Compression driver
Encyclopedia
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. The efficiency can even pass 100%, which is due to the way it measured in loudspeakers. They are almost universally used for high frequency reproduction in high power sound reinforcement loudspeakers.
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.
and studio monitor
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, UREI, Cerwin Vega 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 the filament consumes an increasing share of the total power thus limiting the power available to the compression driver.
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
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
Sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
level applications. They are normally used in conjunction with acoustical horns
Horn (acoustic)
A horn 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 transferred to the air...
. Horn-loaded Compression Drivers can achieve extremely high efficiencies compared to direct-radiating loudspeakers. The efficiency can even pass 100%, which is due to the way it measured in loudspeakers. They are almost universally used for high frequency reproduction in high power sound reinforcement loudspeakers.
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.
Compression driver protection
In some sound reinforcementSound reinforcement system
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
and studio monitor
Studio monitor
Studio monitors, also called reference monitors, are loudspeakers specifically designed for audio production applications such as recording studios, filmmaking, television studios and radio studios where accurate audio reproduction is crucial....
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
Electro-Voice
Electro-Voice is a manufacturer of audio equipment, including microphones, amplifiers, and loudspeakers. A subdivision of Telex Communications Inc., Electro-Voice markets its products for use in small or large concert venues, broadcasting, houses of worship, and in retail situations.-History:On...
, Community, UREI, Cerwin Vega 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 the filament consumes an increasing share of the total power thus limiting the power available to the compression driver.