The Renkus-Heinz CoEntrant transducer
Apr 1, 1999 12:00 PM, Ralph Heinz
The acoustically ideal transducer is sometimes described as a mass-lesspoint source with a three decade bandwidth. Despite many attempts toapproximate the ideal using exotic materials and unusual shapes, thefamiliar paper cone woofer and metal dome compression driver remain thesound reinforcement standards.
Materials science and physical laws limit these devices to a bandwidth ofone decade. Large woofers reproduce 20 Hz to 200 Hz, but their heavy coneslack the stiffness to vibrate much faster. For the 200 Hz to 2 kHz band, asmaller, stiffer cone driver is typical. For the top decade from 2 kHz to20 kHz, the device of choice is a compression driver with a light, stiffdiaphragm. Large-diaphragm 2 inch (51 mm) throat drivers for high-frequencyapplications can be used down to 500 Hz, but their output falls off rapidlyabove 10 kHz, and they tend to produce audible subharmonics. The 1 inch (25mm) throat drivers can have flat response to 20 kHz, but cannot be usedmuch below 1.5 kHz. In between are designs with 2 inch (51 mm) or 1.4 inch(36 mm) throats and medium-sized diaphragms. As expected, their upper andlower frequency limits fall in between the two types described above.
Intelligible sound reinforcement demands pattern control, which usuallyrequires a horn. The lower limit of a horn's effectiveness is about 1/2wavelength. Therefore, a true 20 Hz horn would have to be 28.3 feet (8.6 m)at its mouth. An effective 200 Hz horn would need to be about 2.8 feet (853mm) across, and a (2 kHz) horn might be as small as 3.5 inches (89 mm).
We now have a three-way system producing the entire audible spectrum, butthe horns will physically separate our three drivers. This separation makesit impossible for thetransition between the mids and highs (typically inthe 1 kHz to 2 kHz range) to remain coherent within the intended coveragearea. When the distance between two adjacent devices is greater than 1/4wavelength, point-source performance is no longer possible. Polar lobingand waveform distortions become increasingly severe as the distanceapproaches 1/2 wavelength.
In short, the system designer faces two conflicting requirements-the hornmust be about 1/2 wavelength to provide pattern control at crossover, butthe devices must be closer than 1/4 wavelength in order to avoiddestructive interference. Co-axial horn-with-a-horn designs eliminate theseparation between drivers along one axis only, and retain most of theproblems observed when two driver/horns are mounted on the front baffle.
The Renkus-Heinz CoEntrant transducer addresses this problem by integratingthe acoustic outputs of a midrange cone and a compression driver. Thedesign employs a compound manifold to couple multiple midrange andhigh-frequency drivers to a single waveguide. This eliminates spatialseparation on both axes and achieves point-source performance through thecrossover region. Within the compound manifold, the mid- and high-frequencyacoustic path lengths are identical. This creates temporal coincidence,spatial coherency and consistent coverage over two decades of output.
The CoEntrant driver evolved out of this concept. It replaces the multipledrivers on a complex manifold with a single wide-bandwidth point-sourcedevice. A unique compound phase plug integrates the outputs of a conetransducer and a compression driver into a single source. The CoEntrantdriver eliminates the need for a compound manifold, so it can be bolted toexisting horns. The first product in this range is the CDT-1, using an 8inch (203 mm) cone and a 1 inch (25 mm) high-frequency driver with astandard 2 inch (51 mm) throat.
By integrating two types of driver into a single point source, theCoEntrant transducer reaches a two-decade bandwidth; this means that theCDT-1 reproduces sound both an octave lower and an octave higher than 2inch (51 mm) compression drivers typically used with medium- andlarge-format horns. The CDT-1 also has higher power handling and lowerdistortion, resulting in far more natural sound, approaching referencequality even in systems designed for large-scale sound reinforcementapplications.