Community Professional's M4
Apr 1, 1999 12:00 PM, Chuck McGregor
Higher SPL levels are certainly desirable in today's pro audio systems, butimproved quality and low distortion are also important. The Community M4compression driver, one of the only compression drivers designed toreproduce the critical 200 Hz to 2 kHz midrange decade, was designed forhigh efficiency with continuous output capability well above 135 dB, but itmaintains low distortion levels. With multi-way systems increasinglyrelying upon midrange drivers to bridge the gap between LF drivers thatcannot produce the necessary midrange quality and extended-range HF driversthat cannot maintain enough SPL at fundamental midrange frequencies, the M4fits into this niche. Compression drivers are, by design, higher in outputthan cone drivers, so the development of a high efficiency device in the200 Hz to 2 kHz range with the necessary sound quality alleviates the needfor multiple, low-efficiency midrange cones in a given application. Withthe introduction of the M4, Community was the first to manufacture a deviceof this size (8 inch or 203 mm diaphragm) as a production model.
A main design goal for any loudspeaker diaphragm is to maximize thestiffness-to-mass ratio, especially critical for mid- and high-frequencycompression drivers. A variety of materials have been used for compressiondriver diaphragms in an effort to achieve this goal, with beryllium andtitanium metals certainly among the most popular. There are, however, otherproperties 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 higheracceleration for any given frequency. As a result, diaphragms can besubjected to high flexural forces that can lead to material fatigue,performance changes and fractures. Suspensions are affected similarly withadditional tensile forces that can fatigue them or even pull them apart.Finally, any contact with the phase plug usually results in immediatefracture of metallic diaphragms.
The Community M4 diaphragm was designed to take advantage of newercomposite carbon fiber materials. The goal in using a carbon fiberdiaphragm for the M4 was to provide not only a high stiffness-to-massratio, but also the flexural strength and impact resistance necessary toeliminate failure modes associated with these anomalies. The Mylar materialused for the suspension provides the high flexural and tensile strengthneeded for high power operation.
Measurements of the M4 with a carbon fiber diaphragm show its mettle. Thethird harmonic distortion at 25% power input stays 50 dB to 60 dB below thefundamental (0.1% to 0.3% distortion) throughout its operating range.Efficiency for the M4 measures more than 33% at 500 Hz. This level ofperformance complements the durability of the entire moving system forextended high-power use.