Compression driver
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 are 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 reinforcement loudspeakers.
History
History of the compression driver starts in 1924 with the work of Hanna and Slepian. [1] They 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 realized 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 was made by E.C. Wente and A.L Thuras published their paper "A High-Efficiency Receiver for a Horn-Type Loudspeaker of Large Power capacity" in the Bell System Technical Journal, 1928. [2] It 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 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.[3]
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.[4]
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 [5]. 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 Fancher Murray [6]. 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 [7] 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.
Compression driver protection
In some sound reinforcement and studio monitors the high frequency drivers are protected with current sensing / self-resetting breakers. When too much power is provided to the driver, the breaker trips. After a short time of acceptable power, the breaker resets itself. An older circuit protection technique which was used by Electro-Voice, Community, UREI, Cerwin Vega and others is a is a light bulb put in series with the driver which acts as a "current-to-light" conversion circuitry. As the power applied to the driver increases, the filament in the light bulb heats up and emits light. As the current is increased, the resistance of the filament also goes up as it heats up. This in turn causes the light bulb to dissipate more of the current.[8][9]
References
- ^
Hanna, C. R. (1977 (originally published 1924)). "The Function and Design of Horns for Loudspeakers (Reprint)". The Journal of the Audio Engineering Society. 25: 573–585.
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Wente, E. (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.
{{cite journal}}: Check date values in:|year=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help)CS1 maint: year (link) - ^ http://invalid.ed.ntnu.no/~dunker/refs02.html MORE REFERENCES ON HORN LOUDSPEAKERS
- ^ http://amplioaudio.blogspot.com/2007/05/short-history-of-audiovideo-technology.html The Short History of Audio/Video Technology
- ^
Smith, B. (1953). "An Investigation Of The Air Chamber Of Horn Type Loudspeakers". The Journal of the Acoustical Society of America. 25: 305–312. doi:10.1121/1.1907038.
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Murray, Fancher (1978). "An Application Of Bob Smith's Phasing Plug". Presented at the 61st convention of the Audio Engineering Society. preprint 1384.
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Dodd, M. (2007). "A New Methodology for the Acoustic Design of Compression Driver Phase-Plugs with Concentric Annular Channels". Presented at the 123rd Convention of the Audio Engineering Society. preprint 7258.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ http://www.audioxpress.com/magsdirx/voxcoil/addenda/media/seaandland.pdf Sea & Land's Speaker Protection Devices
- ^ http://www.patentstorm.us/patents/6201680/description.html US Patent 6201680 - Adjustable high-speed audio transducer protection circuit