Tapped Horns

Horns have been used to increase the loading for a loudspeaker driver. This is provided to increase the power transfer from the driver to the radiated environment. To provide the maximum power transfer, it is required to match the impedance between the driver and the free air. A horn is the means of matching of this impedance. The only problem is for a horn to operate properly, it must be acoustically sized to the frequencies that the driver will produce.
 
The initial requirement of a horn needed to be at least one-half wavelength long at the low frequency cut-off, and the mouth circumference must be at least one wavelength as well. Using a horn for a subwoofer application, the low frequency reproduction can make this horn very large and becomes impractical to build. The horn becomes so large that it is not suited for real world applications.
 
In sacrificing some performance, a common practice today, is to size to only one-quarter of a wavelength long at the low frequency cut-off. For a conventional subwoofer design, this will reduce the horn size to make more suitable for real world applications. For a quarter wavelength horn design to be efficient, it is required to understand the conditions present to match the horn’s throat to the driver for maximum power transfer.
 
The quarter wavelength resonance application will have a minimum velocity at the throat compared to the quarter wavelength resonance that will have the maximum velocity at the throat. The minimum velocity condition requires that the driver to have a much stronger motor (BL) and larger moving mass (MMS) than conventional horn theory dictates. In a tapped horn subwoofer application, the driver is mounted at the tapped location of the horn and is radiating into the mouth of the horn. At this tapped location, the horn allows the radiation from the rear side of the driver to radiate at the mouth of the horn and is sufficiently far away from the throat.
 
Because 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 this radiation does not affect the total output. Now, as the frequency increases the rear of the driver begins to be coupled to the horn.
 
When the frequency of one-half wavelength long, the rear of the driver is fully coupled to the horn. The radiation from the front and rear of the driver are of reverse polarity; a 180° phase shift at all frequencies. The radiation from the front of the driver at the throat, and the radiation from the rear of the driver, close to the mouth, are now approximately one-half wavelength apart. At this frequency both the front and rear of the driver is driving the horn in phase. At this point, the driver’s radiating surface area (Sd), 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. When measuring SPL for a conventional vented horn and a tapped horn design, the diaphragm excursion of the driver for the tapped horn is greatly reduced due to acoustical loading of the horn. This decrease in excursion will translate into lower distortion and far higher output capability for a tapped horn application.
 
My original comments:
A tapped horn is unlike other horns that uses the radiation from both the front and rear of the driver that is combined at the mouth. This allows for greater efficiency within a smaller enclosure with deeper extension. Because of the acoustic load placed on the driver, excursion is reduced, leading to increased maximum SPL and lower distortion.
 
When using a tapped horn design, one important factor to remember is the low frequencies that this pass band can produce. Just like any other subwoofer, a high pass filter is require for protection. The reason is that excursions rapidly increases as the frequency decreases.
 
94122_1438085680 101850_1411397808 THAM18 xxx
  Figure 1  Figure 2  Figure 3

Figure 1 was the first design that I seen but my personal
beef is that I like to use all of the front with a flare of my subwoofer for improved coupling issues. Dimensions: 15″ – 28H x 17W x 23D

Figure 2 improved this feature and usable but does not satisfy need requirements 100%.   Dimensions: 18″ – 39H x 21W x 28D | Dimensions: 15″ – 32H x 18W x 23D

Figure 3 improved greatly by using all of the top as part of the flare but I have been thinking about doing the same at the bottom as well. Dimensions: 18″ – 31H x 23W x 25D |  Dimensions: 15″ – 26H x 19W x 20D

I built a demo of Figure 3 for a 12″ with Dimensions: 24H x 18W x 24D. My results was +10 dB above raw speaker free air SPL. WOW!!

When I build a 15″ version of Figure 3 – Dimensions: 28H x 20W x 24D

My goal is for the full face of the horn with no flat surfaces as in the first two figures. At the same time I wish to provide a lower profile horn and elongate the depth to provide the same results as shown below.

PerkAudio’s House of Horns Tapped Horn
Drawn by Lloyd R. Perkins

Comparing the THAM18 – traditional THAM with the new flared version


Links:

Danley’s Tapped Horn and Synergy Horn Technologies

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