Community Professional's M4
Apr 1, 1999 12:00 PM, Chuck McGregor
Higher SPL levels are certainly desirable in today's pro audio systems, but improved quality and low distortion are also important. The Community M4 compression driver, one of the only compression drivers designed to reproduce the critical 200 Hz to 2 kHz midrange decade, was designed for high efficiency with continuous output capability well above 135 dB, but it maintains low distortion levels. With multi-way systems increasingly relying upon midrange drivers to bridge the gap between LF drivers that cannot produce the necessary midrange quality and extended-range HF drivers that cannot maintain enough SPL at fundamental midrange frequencies, the M4 fits into this niche. Compression drivers are, by design, higher in output than cone drivers, so the development of a high efficiency device in the 200 Hz to 2 kHz range with the necessary sound quality alleviates the need for multiple, low-efficiency midrange cones in a given application. With the introduction of the M4, Community was the first to manufacture a device of this size (8 inch or 203 mm diaphragm) as a production model.
A main design goal for any loudspeaker diaphragm is to maximize the stiffness-to-mass ratio, especially critical for mid- and high-frequency compression drivers. A variety of materials have been used for compression driver diaphragms in an effort to achieve this goal, with beryllium and titanium metals certainly among the most popular. There are, however, other properties that are desirable besides the need for a light, stiff material, particularly for high-powered applications. In high-output applications, driver diaphragms undergo more extreme excursions and considerably higher acceleration for any given frequency. As a result, diaphragms can be subjected to high flexural forces that can lead to material fatigue, performance changes and fractures. Suspensions are affected similarly with additional tensile forces that can fatigue them or even pull them apart. Finally, any contact with the phase plug usually results in immediate fracture of metallic diaphragms.
The Community M4 diaphragm was designed to take advantage of newer composite carbon fiber materials. The goal in using a carbon fiber diaphragm for the M4 was to provide not only a high stiffness-to-mass ratio, but also the flexural strength and impact resistance necessary to eliminate failure modes associated with these anomalies. The Mylar material used for the suspension provides the high flexural and tensile strength needed for high power operation.
Measurements of the M4 with a carbon fiber diaphragm show its mettle. The third harmonic distortion at 25% power input stays 50 dB to 60 dB below the fundamental (0.1% to 0.3% distortion) throughout its operating range. Efficiency for the M4 measures more than 33% at 500 Hz. This level of performance complements the durability of the entire moving system for extended high-power use.
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