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Technology - Tapped Horn

Horns have been used for decades in sound reinforcement to increase the loading on the loudspeaker driver. This is done to increase the power transfer from the driver to the environment in which the sound is radiated. For maximum power transfer to occur an impedance match between the loudspeaker driver and the free air in which it is operating must be established. A horn is the means to this impedance match. For a horn to operate properly it must not be acoustically small compared the frequencies (wavelengths) it is to radiate.

Conventional horn design is based on this assumption. To meet this requirement the horn needs to be at least one-half wavelength long at the low frequency cut-off of the horn and the circumference of the mouth must be at least one wavelength. When used for low frequency (long wavelength) reproduction this can make for a very large unwieldy horn that can becomes impractical to build or to move in real world applications.

A common practice used in conventional subwoofer design is to reduce horn size to only one-quarter of a wavelength long at the low frequency cut-off. This “short cut” has some very interesting and not entirely desirable effects on a horn’s performance. The net result is that, while there is output from a "short cut" subwoofer horn design in its low frequency range, the horn will not yield efficient power transfer until it begins to reproduce higher frequencies closer to one-half wavelength long relative to the horn dimensions.

In order for a quarter wavelength horn design to be driven efficiently, it is imperative that we understand the conditions presented to the driver at the horn's throat and match these conditions for maximum power transfer.

A quarter wavelength resonance will have a velocity minimum at the throat compared to the half wavelength resonance that will have a velocity maximum at the throat. The velocity minimum condition requires that the proper loudspeaker driver have a much stronger motor (larger magnet) and a larger moving mass than conventional horn theory dictates. The downside is that this driver is not at all well suited to drive a conventional horn, and once the frequencies present in the audio program increase to the point that the horn is at least one-half wavelength a conventional horn is exactly what we have. Any efficiencies gained in the extremely low end with a heavier driver is quickly lost as frequency rises.

Since it is obvious that no loudspeaker driver that can change physical size, weight and mechanical parameters depending on frequency, the solution is to reinvent the horn, not the driver.

Enter the Tapped Horn Subwoofer Technology
The Elevation Series Tapped Horn Subwoofer (patent pending) allows the radiation from the rear side of the driver to enter the horn at a location (tap) sufficiently far away from the throat (where the front side is driving the horn) and closer to the mouth.

Since the rear of the driver is much closer to the mouth of the horn, at very low frequencies it is effectively de-coupled from the system and its radiation does not affect the total output. As frequency increases the situation changes and the rear of the driver begins to be coupled to the horn.

When the frequency is such that the horn is one-half wavelength long the rear of the driver is fully coupled to the horn. The pressure from the front and rear of the driver are of reverse polarity; a 180° phase shift at all frequencies. The pressure from the front of the driver (at the throat) and the pressure from the rear of the driver (close to the mouth) are now approximately one-half wavelength apart. This represents a phase shift of 180°. At this frequency both the front and rear of the driver are driving the horn in phase. When this happens the driver’s radiating surface area (Sd), as far is the horn is concerned, had significantly increased (almost doubled). Since the driver radiates from the front and back of the diaphragm, this yields very different driver parameters than when at the one-quarter wavelength resonance condition.

In real world applications where the measured SPL is comparable for a conventional vented horn and a tapped horn design, the diaphragm excursion of the driver is greatly reduced due to the acoustical loading of the horn. This decrease in excursion will translate directly into lower distortion and far higher output capability from the Tapped Horn.

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