User manual ELECTRO-VOICE BGM DESIGN GUIDE

[. . . ] Paging/Background Music Systems How to Design and Sell Paging and Background Music Systems from EV & Dynacord DISCLAIMER & COPYRIGHT The material and instructions covered in the document are subject to change without notice. does not guarantee the suitability of the material contained herein for your particular application. Copyright © 2003 Telex Communications, Inc. All rights reserved. Table Of Contents Table Of Contents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Business Audio "Business". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] Budgetary ElectroVoice/Dynacord BGM Guide Page 17 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. 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. 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!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 listeningplane 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. [. . . ] You must take the amplifier's bridged, 8-ohm power rating as the reference maximum power figure for loading calculations. Remember that the minimum load impedance for a bridged power amplifier is twice that of each individual channel. The bridging power figure will be specified accordingly, and may also be de-rated to reflect performance limitations of the amp's power supply circuitry. For this reason, a 200 watt per channel amplifier (or thereabouts) may actually be required, even though the 150-watt amplifier could develop sufficient voltage. Single-Channel Direct Drive As we have seen, the 70-volt line standard discourages direct drive from a single amplifier channel, if for no other reason than economy. [. . . ]