Compression driver

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A Compression Driver is a type of loudspeaker driver which uses the technique of "compression" to achieve high efficiencies. In this context compression refers to the fact that the area of the loudspeaker diaphragm is significantly larger than the aperture through which the sound is radiated. Compression Drivers most commonly used in high sound pressure level applications. They are normally used in conjunction with Acoustical Horns. Horn-loaded Compression Drivers can achieve extremely high efficiencies compared to direct-radiating loudspeakers. They are almost universally used for high frequency reproduction in high power sound reenforcement loudspeakers.

[edit] History

Hannah and Slepian ^[1] are 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 realize that this arrangement results in a significant increase in the radiation resistance resulting in an increased efficiency. In the Hannah and Slepian proposal the compression cavity is directly connected to the throat of the horn, Wente and Thuras ^[2] later devised a plug placed in-front of the 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 outline criterion for the design of the channels in the plug and suggest their path-length based design approach to maximize the bandwith. 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.

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 ^[3]. In this paper Smith thoroughly analyzed the acoustical resonances occurring in the compression cavity and devised a design methodology to suppress the resonances by careful positioning and sizing of the channels in the phase-plug. Regrettably this work was largely ignored by his contemporaries and was only later popularized by Murray ^[4]. 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 ^[5] 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 Acoustical Finite Element Analysis, a luxury that was not available to Smith at the time of his paper.

[edit] Citations

1. ^ Hanna, C. R.; Slepian, J. (Sep 1977 (originally published 1924)). "The Function and Design of Horns for Loudspeakers (Reprint)". the Journal of the Audio Engineering Society 25: 573-585. 
2. ^ Wente, E.; Thuras, A. (Mar 1978 (originally published 1928)). "A High-Efficiency Receiver for a Horn-Type Loudspeaker of Large Power Capacity (reprint)". the Journal of the Audio Engineering Society 26: 139-144. 
3. ^ Smith, B. (Mar 1953). "An Investigation Of The Air Chamber Of Horn Type Loudspeakers". The Journal of the Acoustical Society of America 25: 35-312. 
4. ^ Murray, F. (Oct 1978). "An Application Of Bob Smith's Phasing Plug". presented at the 61st convention of the Audio Engineering Society preprint 1384. 
5. ^ Dodd, M.; Oclee-Brown, J. (Oct 2007). "A High-Efficiency Receiver for a Horn-Type Loudspeaker of Large Power Capacity (reprint)". presented at The 123rd Convention of the Audio Engineering Society preprint 7528.