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Speaker selection


Russ Buck:
Is wondering it there is some kind of tool that you could set at the speaker angle and would have some sort of laser lights or something where you could see where the speaker is going to cover?  lets say the speaker cover 45 x 60 you could set it at 60 and hold it in place and see what it covers and so forth?  

Dick Rees:

It's easy enough to simply draw out your floor plan, then make some small overlays which match the advertized dispersion of the speaker out of stiff paper.  Simply place the overlay on the floor plan to approximate the coverage.


Russ Buck:
Would it be best to use clear transparency for the overlays? and how do you know how far a speaker will throw in distance?

Lee Buckalew:
First, coverage angle across the entire frequency range of the speaker is not the listed nominal angle that you see in the speaker specs.  There are very few speakers that will carry the majority of their frequency response within the listed nominal angle.  It's important to know that and to know what the coverage angles are at various frequency ranges.

Second, throw has to do with loss over distance and also the reverberent field (among other things).  Two factors will come into play.  The inverse square law (to a certain extent since you will not be in a free field nor will you be using a point source) stating that you will experience approximately a 6dB/SPL drop in pressure for every doubling of distance from a standard (non line array) speaker.  The loss over distance, after you are in the reverberent rather than the near field is not fully 6dB/SPL per doubling of distance.  Where more than one identical speaker cover the same area with the same SPL and do not have comb filtering or other phase problems the result will be an increase of 3dB/SPL at each measured distance.

The second issue is high frequency loss through air absorption.  This varies tremendously based upon distance and humidity (air density).  In a smaller room this is relatively unimportant.

Hope this helps some,
Lee Buckalew

Brian Ehlers:
While ray tracing (2D on paper or 3D with lasers) tells you some good information, there are enough factors complicating matters that you really need a simulation tool, such as EASE, to design coverage with any kind of useful accuracy.  My comments will overlap a little with what others have said.

If a loudspeaker is 2-way, the stated coverage pattern refers only to that of the HF horn, which likely only goes down to about 1.5 to 2.5 kHz.  Only if you have a 3-way speaker with a large MF horn will you achieve any kind of directivity down to, say, 300 Hz.

Also, the stated coverage pattern is as it is applied to the surface of a sphere at some distance.  Go read the article linked on the PSW homepage about designing distributed ceiling speaker systems.  It gives good examples of how a stated 6 dB down at, say, 60 degrees off axis actually results in more like 13 dB down when your listening plane is flat, not a sphere.  And that's when the listening plane is perpendicular to the speaker's axis.  Now imagine what happens to the response when the listening plane is at a shallow angle to the speaker's axis, as it is in most PA applications.  That's why when you have a single speaker covering the depth of a room (no delay speaker), you typically aim the axis of the speaker at the last row of people, thus "wasting" the entire top half of the horn's coverage pattern, making it hit the back wall instead of people.


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