Compression Driver

What a compression driver is

A compression driver is a small, specialized diaphragm loudspeaker that generates sound for a horn. Its diaphragm is a dome, usually titanium and up to 4 inches in diameter, vibrated by a voice coil in the gap of a cylindrical magnet. The dome is much larger than the opening that leads to the horn, the throat. A large 4 inch diaphragm, for instance, may feed a 2 inch throat. The air in the chamber in front of the dome is therefore compressed, which raises the acoustic impedance seen by the diaphragm and lets it couple efficiently into the horn. Wikipedia puts horn-loaded compression drivers at about 10 times the efficiency of direct-radiating cone loudspeakers.

How a compression driver works

Between the dome and the throat sits the phase plug, a slotted plug whose channels are machined so that the path from every part of the dome to the throat has the same length. Lenard Audio notes that above 6 kHz wavelengths shrink below 50 mm (2 inches) and sound from different parts of a dome would otherwise cancel. E. C. Wente and A. L. Thuras of Bell Labs introduced a plug in front of the diaphragm in 1928 to extend bandwidth. Bob Smith's 1953 Journal of the Acoustical Society of America paper analyzed resonances in the compression cavity and gave a way to suppress them by sizing and placing the channels, and most modern drivers follow that approach.

The horn finishes the job. Lenard says a compression driver's horn must be one wavelength long at its lowest operating frequency; at 800 Hz that is 18 inches (440 mm). A horn that is too short lets the diaphragm move too far, and a dome, unlike a cone, cannot flex and may fracture. Practical drivers come in two sizes: 2 inch units that cover 800 Hz to 8 kHz at 80 to 100 W, and 1 inch units that cover 1 kHz to 10 kHz at 30 to 50 W. Lenard puts compression drivers at about 20% efficient, or 108 to 112 dB per metre per watt, and compression tweeters at 106 to 110 dB at 50 to 100 W. Never connect them directly to an amplifier: a series capacitor protects the diaphragm from turn-on pulses and DC offset, and adds 6 dB of roll-off to the crossover.

Bell Labs, 1928, and cinema

Western Electric's compression driver was described in the Bell System Technical Journal in January 1928. The abstract says the moving-coil receiver took 30 W of continuous input, against about 5 W for the largest earlier unit, with a conversion efficiency of 10 to 50 per cent from 60 to 7,500 Hz. Most loudspeakers of the day, cone or horn, averaged about 1 per cent. The authors calculated a 50-fold gain in efficiency, and with the extra power capacity, a single unit with 250 to 300 times the sound output of anything before it. It was already in use in Vitaphone and Movietone talking pictures. The first commercial compression driver, the Western Electric No. 555, went into a Bell Labs two-way loudspeaker in 1933.

Compression driver vs dome tweeter

A dome tweeter radiates directly into the room; a compression driver feeds a horn that controls where the sound goes. Soundcertified puts compression tweeters at 105 to 110 dB at 1 W and 1 m against 90 to 94 dB for soft and hard domes, a gap of up to 20 dB, which is up to 100 times the sound output at the same power. The horn also shapes dispersion, so the speaker radiates less sound at the walls, floor and ceiling. The costs are the horn's size and the need for a crossover high enough for the small dome (1 kHz or above for a 1 inch unit). Treat sensitivity figures carefully, because a finished speaker's rating covers the whole system. The woofer, crossover and cabinet limit it, and a bare driver number of 105 to 110 dB does not carry over.

Where compression drivers are found

They are standard in cinema and pro sound, in PA speakers, and in stage monitors. In home theater, Klipsch's Reference Premiere RP-600M II is a documented example. It pairs a 1 inch titanium LTS vented tweeter with a 90 x 90 degree hybrid Tractrix horn and a computer-modelled phase plug, crosses over at 1.5 kHz, and has a rated sensitivity of 94.5 dB at 2.83 V and 1 m, an 8 ohm nominal impedance, 44 Hz to 25 kHz (plus or minus 3 dB) response and 100 W continuous power handling. Erin's Audio Corner measured the same model on a Klippel Near-Field Scanner in CTA-2034 format. The related horn-loaded speaker entry covers the cabinet-level version, and controlled-directivity waveguides cover the shaped-horn approach used in home and studio speakers. Altec's 604 and the Tannoy Dual Concentric both put a compression driver behind a 15 inch woofer, which links this entry to coaxial drivers.

Common confusions

A compression driver is not a tweeter with a horn stuck on. The compression chamber and phase plug are what make it a compression driver; a piezo horn tweeter or a ribbon with a flared body is a different animal. "1 inch" and "2 inch" refer to the throat, not the dome. Lenard cautions that, against marketing claims, compression drivers do not successfully produce energy much above 6 to 10 kHz, so super-tweeters are often added. And efficiency does not mean low distortion on its own: the diaphragm travel is tiny, but the horn throat sees very high pressure, which is why the throat design and phase plug matter as much as the dome. For the effect on amplifier choice, see sensitivity: a 3 dB gain halves the amplifier power needed for the same volume.