A Tapped Horn Experiment

[Triticum Agricolam]

So several weeks ago I started construction on a tapped horn subwoofer as a bit of an experiment. My goal was to get as much output as possible out of a Sundown E-10v2D2, while still having it sound decent. We are all used to ported boxes and, to a lesser extent, bandpass box here, but there is more that one way to get loud and horns are something that I think deserve more attention. Its very coincidental that Patrick Bateman started a thread on creating a tapped horn for his vehicle just about a week or so after I started work on this horn.

This design is not something I came up with by myself. I started with an existing home theater tapped horn design and modified it to work better for car audio use. Here is what the internal layout looks like:

The sub mounts to the hole towards the bottom right, just inside of the horn mouth. Here it is closed up and ready for finish, you can see where the sub mount pretty plainly:

Here is the finished enclosure with the sub mounted, I finished it in white Duratex with red splatter just for something different:

So, now on to what this thing can do. The horn is tuned to about 31 Hz for good low end output. Yesterday I was able to get the sub and all my measuring equipment outside for a little outdoors testing. This is done to try to eliminate any reflections that might skew the results. I'm looking to measure just the raw output from the sub. Here is what we got:

That's the 1 watt at 1 meter measurement. In my opinion I think it looks pretty good. Its got a little dip around 65 Hz and then a gradual trailing off below 50 Hz until you get to the lower knee point at about 30 Hz. When the sub is in a vehicle cabin gain should boost the low end giving a (hopefully) smoothly rising low end frequency response. The upper end ripple is what you usually get with just about any horn and is above the passband of where the sub is going to be used, so it's not really important.

Before I built this enclosure, I played around with it quite a bit in HornResp, which is horn/transmission line modeling software. Here is the measured frequency response compared to what HornResp says it should do:

The green line is the measured response and the blue line is what HornResp predicted. I think the predicted response is really close to what I actually got. The measured response shows a small hump in the 35-50 Hz range, but that's no problem, its free output in my opinion and I'd much rather get more output than HornResp predicts than less. I think this graph does a good job of showing just how accurate modeling software can be.

The put this performance into perspective, here is the output of the horn compared to a couple ported boxes:

The green line is the horn again and the yellow line is the E-10v2 in its recommended 1 cu ft box tuned to 32 Hz. The horn pretty much stomps all over it. It's about 6 db or more above what the 1 cu ft ported box does just about everywhere in it usable bandwidth. That's like getting four times as much amp power or cone area for free. Its a beat down. In fact this puts the horn's performance on par with what a 15" sub would do. The blue line in the graph is a E-15v3 in its recommended 3.25 cu ft enclosure tuned to 32 hz. The horn even beats it a majority of the time between 35 - 90 Hz!

While outdoor 1W/1M measurements are interesting (well to me, at least) what we really care about is what it does in a vehicle. I installed this sub in my wife's bone stock 4-door '96 Honda Accord. It being powered by a really old 500 watt amp. Here is the 1 watt measurement I took in the vehicle compared to the 1W/1M outdoor measurement:

The green line is the outdoor measurement and the purple line was with the mic on the driver side dash. The difference between the two lines is the result of cabin gain. As you can see it boosts the output starting at about 90 Hz and the boost it gets is quite significant. The reason output is lower above 100 Hz is probably because there isn't good airflow between the trunk and the cabin and those higher frequencies are having a hard time getting out of the trunk.

I'd really like to know just how loud this sub gets in-vehicle but unfortunately I don't have a TermLab, yet. I can make a rough estimate though. If the sub gets me 110 db at 30 Hz on one watt, at 500 watts it should get me 27 db more so 137 db. In practice it's highly unlikely I'll get that much due to things like power compression so output doesn't scale in a completely linear fashion. However somewhere in the mid 130s is a reasonable guess. Not too bad for a 10” sub on 500 watts at 30 hz out of the trunk of a bone-stock sedan.

So this sub measures very well but how it sound to the ear? Quite nice actually. I've always been a fan of horn subwoofers. To use a bunch of totally subject audio terms, they sound punchy, very tight and effortless. This sub is not exception to that. This may be my favorite car audio sub yet. I really like it a lot.

Another advantage to a horn subwoofer vs a ported design is port compression is MUCH less of a problem. On 500 watts the air velocity within the horn maxes out at around 12 m/sec. A doubling of air velocity requires a 4x increase in input power so that leaves LOTS of room to add more power (if the sub could take it).

Well now that I've spent a bunch of talking about how great this sub is I'm going to talk about what's not so great about it. Everything comes at a price. The first drawback is the size/shape of the box. At 4.45 cu ft gross this box is twice as big as the 2.2 cu ft gross that Sundown's recommended ported box is. In addition to just being large, this style of box is a lot less flexible in terms of shape than ported box is. If you need it to be 2" narrower and an 1" taller that's not an easy thing to make happen. Resizing the design is possible but you have to refold the horn path inside which is a LOT of work.

Another drawback to this style of enclosure is they are VERY picky when it comes to what model of sub will work well in them. There is a certain ratio of motor force to cone area that tapped horns like, outside of that it can be different to get good performance. In the case of this particular design it works best with either a Sundown E-10v2 or Alpine SWS-10. There are a few other subs it will work OK with, but that's about it. Higher power handling 10" subs tend to have too much motor force to work well. There is good news though, it's easy to scale this design up and down. When scaled up for 12" subs there are quite a few choices that will work well. A pair of these enclosures loaded with FI Q12s on about 4K would absolutely slam.

