The Renkus-Heinz CoEntrant transducer
Apr 1, 1999 12:00 PM, Ralph Heinz
The acoustically ideal transducer is sometimes described as a mass-less point source with a three decade bandwidth. Despite many attempts to approximate the ideal using exotic materials and unusual shapes, the familiar paper cone woofer and metal dome compression driver remain the sound reinforcement standards.
Materials science and physical laws limit these devices to a bandwidth of one decade. Large woofers reproduce 20 Hz to 200 Hz, but their heavy cones lack the stiffness to vibrate much faster. For the 200 Hz to 2 kHz band, a smaller, stiffer cone driver is typical. For the top decade from 2 kHz to 20 kHz, the device of choice is a compression driver with a light, stiff diaphragm. Large-diaphragm 2 inch (51 mm) throat drivers for high-frequency applications can be used down to 500 Hz, but their output falls off rapidly above 10 kHz, and they tend to produce audible subharmonics. The 1 inch (25 mm) throat drivers can have flat response to 20 kHz, but cannot be used much 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 and lower frequency limits fall in between the two types described above.
Intelligible sound reinforcement demands pattern control, which usually requires a horn. The lower limit of a horn's effectiveness is about 1/2 wavelength. 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 (853 mm) 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, but the horns will physically separate our three drivers. This separation makes it impossible for thetransition between the mids and highs (typically in the 1 kHz to 2 kHz range) to remain coherent within the intended coverage area. When the distance between two adjacent devices is greater than 1/4 wavelength, point-source performance is no longer possible. Polar lobing and waveform distortions become increasingly severe as the distance approaches 1/2 wavelength.
In short, the system designer faces two conflicting requirements-the horn must be about 1/2 wavelength to provide pattern control at crossover, but the devices must be closer than 1/4 wavelength in order to avoid destructive interference. Co-axial horn-with-a-horn designs eliminate the separation between drivers along one axis only, and retain most of the problems observed when two driver/horns are mounted on the front baffle.
The Renkus-Heinz CoEntrant transducer addresses this problem by integrating the acoustic outputs of a midrange cone and a compression driver. The design employs a compound manifold to couple multiple midrange and high-frequency drivers to a single waveguide. This eliminates spatial separation on both axes and achieves point-source performance through the crossover region. Within the compound manifold, the mid- and high-frequency acoustic 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 multiple drivers on a complex manifold with a single wide-bandwidth point-source device. A unique compound phase plug integrates the outputs of a cone transducer and a compression driver into a single source. The CoEntrant driver eliminates the need for a compound manifold, so it can be bolted to existing horns. The first product in this range is the CDT-1, using an 8 inch (203 mm) cone and a 1 inch (25 mm) high-frequency driver with a standard 2 inch (51 mm) throat.
By integrating two types of driver into a single point source, the CoEntrant transducer reaches a two-decade bandwidth; this means that the CDT-1 reproduces sound both an octave lower and an octave higher than 2 inch (51 mm) compression drivers typically used with medium- and large-format horns. The CDT-1 also has higher power handling and lower distortion, resulting in far more natural sound, approaching reference quality even in systems designed for large-scale sound reinforcement applications.
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