448 Chapter 15 stimulus to the development of subwoofers and the pursuit of an extended bass response in general. If the subwoofer approach is applied to upscale music-listening rather than an audio-visual experience, it is normal not to take chances with the possibility of losing low-frequency stereo information, and two subwoofers are used, for left and right in the usual way. The subwoofers are often placed under the main speakers, or very close to them, to preserve what stereo cues can be extracted from their output. They are not placed almost at random in the listening room in the way that mono subwoofers often appear to be. A classic application of stereo subwoofers is the extension of the bass response of electrostatic loudspeakers, notably those by Quad, such as the ESL-57 introduced in 1955 and the later ESL-63. Since the technology of the hi-fi and the home entertainment subwoofer are similar, they are dealt with together in the next section. 15.5 Home Entertainment Subwoofers When the emphasis is watching television rather than listening to music, it is more common to use a single subwoofer. In multi-channel formats the extra directional information from rear and centre channels means that any lack of stereo in the deep bass is more likely to go unnoticed, and a single subwoofer takes up less space and is easier to fit into a room. In this application the drive units are typically between 4 and 15 inches in diameter. Table 15.1 below gives the vital statistics of a handful of home entertainment subwoofers picked pretty much at random from those on the market now (2010). This does not in any way claim to be a representative selection, but it does give some feel for the basic subwoofer format. Note that ABR stands for Auxiliary Bass Radiator. Domestic considerations require the subwoofer to use as small a box as possible, while at the time being capable of reproducing deep bass. This means that efficiency is inevitably low, and powerful amplifiers are needed to generate the desired sound levels--considerably more powerful than those driving the main loudspeakers. It is common for the subwoofer amplifier to have ten times the power capability in Watts compared with the main amplifiers. Figure 15.2 shows the block diagram of a typical hi-fi subwoofer with its electronics. The facilities offered are subject to some variation, but typical features you might expect to find are: Low-level inputs (unbalanced) Low-level inputs (balanced) High-level inputs High-level outputs Mono summing