Compression Driver
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A compression driver is a small specialized diaphragm
loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...
which generates the sound in a
horn loudspeaker A horn loudspeaker is a loudspeaker or loudspeaker element which uses an acoustic horn to increase the overall efficiency of the driving element(s). A common form ''(right)'' consists of a compression driver which produces sound waves with a small ...
. It is attached to an
acoustic horn An acoustic horn or waveguide is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are tran ...
, a widening duct which serves to radiate the sound efficiently into the air. It works in a "compression" mode; the area of the loudspeaker diaphragm is significantly larger than the throat aperture of the horn so that it provides high sound pressures. Horn-loaded compression drivers can achieve very high efficiencies, around 10 times the efficiency of direct-radiating cone loudspeakers. They are used as
midrange In statistics, the mid-range or mid-extreme is a measure of central tendency of a sample defined as the arithmetic mean of the maximum and minimum values of the data set: :M=\frac. The mid-range is closely related to the range, a measure of st ...
and
tweeter A tweeter or treble speaker is a special type of loudspeaker (usually dome, inverse dome or horn-type) that is designed to produce high audio frequencies, typically deliver high frequencies up to 100 kHz. The name is derived from the high ...
drivers in high power sound reinforcement loudspeakers, and in reflex or folded-horn loudspeakers in
megaphone A megaphone, speaking-trumpet, bullhorn, blowhorn, or loudhailer is usually a portable or hand-held, cone-shaped acoustic horn used to amplify a person's voice or other sounds and direct it in a given direction. The sound is introduced int ...
s and
public address systems A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
.


History

In 1924 C. R. Hanna and J. Slepian 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 In a loudspeaker, a phase plug, phasing plug or acoustical transformer is a mechanical interface between a speaker driver and the audience. The phase plug extends high frequency response because it guides waves outward toward the listener rather th ...
. They also outlined criteria 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 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 eliminate all traces of acoustical resonance in the compression cavity. In this work Smith's derivation is confirmed using Finite Element Analysis, a luxury that was unavailable to Smith.


Compression driver protection

In some
sound reinforcement A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sound ...
and
studio monitor Studio monitors are loudspeakers in speaker enclosures specifically designed for professional audio production applications, such as recording studios, filmmaking, television studios, radio studios and project or home studios, where accurate ...
s the high frequency drivers are protected by current sensing self-resetting circuit breakers. When too much power is dissipated by the driver, the circuit breaker interrupts the flow of electric current. The circuit breaker resets itself after a brief interval. An older circuit protection technique used by Electro-Voice,
Community A community is a social unit (a group of living things) with commonality such as place, norms, religion, values, customs, or identity. Communities may share a sense of place situated in a given geographical area (e.g. a country, village, ...
,
UREI United Recording Electronics Industries (UREI) was a manufacturer of recording, mixing and audio signal processing hardware for the professional recording studio, live sound and broadcasting fields. History Bill Putnam Sr. founded Universal ...
,
Cerwin Vega Cerwin-Vega Inc. is a brand name used on products for professional audio components, as well as home audio speakers, and car audio components. Originally a stand-alone company, Cerwin-Vega was acquired by the Stanton Group after declaring bankru ...
and others is a light bulb placed in series with the driver to act as a variable resistor. The resistance of the bulb filament is proportional to its temperature which increases as current flow through the filament increases. The net effect is that as the power increases the filament consumes an increasing share of the total power thus limiting the power available to the compression driver. US Patent 6201680 - Adjustable high-speed audio transducer protection circuit


References


External links


WESTERN ELECTRIC speakers
{{DEFAULTSORT:Compression Driver Loudspeaker technology