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.
Interesting article, but then why do my magnepans (or martin logan's I've heard) sound so good?
@funboy: Panel speakers merit a whole article on their own. Their dispersion patterns are completely different from conventional speakers. A typical panel speaker radiates sound evenly front and back, but has a null (i.e., no sound radiation at all) at +90 and -90 degrees off-axis. Because the panels are so tall, they're beamy vertically - but it's a very big beam so that's not a problem, and it's actually something of an advantage because there's less reflection from the floor and ceiling.
There are variations - for example, MartinLogan uses curved panels, and on-wall panel speakers tend to have baffles that redirect the sound radiation off the back of the panel.
They sound good in large part because they're radiating sound against the wall behind them as well as directly toward you, thus producing a higher ratio of reflected to direct sound. This tends to give you a bigger soundstage and a more enveloping sound, but typically with less-focused stereo imaging.
Comment & question:
EPI 100: I have a set of these. 8" woofer crossed over by a capacitor (alone) at about 1.8 kHz to a 1" inverted-dome tweeter in a sealed box. Pretty much verifies your dispersion formula - no serious change in sound quality out to 45 deg or so off-axis, and after that it's background music reflecting off the walls anyway so changes don't matter. Sound is awesome and has very little distortion; bass is solid down to 50 hz or below and highs are clean well beyond my (older) hearing. Power handling despite the crude crossover is good - rated 60w - but any amp over 30w is ample. Wish EPI was still around - I want another pair when I go surround, with a sub crossing over around 60-80.
Whizzer cone/other tweaks in a single radiator: I used to have a pair of tiny Goodmans speakers -- 6" with an aluminum dome in the center. They sounded amazing for something that small, and needed very little power to drive (10wpc amp was ample). Yes, the very high treble was rolled off and beamed straight forward, but below 10-12K it didn't seem bad and in a dorm room the beaming didn't matter - bounced off the other wall and splattered rather than dispersing. My mom has them now and loves them. So how do those little tweaks (center dome, whizzer cone) help, or do they just make you *think* you're getting some treble without a real tweeter?
@mook: I can't say with great certainty how much those tweaks help, but I can say that every full-range driver I've measured had really lousy dispersion above 6 to 8 kHz, and extremely uneven frequency response above that, too. That said, they sometimes sound pretty good. I have yet to be swayed by a full-range driver larger than about 4 inches, but I'm keeping an open mind about it.
I've heard some good EPIs in the past, too.
Thanks for this "fundamentals" article. I have an appreciation of great sounding gear without an understanding of how and why it works. This article is a starting point for my education.
My understanding is that the reflections from wide dispersion speakers have a detrimental effect on imaging due to the sound from the reflections arriving at your ear later than the direct on axis sound.
Narrower dispersion speakers don't have the room boundry/reflection issue hence the imaging/sound quality is better.
It seems the best speaker design would be to have a narrow dispersion and good off axis frequency response
@mikeba316: The room reflections created by broader dispersion speakers improve the perceived sound quality. That's why there are few, if any, speakers available on the consumer market that show a conscious effort to narrow the dispersion. This principle has been established through decades of research at Canada's NRC, continued at Harman International, and outlined in depth in Dr. Floyd Toole's book "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers in Rooms." Strongly recommended reading.
this kind of article is dangerous. it is at least misleading and claim to offer 'fundamental' insights when it is in fact based on compromised theories.
the whole 'stereo everywhere' school of thought will get you nothing but mediocre sound everywhere, and with wide dispersion, you are just listening more to your room than what is on the recordings.
for a counter perspective, take a look at:
http://www.sanderssoundsystems.com/technical-white-papers/dispersion-wp
http://www.sanderssoundsystems.com/technical-white-papers/room-acoustics...
@pureo: "dangerous"? That's a strong statement. My article is based heavily on research done at Canada NRC and Harman (widely acknowledged as two of the best and most important audio research facilities in the world), on information taken from some of the best-known speaker design books, and on the general practices of the most recognized and accomplished speaker designers on the planet. Your articles seem to be based primarily on your own anecdotal experiences and pet theories.