To provide sound to a medium size room approximately 40 feet x 50 feet x 15 feet high, or a room volume of 30,000 cubic feet, a system such as the one shown in Figure 11 and 12 could be used. It uses E-V Dominators and will pre duce 106 dB average mid-band levels and peaks of 116 dB. Not that the additional efficiency of the Dominator relative to the S12-2 or S15-3 has just made up for the SPL loss in going from the small to medium size room. If you don't need 106 dB average levels in the medium size room you could use the S12-2 or Si15-3 for 101 dB levels. If you need more than 106 dB with the Dominators, you could go from the 50-watts-per-channel amp t l50-watts-per~hannel and get 111 dB.
In the case of a medium size room, some of the earlier discussions about SPL drop with distance, the reverberant field, a~ dispersion come more into play. The job can be done with high-efficiency, all-in-one system such as the Dominator However, it is more appropriate to do it with a suitable component system such as the one shown in Figures 13 and 14.
The all-in-one system shown in Figures 11 and 12 is capable of producing 106 dB average sound pressure levels and peak sound pressure levels of 116 dB. The component system shown in Figures 13 and 14 can produce average levels of 108 dl with peak levels of 118 dB. For higher levels, a larger air could be used on the high frequency horns, such as the TAPCO CP500 at 150 watts per channel. This would increase the sound pressure level to113 dB, with peaks of 123 dB.
The component system has important advantages over the all-in-one layout using the Dominators: 1. The narrower, controlled dispersion of the component system better fits the geometry of the room. The medium-throw 600 x 400 coverage of the HR60 horn directs more sound to the rear of the room. This gives: A. More uniform SPL throughout the room (helps get around the inverse square law.) B. More direct sound at the back of the room for clear, intelligible vocals there (helps keep room reverberation from swamping your voice.) 2. The component system is modular. It has the flexibility needed to expand to larger and more complex environments in the future. Using the component system requires an electronic crossover (such as the E-V XEQ-l). This makes the system a "bi-amplified" system. Bi-amplification is a method by which crossover is placed ahead of the power amplifiers feeding the speakers. First, let's be certain that we know that a "crossover" is a device which separates full-range program material into the appropriate low-frequency and high-frequency portions for a two-way speaker system. The crossover directs the material to the speaker designed to reproduce it: the bass to the low-frequency speaker and the treble to the high-frequency speaker. Otherwise, the performance of the complete speaker system would be compromised, including blowing up the high-frequency driver with bass frequencies it can 't handle. The frequency response of a typical, idealized two-way crossover is shown in Figure 15.The point where the curves "cross over" is called the crossover frequency. The slope rates of 6-, 12-, and 18-dB-per-octave designs are shown for reference.
Usually, the crossover is placed after the power amp, between the amp and the speaker components in question. Such a crossover is often called "high level" (meaning it comes after the relatively large voltages and power levels of the power amp) and "passive" (meaning it has no electronics or a need to be plugged into an AC outlet). In a bi-amplified system, the low and high division occurs ahead of the power amps, so that separate low- and high-frequency amps connected directly to the corresponding low- and high-frequency speakers are required. This is where the term "bi-amplification" or "bi-amp" comes from. Also, the crossovers used in bi-amped systems usually incorporate electronic components in them and must plug into an AC outlet, so they are called "active" crossovers. They are "low-level" active crossovers because they work on the few volts which come out of the mixer, instead of the high-level signals delivered by the power amps. Bi-amplification is a necessity for a flexible component PA system. High-frequency horn/driver combinations are usually much more efficient than bass boxes - by a factor of 3-to-5 times. Only separately controlled high- and low-frequency power amps can efficiently compensate for this difference so that a smooth response in the room will result. Bi-amplification, with its separate high-frequency amps, also provides a means to balance short-, mid-, and long-throw horns in the more elaborate component setups (see the "Large Size Room" section beginning on page 13). Also, bi-amplification reduces the audible effects of overdriven power amplifiers, a situation which occurs from time-to-time in even properly designed and operated systems. For example, in a conventional full-range system, if low frequencies clip the power amp the unpleasant high-frequency distortion products which are generated are reproduced all-too-cleanly by the speaker's high-frequency components. In a bi-amped system, these distortion products are fed only to the woofers, which do not reproduce them so well. Similarly, if high or mid frequencies clip the arm in a conventional full-range system, low-frequency distortion products may be generated which will be reproduced by the system woofers. In a bi-amped system, these distortion products would never get to the woofers for reproduction. (Some power amplifiers essentially eliminate these clipping-related distortions by preventing the amp from going into clipping in the first place. Examples are the TAPCO amps with PowerLock.) When using a bi-amplified system such as the one shown in Figures 13 and 14, the sound level of the high-frequency horns must be balanced to the sound level of the low-frequency speakers. One simple method of accomplishing this is to turn the gain controls of all power amplifiers all the way up and turn the high-frequency level control on the crossover all the way down. (If the crossover has no such control, use in its place the gaincontrol on the high-frequency power amp.) Then, while speaking or singing into the microphone you usually use, bring the high-frequency level control on the crossover up until a natural, balanced sound is obtained. This balance may also be achieved in a more scientific way by employing acoustic instrumentation such as a one-third-octave real-time spectrum analyzer- which will display the frequency response of the system in the audience area. Adjust the crossover high-frequency level control for the most uniform response in the crossover region (800 Hz).
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