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constraints tend to favor sacrificing density, so the optimum center-to-center configuration is, in
practice, the least common of the three.
Ceiling Speaker Coverage
The main objectives in deciding about the placement pattern and density of loudspeakers in a
distributed system are covering the area effectively, providing sound that is audible and intelligible
over the entire listening area, and making sure the system is capable of sustaining whatever
average and peak sound pressure levels the application requires.
A misunderstanding about the coverage angle specification of loudspeakers can easily result in
system design mistakes. It is very common to see a “polar coverage” spec and assume that the
speaker will actually cover this angle. Loudspeakers actually cover less area than their spec
sheets would imply. (Let me clarify that the coverage angle is typically the angle at which the
sound level is 6 dB down from the on-axis sound level.)
Polar vs. Listening-Plane Coverage. There are two different types of coverage measurements
that often get confused for one another. It is standard in the loudspeaker industry to state the
coverage in a polar pattern — in a sphere that is 1 meter from the speaker in all directions. The
angle where the sound level is down 6 dB from the on-axis level is called the edge of the polar
coverage pattern. This is what appears on spec sheets.
It's a legitimate specification, but it does not represent what the coverage will be over a flat
listening plane, as in any room, because it doesn't take into account the difference in distances
that people are from the speaker. For speakers projecting from a ceiling onto a flat listening
plane, the sound has to travel farther off-axis (to the sides) than it travels on-axis (directly below
the speaker) resulting in a much greater drop-off of sound level off-axis. The result is that the
actual coverage angle (at -6 dB) on the listening plane is more narrow than the polar spec. Some
ceiling speaker manufacturers use their polar measurement to claim extraordinarily wide
coverage. Do not use this specification to lay out coverage patterns of ceiling speakers!
To Illustrate. Imagine a loudspeaker with a 180° polar spec. If you were to incorrectly interpret this
as 180° coverage on the listening plane, then one speaker would be all you would ever need for
any application. But imagine a single speaker trying to cover an entire department store or
restaurant. In fact, you will see that unless a speaker can send more sound to the sides than it
does directly on-axis, it never covers more than 120°.
The sound system designer needs to work with the actual coverage over a flat listening plane
because that is the plane in which we live, listening at a height of 3 to 6 feet above the floor,
depending on how tall we are and whether we're standing or seated. This is called the listening-
plane coverage specification of the speaker. The listening-plane spec represents the reality of the
speaker's coverage for the listeners. Laws of physics dictate that the listening-plane coverage is
always more narrow than the polar coverage pattern.
Let’s take a speaker that has a 140° polar coverage (i.e., its 6dB down points) as an example. It
would be a mistake to assume that this speaker can cover 140° over the listening plane. In fact,
the level at the edges of a 140° pattern is actually more than 15 dB down compared to on-axis —
not 6 dB down. It's interesting to note that the same proportions hold true for any ceiling height:
No matter how high the ceiling is, the off-axis distance is even farther away by the same
proportion. So for the loudspeaker in this example, whether the ceiling height is 8 feet or 20 feet,
the listener who is at the edge of the 140° pattern, who you might think is at the 6dB down point is
really 15 dB down.
The actual listening-plane coverage depends on the polar plot of each speaker. On average, the
coverage of the listening plane from a speaker with a 140° polar coverage is usually between 90°
and 110° .
ElectroVoice/Dynacord BGM Guide Page 18