We tend to think of speakers as devices that blast sound at us. But they actually blast sound in every direction, and that’s a good thing. In fact, if they don’t blast sound in every direction, it can be a problem.
A speaker's characteristic sound projection pattern, broad or narrow, is referred to as "dispersion."
In home applications, broad dispersion is generally considered good, while narrow dispersion is generally considered bad. Audio geeks refer to a speaker with narrow dispersion as “beamy.”
Think about the last hi-fi speaker you heard. It probably sounded pretty good, in part because most of today’s speakers have decent dispersion. Now think about the last bullhorn you heard. It surely sounded bad, partly because bullhorns are designed to emit sound in a narrow beam.
Dispersion changes with frequency. In general, the bigger the driver, the narrower its dispersion at higher frequencies. You can easily calculate the frequency at which a driver’s dispersion starts to narrow. Just divide 13,512 (the speed of sound in inches per second at sea level) by the diameter of the driver in inches. Thus, a 5-inch driver’s dispersion starts to narrow at about 2.8 kHz. The higher the frequency above that, the narrower the dispersion will be. Conversely, at lower frequencies of about 1 kHz and below, dispersion generally isn’t an issue.
I can think of five reasons:
Dispersion occurs horizontally, vertically, and every which way in between, but speaker designers are generally most concerned with horizontal dispersion. This is because a seated listener’s ears will typically be about at the same level as the speaker’s tweeter, so they’ll be on roughly the same vertical axis. But it’s harder to predict the listener’s horizontal position. Maybe she’s on a couch in the center of the room, on-axis with the speaker. Or maybe she’s in a chair way off to the side. You need consistent horizontal dispersion in order to deliver good sound quality to all those listening positions — and, of course, to multiple listeners.
Interestingly, some monitor speakers designed for recording studio control rooms have relatively narrow dispersion. In that application, broad dispersion generally isn’t necessary because the most important listener — the engineer at the mixing board — usually sits very close to the speakers, and much of the control room’s wall surface is covered with absorptive material. Thus, most of what the engineer hears is the direct sound from the speakers.
Brent Butterworth and Geoff Morrison combine their years of gear testing and knowledge in one überblog of irreverence and techiness.
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