PSW-6: The First Directional Subwoofer in History
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NOTE: The PSW-6 is designed as a low-frequency companion cabinet to Meyer's MSL-6. A new sequence of comprehensive outdoor tests verified that Meyer Sound's new cardioid subwoofer, the PSW-6, exhibits unprecedented directional control across more than two octaves, from 27 Hz to 125 Hz. This is the first time ever that a speaker manufacturer has achieved such a profound degree of low frequency directional control. During the past ten years, Meyer Sound tested delay-based techniques for front-to-back directional control. This, however, only proved effective to 1/2 octave over the cancellation zone. We used this technique successfully with the Grateful Dead on their last tour to suppress a single bothersome note. But of course it was insufficient for suppressing other notes, which meant a much broader based solution was needed. The latest tests were conducted at Golden Gate Fields, a horse racing track located about three miles from Meyer's Berkeley headquarters. This large, open space allowed extensive testing at various distances and angles, employing both a single PSW-6 cabinet and an adjacent pair of cabinets. Earlier, preliminary tests were conducted in Meyer Sound's parking lot and employed only a single prototype cabinet. As shown in the accompanying diagrams, the revolutionary cardioid design of the PSW-6 attenuates sound emitted toward the rear by as much as 20 dB. (The polar plots show response predicted by MAPP software, with actual measured response shown by the dots at 30 or 45 degree increments.) The new tests also reveal that lobing effects are almost non-existent at 125 Hz, the upper limit of the PSW-6's specified response. This was a pleasant surprise, since preliminary tests and predictive data indicated slightly more pronounced lobing. A Breakthrough for Creating Predictable "Quiet Zones" "At this point we have no doubt that the PSW-6 marks a breakthrough in concert level reproduction of low frequency musical information," says Meyer Sound's president and founder, John Meyer. "For the first time, the PSW-6 will enable concert system designers to create effective 'quiet zones' within the venue-- on stage, for example-- that extend all the way down to the lowest octave." This "quiet zone" phenomenon was strikingly evident during the initial round of PSW-6 research in Meyer's parking lot. Two guests invited to attend the tests found that the high sound pressure levels in front of the PSW-6 made conversation virtually impossible, yet they could easily converse in normal voice simply by moving around behind the speaker cabinet. A Decade of Research According to Meyer, the most recent tests mark a huge step forward in development of the first cardioid subwoofer suitable for large scale sound reinforcement. The PSW-6 project is the culmination of over ten years of research at Meyer Sound into precise directional control of low frequency information. "We did some experiments with the delay-based control techniques in Bergen, Norway back in the 80's," says John Meyer. "We placed a delayed element in the back of the cluster and, using EQ and a digital delay line, we were able to create the desired effect over about 1/3 octave." In order to extend the directional control bandwidth by a factor of four, Meyer engineers moved away from using separate cluster elements and external electronics in favor of a single, integrated unit which took into account all relevant factors: driver type and orientation, relative spacing between drivers, cabinet acoustics, and the delay electronics. According to Meyer, the company's new MAPP software (Multi-purpose Acoustical Prediction Program), now in alpha testing within the company, was instrumental in establishing the validity of the PSW-6 concept and in designing prototypes. Although the basic design remains unchanged, subsequent tests of actual performance have led to refinements such as alteration of speaker placement within the cabinet. MAPP: Testing Different Notions to Get More Refined Source Data The latest round of PSW-6 tests also contributed to further refinement of the MAPP program itself. MAPP modeling is strictly based on guidelines set forth in AES information document 5id-1997, which describes recommended procedures for measuring, presenting and predicting polar data from single acoustic sources or from an array of acoustic sources. A crucial element in MAPP's continuing development involves careful comparison of predicted performance versus actual measured performance. For example, tests earlier this year (on a UPA-1P speaker) showed that MAPP predicted performance fell within the +/-3 dB window defined by AES-5id. However, the most recent PSW-6 tests revealed an error build-up extending outside the window when two adjacent PSW-6 prototype cabinets were measured. Investigating the discrepancy, Meyer engineers believe that the initial source data used for the predictive models was insufficient when applied to the complex responses of the arrayed system. "Our first models were based on data gathered from tests of 15-inch speakers each in a separate cabinet," says Meyer. "These measurements did not take into account the unique coupling characteristics of two speakers within the same cabinet. So we performed another round of tests in our anechoic chamber, this time of dual 15-inch speakers in the same cabinet. We used this data along with that from the single speaker configurations to refine our model for the PSW-6. These predictions came out much closer to the actual observed performance." Narrowing the Gap Between Predicted and Measured Performance According to Meyer, this experience emphasizes the need for acquiring extremely accurate data on system components ("objects"), since object information comprises the essential building blocks upon which predictive models are constructed. Object tests in Meyer's anechoic chamber are conducted as specified in AES 5-id, acquiring data at 1/32 octave resolution and with angular rotation of a single degree. "The accuracy of the object is absolutely critical to building accurate predictive models," says Meyer. "Our MAPP program is unique in its ability to perform complex summations and create accurate models of large loudspeaker arrays. But the accuracy of that complex model is dependent upon the degree of precision in the source data. If you are just one dB off in one component speaker, that discrepancy can be multiplied many times over in a large concert system." Refinement of MAPP will continue through measurement of incrementally larger objects (full cabinets and arrays) for use as source data. Models created from this data will then be compared with actual performance in field tests. Analysis of any discrepancies will contribute to further development of the program. April, 1998 |
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