This is the text that John posted a few years back at Triode Mafia. John allowed me to post it here warning that he have learned more about tapped horn since then and hat his opinion might have changed since then.
As a nice start, I would highly recommend reading Tom Danley’s white paper on what tapped horn technology is. It is a very interesting concept. Might I add that it works quite well.
Tapped horn white paper -->
If for some reason this link doesn’t work for you, just go to the technical download area of Danley Sound Labs.
It is also very useful to read the pending international patent.
Tapped horn patent application -->
The short version is that a tapped horn uses the secondary side of a woofer to fill in the dips in the horns response due to an undersized mouth. This is done by aligning the phasing properties of the horn in relation to the T/S parameters of the woofer. As the wavelength of the note played changes, so does the phase relationship of the woofer in regards to the horn. At higher frequencies, the horn only makes use of the front side of the woofer cone. As frequency goes down, the back side of the woofer cone progressively starts to contribute as additional driving surface area for the horn. It’s like having a variable woofer cone, but it is done automatically as frequency and phase changes. Pure genius in my opinion. Don’t worry, its easy to design for.
Before I go into how I designed my tapped horns I would like to touch on some very important things that separate tapped horns from conventional horns. I will break this down point-by-point. I will point out both positive and negative aspects of the tapped horn. I will continue to add to this thread as I have more to add. I hope you guys find this interesting and it motivates you to build your own horns.
1.) The primary reason behind using a tapped horn is its size. It is now possible to make a horn extremely small in relation to the lowest frequency it will play. For all practical purposes, you only need a mouth large enough to allow you to mount your woofer. That’s it, it can be quite small.
2.) The tapped horn does not suffer from group delay as bass reflex and conventional front loaded horns. Group delay is the primary culprit that causes “slow” bass. If you have ever heard bass that sounds like it is half a beat behind, you have heard the effects of group delay. The tapped horn avoids this by virtue of its design. Because the backside of the woofer is in close proximity of the mouth, time of flight for the sound is the same as a direct radiator. Due to the air volume inside a tapped horn being much less than a conventional horn, it can quickly pressurize this air and keep proper phase with the back side of the woofer. This has allowed me to finally time align my horns the way I wished I could. Integration is much better than my old 37Hz subwoofer horns. Not to mention the tapped horns are ¼ the size.
3.) One of the most magnificent benefits of tapped horns comes again from the woofer being in close proximity of the mouth. I have come to realize that this can contribute to much more interesting sonic textures. The tapped horn allows the higher harmonic content of instruments to come through. In a conventional front loaded horn, all the folds act as a muffler and acoustically attenuate these harmonics. What I have found is these harmonics are important in reconstructing a convincing field of depth. The realism is greatly enhanced with these harmonics present. A very interesting effect. This works well in my 40Hz tapped horns because the horn’s overall sensitivity is within a dB or two of the woofer’s own raw sensitivity. I’m using B&C Speakers 8PE21 woofers with 98dB sensitivity. The horns overall sensitivity is 100dB. The combined high sensitivity with the detailed light weight cones is capable of resolving the complex textures involved. In addition, the light cones help control over shoot and stored energy smearing of the signal. You may be thinking that the tapped horn is not providing much loading being it is only doing 100dB and the woofer was already 98dB. Here’s the deal, high efficiency woofers have a falling low frequency response. The 8PE21’s sensitivity is only 70dB at 40Hz, while in the tapped horn it is doing 100dB. That’s 30dB of gain on the low end. This type of gain is hard to come by even in a conventional front loaded horn.
4.) Now for a few issues you must consider with tapped horns. The most pressing issue to me is distortion. While the tapped horns still have much less distortion than a bass reflex, infinite baffle, or acoustic suspense subwoofer, it still has more than a conventional front loaded. Let me explain why. In a conventional front loaded horn you usually have a rear chamber that has its volume tuned to resonate the woofer at the horn’s flare cutoff. This is all part of annulling the throat reactance to get the lowest frequency output from the horn. Due to the design nature of the tapped horn, there is no rear chamber. In a tapped horn you must use the back side of the woofer in a phase additive manner to fill in the dips in response. Since there is no back chamber acting as an air spring to help control cone motion, the result is more even order distortion. This is because the uneven excursion (dumax) becomes a little pronounced in a tapped horn. When the pressure wave inside the horn is phase additive, it pushes on the cone, when the pressure wave is phase subtractive, it pulls on the cone and exaggerates uneven excursion. So, how bad is the even order distortion of the tapped horn? Well, I don’t have exact numbers, but from all indications and past experience, I would say if all things were equal (but size of the horns of course) a conventional horn would have 1.5% distortion, then the tapped horn would be in the neighborhood of 8% - 10%. That does look bad, but it is almost all even order harmonics. Remember people have been living with fart machines for subwoofer for a long time. Most home theater subwoofers push 20% or more distortion at high excursion.
5.) Distortion is not all gloom and doom for the tapped horn. If we are smart we can still design a tapped horn with distortion almost as low as a conventional front loaded horn with a rear chamber. The way around this is to use push-pull woofers. The two woofers share a common chamber that feeds the horn throat. One woofer is reverse mounted and also has its polarity reversed. This sets up a push-pull pair that will cancel the uneven dumax of the woofers. It will also cancel even order harmonics just the same as it happens in push-pull amplifiers. In addition, it linearizes excursion cased Bl fade. As one woofer’s coil begins to leave the gap, the other drives deeper.
6.) Having built both push-pull and single woofer tapped horns; I can say the push-pull pair is the way to go. The sound difference can be heard if you listen to recordings with very dry and tight bass. The even order distortion of the single woofer horn rounds and smoothes the attack of the bass notes. It is not the end of the world, but well worth the extra effort to design with a push-pull pair.
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche