smaller that the recording environments. If one wishes to evaluate the sound in these environments then it is necessary to make them suitable for this purpose.
In Chapter 6 we have seen how to analyze existing rooms and predict their performance. We have also examined methods for improving their acoustic characteristics. However, is there anything else we can do to make rooms better for the purpose of critically listening to music? There are a variety of approaches to achieving this and this section examines: optimal speaker placement, IEC rooms, room energy evolution, LEDE rooms, non-environment rooms, and diffuse reflection rooms.
7.1.1 Loudspeaker arrangements for critical listening
Before we examine specific room designs, let us first examine the optimum speaker layouts for both stereo and 5.1 surround systems. The reason for doing this is that most modern room designs for critical listening need to know where the speakers will be in order to be designed. It is also pretty pointless having a wonderful room if the speakers are not in an optimum arrangement.
Figure 7.1 shows the optimum layout for stereo speakers. They should form an equilateral triangle with the center of the listening position. If one has a greater angle than this the center phantom image becomes unstable — the so-called "hole in the middle" effect. Clearly, having an angle of less than 60º results in a narrower stereo image.
FIGURE 7.1 Optimal stereo speaker layout.
5.1 surround systems are used in film and video presentations. Here the objective is to provide both clear dialog and stereo music and sound effects, as well as a sense of ambience. The typical speaker layout is shown in Figure 7.2.
FIGURE 7.2 Typical speaker layout for 5.1 surround.
Here, in addition to the conventional stereo speakers there are some additional ones to provide the additional requirements. These are as follows:
- Center dialog speaker: The dialog is replayed via a central speaker because this has been found to give better speech intelligibility over a stereo presentation. Interestingly the fact that the speech is not in stereo is not noticeable because the visual cue dominates so that we hear the sound coming from the person speaking on the screen even if their sound is coming from a different direction.
- Surround speakers: The ambient sounds, and sound effects, are diffused via rear mounted speakers. However they are, in the main, not supposed to provide directional effects and so are often deliberately designed, and fed signals, to minimize their correlation with each other and the front speakers. The effect of this is to fool the hearing system into perceiving the sound as all around with no specific direction.
- Low-frequency effects: This is required because many of the sound effects used in film and video, such as explosions and punches, have substantial low-frequency and subsonic content. Thus, a specialized speaker is needed to reproduce these sounds properly. Note: this speaker was never intended to reproduce music signals, notwithstanding their presence in many surround music systems.
More recently systems using six or more channels have also been proposed and implemented; for more information see Rumsey (2001).
As we shall see later the physical arrangement of loudspeakers can significantly affect the listening room design.