Boxoffice-November.2001

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Desirable surround frequency response, measuring flat to 12.5 kHz, shown behind the theoretical "H" curue, rolled off aboue 2 kHz. o 250 500 1000 2000 4000 Frequencies on octaue centers Figure 5 ohm from the amplifier to the speaker and back. A surround speaker with a minimum impedance (perhaps most easily understood as the total of the electrical factors that impede the current flow) of 6 ohms will therefore add about 6 ohms of resistance to the speaker wiring if it is in series with another speaker. The more speakers one wires resultant load on the amplifier would be reduced from 6x5 ohms, or 30 ohms, to about 7 ohms, and the effective wire resistance between a speaker and an amplifier would be reduced from 24 ohms to about 6 ohms in the group of two speakers and about 12 ohms in the group of three. When one wires five surround speakers with a minimum impedance of 6 ohms entirely in parallel, the amplifier will see a very low impedance load, perhaps as low as 1 .2 ohms. This is typically far below what an amplifier can live with. Most amplifiers can easily accommodate a load as low as 4 ohms. The best amplifiers can drive 2 ohm loads. An amplifier with a 2 ohm minimumload capability makes the job easier for the sound system designer, because more speakers can be wired in parallel with each other before the amplifier might become compromised. In the case of theatre "A," the surround speakers were specifically designed to have a minimum impedance of about 10 ohms. As such, five of these speakers can be wired in parallel, presenting a load on the amplifier of an together in series, the worse it gets. If five of these surround speakers were wired in series, each speaker would see an effective resistance of as much as 24 ohms between itself and the amplifier, entirely acceptable 2 ohms. Loading the amplifier at its minimum 2 ohm capability or 96 times greater than it should be. In also means that the amplifier will order to reduce this series-parallel resistance problem, groups of speakers are deliver its maximum power, nearly twice the power it would deliver into an 8 ohm wired in several parallel groups of load and still more should the amplifier's speakers wired in series. power supply permit. If in the above example we wired our Theatre "B" had seven surround five speakers in two series groups, one of three speakers and the other with two, the speakers per surround channel. A group of four speakers wired in series was paralleled with another group of three speakers. (See Figure 3.) These speakers have a nominal impedance of 8 ohms. One can assume that the minimum impedance, though not specified, is somewhere around 4 to 6 ohms. This would have presented a load for the amplifiers of about 6.9 to 10.3 ohms. If this load could have been reduced to 2 ohms, the available amplifier power would have increased significantly. Another problem with surround systems wired in series-parallel groups is uneven power distribution among the speakers. This is due to the uneven impedance that results between the two groups of speakers when they contain a different number of speakers. Because of the series-parallel wiring in theatre " some surround speakers were louder than others. This is usually avoided by adding or subtracting enough speakers to ensure an equal number per series group. But adding unnecessary speakers is wasteful, and eliminating them reduces the surround coverage in the theatre. The all-parallel wiring in theatre "A," however, guaranteed that all the surround speakers would play at the same level. So how could all the surround speakers in theatre "B" be wired in parallel? Well, they couldn't unless something else was added: an impedance-matching autotransformer—and a rather hefty one at that. Wired in parallel, a group of seven of the surround speakers in theatre "B" would have presented the amplifier with a load of .57 to .86 ohms. Let's assume the lower number of .57 ohms to be safe. By inserting an impedance matching transformer wired to multiply this by 4, we present the ampli fier with load of 2.28 ohms. (See Figure 4.) The benefits are two to four times more amplifier power as well as cleaner sound with more punch from the speakers themselves thanks to the lower effective resistance in the speaker wiring. Another well-known approach for wiring surround speakers employs 70- volt transformers. This tried and true system will also work well. Its only disadvantage is cost, because a transformer is required for each speaker. 58 BoxoilKI
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Boxoffice-November.2001

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