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I have received some questions and criticism about tapped horns. In order to spread a better understand of tapped horns, I’ll restate the questions and try to answer them.

1.) The mouth of the tapped horn is too small to be useful.

Answer: A small mouth size is exactly one of the reasons to use a tapped horn. You no longer have to have a horn the size of a refrigerator to enjoy clean and powerful horn bass. The tapped horn gets around this with its special phasing properties. Usually when you have a mouth that is too small, the result is a very rippled response. See below graphic.

These are the same horn and driver. The only difference is the mouth was made too small on the first one. Their length is ¼ wavelength at 40Hz. The ripple peaks in the first horn occur at harmonics of the ¼ wavelength resonance. In the tapped horn the rear side of the woofer is very close to the mouth. This distance is too short to be acoustically visible to the horn. However, as frequency raises, the rear side of the woofer progressively begins to come more in-phase with the horn. The effect this has is the radiating surface of the woofer (Sd) also progressively gets larger until is becomes 2 X Sd. Because of the increase in driving Sd, the woofer is able to completely fill in the ripple valley in the response. This is a wonderful thing!

2.) The length of the tapped horn is too short to do what you say.

Answer: The tapped horn does not define its length like a conventional horn. In a conventional front loaded horn the length is defined as the distance from the throat to the mouth. This is not the case with the tapped horn. Since you “tap” into the horn down from a closed throat, this distance gets added twice. This is because the sound travels from the “tap” point toward the closed throat, and then gets reflected back down the horn toward the mouth. In addition, the distance from the back side of the woofer to the mouth also gets added to the over all length. So, a tapped horn is acoustically longer than it would first appear. With a tapped horn, all these sections just need to add up to approximately ¼ wavelength of the low frequency of interest.

3.) The tapped horn suffers from cavity resonances. That’s what all those peaks are above 150Hz.

Answer: The tapped horn suffers no more from resonances than a conventional folded front loaded horn. In any folded horn you have to deal with 90 and 180 degree bends, and the resonances associated with their ½ wavelength cancellations. This is just normal life with a folded horn. It is possible to build a straight unfolded front horn and avoid all this. However, you better have a very large room and a very forgiving wife. It is completely impractical for 95% of people to have a straight front loaded bass horn.

Actually, you can use the ½ wavelength cancellations to your advantage with the tapped horn. You can model it in HornResp and see where the first high frequency peak occurs. Now just design the horn to have a fold that makes the distance between the fold and the mouth or throat ½ wavelength. This will cancel the peak and then some. Lemonade from lemons! It is also important to remember that the peaks HornResp shows for the tapped horn are exaggerated. I have measured and found the peaks to be at least half the size as calculated from HornResp. In addition, these peaks occur outside of the frequency band of interest. You should not be using a bass horn much above 80Hz anyway. It becomes too directional above 80Hz in my opinion.

Rgs, John

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