In summary, here is a list of the advantages and disadvantages of tapped horns for car audio use.

Advantages:

- Significantly higher efficiency/output

- Wide bandwidth

- Great sound quality

- No port compression to worry about

Disadvantages:

- Much larger size vs ported box for same sub

- Reduced flexibility for box size/shape

- More complex to build

- Picky about what sub driver is used

I know this was a lot to read, hopefully someone found it interesting, if not useful.

------------------------------------

UPDATE:

I have TermLab results of this box on page 7!

[baron_of_bass]

Great read! Man, once my tahoe is all bagged up Ima going to have a huge she'll, you know damn well Im going to need your help on what to do with all that space. 2 seater Tahoe.... The possibilities

[diegoclass2010]

this is great man! nice write up! something different. i like it .... keep up the good work!

[Karkov]

Wow, amazing write up, over my head, but amazing. I like how with the finished product you can see the sub RIGHT there...

EDIT: when these get more common, or easier to get a design for one, the question/debate will come from people like me who own a full size SUV and I am looking at a small set up but have room to fit a box like that horn. With a proper design I can easily assemble it myself, and would save money buying only one (or perhaps 2) drivers instead of 2 or 3 or even 4 if the output was the same. Depends on IF* the output was comparable though, and if time meant nothing to me (box building aspect) or how easy it would be to acquire a design. so cost vs output vs time vs etc. etc. Kind of interesting. I love the science behind all aspects of car audio, I don't understand very much of it though, thinking about some aspects make my head dizzy, lol.

[tboorn]

very nice study indeed

[ShadeTreeMechanic]

Love your work man! I have always liked the horn concept and with their output I can see this becoming commonplace in car audio. Thanks for sharing!

[Triticum Agricolam]

Wow, amazing write up, over my head, but amazing. I like how with the finished product you can see the sub RIGHT there...

EDIT: when these get more common, or easier to get a design for one, the question/debate will come from people like me who own a full size SUV and I am looking at a small set up but have room to fit a box like that horn. With a proper design I can easily assemble it myself, and would save money buying only one (or perhaps 2) drivers instead of 2 or 3 or even 4 if the output was the same. Depends on IF* the output was comparable though, and if time meant nothing to me (box building aspect) or how easy it would be to acquire a design. so cost vs output vs time vs etc. etc. Kind of interesting. I love the science behind all aspects of car audio, I don't understand very much of it though, thinking about some aspects make my head dizzy, lol.

Tapped horns are a relatively new creation. Tom Danley at Danley Sound Labs was the first person to implement them and actually has a patent on them. This style of horn is what makes car audio use feasible since they are much smaller than front loaded horns.

I think for situations like your full size SUV a tapped horn could work really well. They would let you use either a smaller number or less expensive subs and still get the same level of output like you are saying.

[Patrick Bateman]

Due to Hoffman's Iron Law, box size basically dictates output level.
For instance, I can juggle the parameters in hornresp and make a tapped horn with *exactly* the same F3 as a ported box, and if the box size is the same, the efficiency will also be the same.

I think the advantage of a tapped horn is realized when you're pouring a lot of power into a sub. Wayne Parham ran some measurements that really illustrate this, they show that horns don't compress the way that vented boxes do.

This makes sense, because the only thing moving at the tuning frequency is the air in the port. IE, if you have a vented box with a 12" woofer tuned to 30hz, the only thing moving at 30hz is the air in the port. The cone isn't moving. Due to this, you need a REALLY efficient port at high SPL, and a tapped horn is basically a giant port.

The other cool thing about bandpass boxes, tapped horns, transmission lines and front loaded horns is that you can basically "dial-in" any response curve you want. If you wanted to, you could make a 4" woofer play down to 20hz! (The efficiency would be terrible of course.)

[Triticum Agricolam]

My problem with Hoffman's Iron Law is its too simplistic. It doesn't account for frequency response or port displacement. Sure I could build a ported box the size of my horn and probably get the same efficiency at 30 Hz, but what about above that? A 3.5 cube ported box with my E-10 in it is going to be one peaky little bastard and I'd probably destroy the sub if I put full power to it. So while the efficiency may be the same at 30 Hz, overall it's not really the same. Port displacement is also a problem. If I make the port larger on a ported box the box now takes up more space but isn't any more efficient unless there was port compression with the smaller port.

If you are feeling up to a challenge, juggle the parameters in HornResp and come up with a ported box that has the same efficiency AND frequency response as my horn :-)

[Patrick Bateman]

Here's the frequency response of a ported box versus a tapped horn. I set the box size to be equal, and tuned both of them to the same frequency. You can see that the efficiency is virtually identical. This is especially easy to do with transmission lines; I can make a transmission line that has the *exact* same response curve and efficiency as a ported box of equal size.

There were a few years where I believe that a horn had an efficiency advantage over other box types, but it's just not true. Hoffman's Iron Law doesn't bend, and efficiency is simply tied to box size. (Of course, horns DO WIN when you make them bigger than ported boxes; the Danley BC-218 is the size of a small car and weighs a quarter of a ton.)

Where things get interesting are at high power, because that's when port compression plays a big role, and thermal effects too. Wayne Parham's work is really interesting; he showed that doubling the power only added about 1dB at the limits. (He literally cranked up the power until the drivers melted down.)

By the way, the ported box in that sim is one half of a PK Sound CX800 and the tapped horn in that sim is the tapped horn in my car.