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How horn loudspeakers work

How they work

How do horns work? Horns are basically acoustic transformers. Because of this transformation, the horn driver is a fundamentally different design to most radiating-type drivers.

Horns can have front and/or rear compression chambers, depending on the design, or no chamber at all. Chambers alter the frequency characteristics of the horn and change the operating conditions for the driver.

Some horns act as waveguides, primarily directing the radiation pattern. You still get increased SPL from the greater directivity of the wavefront (sound is not "lost" to the sides). Constant Directivity (CD) horns aim to shape the radiation pattern (lobe) to provide constant dispersion across the entire bandwidth. In other words, you can physically move around (to different locations) and still hear the same frequency response. No small sweet spot here!

A more complete theory of horn loudspeakers can be found in the bibliography of the references section. One of the most important papers is by W. Marshall Leach, Jr., On the Specification of Moving-Coil Drivers for Low-Frequency Horn-Loaded Loudspeakers. Available at the AES website as Preprint number 1405. This paper contains a model of horn loading based on electrical circuit parameters. Thomas Danley, a horn and sound pressure expert from Servodrive says this model is the most accurate for horn design that he has come across.

A new book on loudspeaker theory by Earl Geddes contains a chapter on waveguides (part of horn theory). Geddes' waveguide theories lead him to question the common Webster equations for horns.

A white paper called The Quadratic Throat Waveguide by John Murray gives an explanation of horn theory. Murray describes how the increased efficiency and bandwidth of modern compression drivers has resulted in changing priorities in horn design. No longer do you need the most efficient horn possible for impedance transformation, now you can concentrate on developing "waveguides" which control the dispersion of the wavefront. The idea is to get better polar distribution patterns at the expense of some efficiency. However Murray's paper gives no mention to the inventor of the waveguide approach, Earl Geddes (also in the references section).

These new theories develop a waveguide that starts with a curved contour which then develops into a conical horn. I believe the idea is to get suitable matching from a vibrating plane source (the diaphragm) in the curved throat, which then develops into a conical horn which guides the waves towards the listener with minimal distortion. It looks like some PA applications might be using these waveguide theories, but so far the horn Hi-Fi crowd haven't tried them. Most successful front horns are still based on the Tractrix contour, see for example Bruce Edgar's Edgarhorn Loudspeakers and Sierra Brooks.

Horn Drivers

Horn loudspeaker system is the oldest design in sound reproduction. Even till today, it is the least known design in the sound industry. Most of us audiophiles don't really know what goes on behind that protruding horn that emits sound so effortlessly.

It started more than a hundred years ago, when Emil Berliner presented his gramophone to the public for the first time. The horn of the gramophone amplified the mechanical oscillations of a pin running along a groove in a disc producing a sound that could be heard by the human ear.

Common knowledge tells us that horns are transforming pressure by using a sound source, which means a driver. Thus they work in principle the opposite to a human ear. The ear consists of the outer, the middle, and the inner ear. The outer ear gradually decreases in diameter via the auditory canal towards the eardrum. The pressure increases accordingly. Thus, by means of this design, we can perceive infinite pressure variations, which enables us to hear far better.

Horn loudspeakers, however, are supposed to work the other way around. For this purpose, a sound emitting membrane is placed at the beginning of the horn, which means the horn throat. Compared with the human ear, the horn is used from the other side. The operational membrane has to oppose an increased pressure induced by the horn shape. Dynamic pressure amplitudes are reproduced extremely fast and the membrane very quickly reaches back its neutral position. Due to this fact, the quick suppression of post oscillations in horn loudspeakers results in an exceptionally audible resolution of infinite details.

Music consists of constantly changing intensities, which means sound levels can vary in their intensity depending on the music. A loudspeaker has to follow these impulses accordingly, and should reproduce them as precisely as possible.

Due to the significantly reduced membrane excursion a horn loudspeaker is extremely quick. This does not refer to its acceleration behavior, but this as well applies to its deceleration capacity, since the acoustical attenuation through the horn causes considerably reduced amplitude of post oscillations. Moreover the reduced membrane excursion and the acoustical attenuation through the horn avoid critical partial oscillations of the membrane. Thus, distortions are nearly nonexistent and dynamic compression does not occur.

Horn loudspeakers are the most natural physical concept for the sound reproduction. Even today its characteristics offer indisputable advantages compared to other loudspeaker systems. Horn loudspeakers concept has the ability to transfer low electrical power into high sound levels.

Horn loudspeaker pioneers designers as Gustavas, Webster, Klipsch and Voigt required decades in order to explore the laws of the horn technology. Paul Voigt submitted his first tractrix horn to the British Patent Office in 1926.

Famous classics horn loudspeakers designs were "The Voice Of The Theatre" designed by Altec Lansing, the Klipschorn by Paul Klipsch, the Imperial Hyphex Horns by Jensen Manufacturing Company, the Voigt Domestic Corner Horn by Paul Voigt of Voigt Patents Ltd and Acousta and Audio-vector by Lowther.