By now it is absolutely obvious that in sound reproduction the room and the loudspeaker are inseparable; they operate as a system. Yet, loudspeakers must be designed as separate devices, so it is necessary to develop measurements and interpretations for those measurements that allow us to anticipate how the loudspeakers are likely to sound in normally reflective rooms.
Chapters 12 and 13 demonstrated several ways in which the room imposes its considerable will at low frequencies. Still, woofers and subwoofers must be measured, just to give us a starting point. The wavelengths at the frequencies of interest are long compared to the dimensions of conventional box loudspeakers and subwoofers, so they can be considered to be approximately omnidirectional radiators. This means that a single curve can describe the frequency response in the range below about 100 Hz.
The fact that the room resonances and standing waves wreak havoc with the sounds arriving at different listening locations is a separate challenge, but Chapter 13 shows that it can be managed. However, this can only be done after the system is set up in the room because all rooms are different, and the locations of subwoofers and listeners are crucially important.
At higher frequencies, we must be concerned with the nature of sounds radiating in different directions from the loudspeaker because they constitute the direct and reflected sounds arriving at listeners' ears. This means that many measurements must be made, and a system for organizing, processing, and displaying the data must be developed to allow it to be usefully interpreted. No single curve will be sufficient to describe the complex interface between the loudspeaker and all of the reflecting surfaces in a room.
18.1 TWO SIMPLE SOURCE CONFIGURATIONS
At the end of Chapter 16, the topic of propagation loss was introduced as a significant factor in what is heard in multichannel audio systems in small rooms. It was suggested that anything that could be done to deliver more uniform sound levels as a function of distance from the loudspeakers would be beneficial. It is convenient, therefore, to start this chapter with a description of the two basic radiation patterns of sound sources.