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Meyer Successfully Tests Unique Cardioid Subwoofer Design
Meyer Sound recently completed preliminary testing of a revolutionary self-powered subwoofer that exhibits unprecedented directional control of frequencies below 150 Hz. Conducted outdoors at Meyer's Berkeley headquarters, the tests clearly establish the viability of the concept and provide a framework for final development. Meyer's new PSW-6 subwoofer will be introduced in late spring of this year. The PSW-6 is the first low frequency loudspeaker system to successful apply the same basic acoustical principles used in cardioid (uni-directional) microphones. Cardioid design effectively reinforces the sound projected out from the front of the cabinet while drastically limiting sound emitted toward the rear. The PSW-6 will serve as an ideal complement to the MSL-6, a full range self-powered system introduced by Meyer Sound in September of 1997. Both systems are optimized for arrayed configurations in large-scale concert sound reinforcement systems. Basic preliminary specifications for the PSW-6 include frequency response of 25-125 Hz and a peak SPL output of 142 dB at 1 meter. The PSW-6: "A cardioid microphone in reverse" However, the most remarkable aspect of the new system is the front-to-back ratio of 15 db with an unusually tight pattern control, as seen in the accompanying diagrams. "Basically, the PSW-6 works like a cardioid microphone in reverse," says Meyer Sound's founder and president, John Meyer. "It's difficult to take a principle that works with a single, small capsule and apply it to a large, multi-driver speaker cabinet. You have to make sure your pattern is symmetrical, so the results are the same on horizontal, vertical and diagonal axes." PSW-6 Preliminary Specifications The PSW-6 incorporates two 18-inch and four 15-inch transducers in a wedge-shaped cabinet: four facing front and two toward the rear to create the cardioid pattern. The reinforced cabinet has the same exterior dimensions as the MSL-6, and houses four 650-watt power amplifiers along with the proprietary electronics required to create the cardioid directional effect. In cardioid microphones, the desired directional characteristics are the result of acoustic phase differences created when the sound takes two separate paths to reach the capsule's diaphragm. (Sound from in front of the microphone goes directly to the front of the diaphragm, while sound from the rear is delayed by ports and arrives out of phase, thus reinforcing sounds from the front and cancelling those from the rear). The principle is the same with the PSW-6, only in reverse, and the delays are created electronically rather than acoustically. The principal benefit realized by the PSW-6's cardioid design is a controlled, highly directional pattern with a front to back ratio that cannot be accomplished with conventional designs. In addition, this exceptional degree of control is achieved without sacrificing energy in critical reproduction bands, as is often the case with bipolar subwoofer designs. "The principle behind a cardioid loudspeaker system is straightforward," comments Meyer, "but making it work effectively is an extremely difficult proposition. You have to get all the elements precisely balanced, and all the spacing exactly right. You have to find the right combination of physical geometry and system electronics, and you have to make sure the same balanced energy goes out on horizontal and vertical axes. All of the elements have to work together coherently in three-dimensional space." MAPP Software Solves Testing Dilemmas Development of the PSW-6 posed a particularly knotty problem because of the difficulty of testing loudspeaker dispersion patterns at very low frequencies. Meyer Sound has a 100 Hz anechoic chamber which has proven crucial in the design and testing of improved mid- and high-frequency systems. But for low frequencies, a large, free-field testing area is required - which inevitably involves time-consuming logistics, particularly during a rainy Northern California winter. Fortunately, Meyer Sound's new Multi-purpose Acoustical Prediction Program (MAPP) - currently in Alpha testing and functioning as an in-house research tool - was available to model performance parameters during developmental stages of the PSW-6. As the accompanying diagrams show, MAPP was used to predict PSW-6 directional behavior both as a single cabinet and in various array configurations. Preliminary free-field tests have shown MAPP predictions holding to within +/-2 dB of actual performance, which is substantially better accuracy than offered by similar programs now on the market. "Seeing" Sound Pressure Levels with MAPP John Meyer is particularly enthusiastic about MAPP's ability to display loudspeaker directional characteristics as linear pressure plots, rather than the logarithmic polar plots typically used to define loudspeaker dispersion patterns. "These MAPP pressure plots are similar to isobar weather maps or topographic maps," he notes. "Because they are linear, they allow you to easily visualize the performance of the speaker in a real acoustic space. You can even put these pressure plots down as an overlay on a bitmapped blueprint of a performance space, adjust the scales to match, and then see the actual sound pressure levels at each location." Sound pressure levels in MAPP can be shown as lines or as graduated grey scales, with gradations as fine as 1dB. The MAPP software package utilizes Autodesk's AutoCAD OEM software as a development platform. In addition to loudspeaker design, MAPP applications include prediction of interaction among arrayed loudspeakers, refining complex audio system designs to accommodate specific acoustical environments, and acoustical design of architectural spaces such as concert halls and recording studios. February, 1998 |
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