History
In 1924 Hanna, C. R. and Slepian, J. were the first to discuss the benefits of using a large radiating diaphragm with a horn of smaller throat area as a means of increasing the efficiency of horn loudspeaker drivers. They correctly surmised that this arrangement results in a significant increase in the radiation resistance (and therefore increased efficiency), because the loading mismatch between the vibrating transducer surface and air is largely corrected, thus allowing for much better energy transfer. In the Hanna and Slepian proposal the compression cavity is directly connected to the throat of the horn.
The next innovation came from E.C. Wente and A.L Thuras in "A High-Efficiency Receiver for a Horn-Type Loudspeaker of Large Power capacity" in the Bell System Technical Journal, 1928. They devised a plug placed in front of a radiating diaphragm to control the transition from compression cavity to horn throat. They found that the bandwidth of the transducer could be extended to higher frequencies using their phase plug. They also outlined criterion for the design of the channels in the plug and suggested a path-length based design approach to maximize the bandwidth. Significantly, their plug moves the coupling point between the cavity and horn away from the axis of rotation. This change significantly improves the transducer response as the effect of the acoustical resonances in compression cavity is reduced. The paper described the first generation compression driver with a field coil magnet and phase plug, It used aluminum diaphragm with an edge wound aluminum ribbon voice coil.
The first commercial compression driver was introduced 1933 when Bell Labs added a Western Electric No. 555 compression driver as a mid-range driver to their the two-way "divided range" loudspeaker which was developed in 1931.
In 1953 Bob Smith made the most significant contribution to modern phase-plug, and hence compression driver design, with his paper published in the Journal of the Acoustical Society of America in which Smith analyzed the acoustical resonances occurring in the compression cavity and devised a design methodology to suppress the resonances by careful positioning and sizing of channels in the phase-plug. This work was largely ignored by his contemporaries and was only later popularized by Fancher Murray . Today the majority of compression drivers, either by inheritance or design, are based on the guidelines outlined by Smith.
The suppression technique of Smith has been recently extended using a more accurate analytical acoustical model of the compression driver geometry. From this work improved phase plug design guidelines have been deduced to completely eliminate all traces of acoustical resonance in the compression cavity. Interestingly, in this work Smiths derivation is confirmed using Finite Element Analysis, a luxury that was unavailable to Smith.
Read more about this topic: Compression Driver
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