502 CH APTER 22 Designing Listening Experiences delayed, or decorrelated signals for more than four loudspeakers. This may require some experimentation. In general, a solo center-rear loudspeaker is not an attractive idea because movies are already center-dominated by the relentless front-center channel. Lateral expansion, spaciousness, or envelopment is a welcome relief. Using a pair of rear loudspeakers is the preferred option, because for the vast majority of the time, they will improve the sense of envelopment in upmixed programs, and they are there for 7.1 discrete program content. Even when they are called upon to do duty as a true "rear" channel, they are not significantly compromised. Because the steered signal is monophonic, listeners on the center line will hear a phantom image in the middle of the back wall. Listeners to the left or right will hear an image to the left or right of the center in the back wall. In all cases, the image, if it is a flyover, will simply proceed to pan to the front of the room as it should. All of these sounds tend to be very brief because moviemakers avoid localizable sounds that dwell at off-screen positions. 22.3 LOUDSPEAKER DIRECTIVITY AND THE ACOUSTICAL TREATMENT OF INTERIOR SURFACES The first requirement of any loudspeaker in a home theater is to deliver strong, high quality direct sounds to all of the listeners. These sounds define the direc- tions of sounds steered to the various channels, and of the phantom images existing between them. As can be seen in Figure 22.2, the front channels meet this requirement with a moderate ±30° horizontal dispersion. The surround channels in the example arrangement need much wider dispersions to reach all members of the audience shown in this example. First lateral reflections can be useful to the front channels, and if it is desired to take advantage of them, the dispersion requirement for these loudspeakers expands considerably. For the direct sounds, variations over the angular spread should be minimal, but the lack of reliable measurement data on loudspeakers makes a technical specifica- tion of the allowable variation futile. Looking to the future, perhaps a measure based on the difference between the on-axis response and an average over the appropriate angular window(s) could be developed for loudspeakers in the dif- ferent roles (e.g., see listening window data in Figure 18.6). Surround loudspeak- ers require a uniform horizontal dispersion exceeding that which can be delivered by many forward-firing loudspeakers. The on-wall bidirectional in-phase design exemplified by the product in Figure 18.19b would appear to be a good choice for this application, although in many theater configurations wide dispersion conventional forward-firing designs would comfortably suffice, especially if they were aimed for optimal coverage. As described in Figure 16.8 for the surround channels there would seem to be advantages to using loudspeakers that exhibit less than the -6 dB/double- distance propagation loss of small-box loudspeakers. Line source loudspeakers,