Simple Tapped Horn Tutorial using Hornresp - Page 2

While we model a conic expansion (both x and y expand), we typically draw and build a parabolic expansion (either x or y expand), which is close enough for most, especially within the ranges of tapers we're usually dealing with. A sequence of varying prismatic sections can be used to approximate conic (or any other) flare rate, it just takes more math to fold things up.

There is a great series of posts explaining this here and I'd rather link to it than try and re-explain it.

A few posts later, here is the answer from Mr. McBean himself.

OK. After a little bit more refining of the model, I made some sawdust and took some measurements.

I'll cut right to it - how does 27-90 Hz. +/- 3 dB of 90 dB @ 2.83 volts sound?

After working out the impulse response and phase issues discussed earlier, the model was revised a bit more. Once I was happy with it, I made a sketchup diagram of a single-folded tapped horn that matched the model as closely as possible.



It's nothing special, it just allowed me to figure out the dimensions and cut-list. If anyone wants the actual file, PM me an email address.

Then - it was time to make some sawdust. Let's just say that it has been a little while since I built a sub - and it shows. It is certainly not a thing of beauty. Ultimately, in translating the model to MDF, I missed a few measurements slightly, and I also had to allow for an access hatch to install the drivers. I measured what I built, then revised the model to reflect the reality. I also broke in two new MCM 55-2421 drivers and measured the Thiele-Small parameters of the pair, as well as an older one that I had sitting around. All three measured very similarly. I averaged the parameters of the two I used and entered that into Hornresp.

Here is the final input screen:



Here is what I measured (2.83 V, 1 Meter, groundplane)



Here it is compared to the model:



Well, it appears that all the bumps are in all the right places, and are a little bit flatter than the model predicted, though the response is also down a few dB from the predicted SPL (which could be my meter or measurement technique, my mic and meter are not perfectly calibrated).

I also ran an impedance sweep with my Woofer Tester 2:



Looks like the resonances may have attenuated the high Q peaks. The slight shift in frequencies could be due to the straight line measurements I used when I calculated the path length.

Anyhow - in a nutshell, this little exercise completely met my expectations and nearly matched the predictions of the simulation. While the drivers may not be ideal for use in a tapped horn, they certainly perform as predicted.

I've not really had a chance for any sort of subjective listening yet, I barely got the basic measurements done and plotted up today.

Let the armchair quarterbacking begin.

Yes, magnets are in the mouth, cones fire into the throat.

That is one of the drawbacks to using this particular driver, it likes a higher compression ratio, which makes the throats pretty small.

Pics or it didn't happen.....

Drivers installed:


Access cover on (and one MESSY shop....):


Somewhat less than ideal 2pi measurement (in the rain):

The test tapped horn was made of 1/2 inch MDF, held together with brad nails and PL Premium. No damping material was used. I did use bracing on both sides of the baffle, but I did not install any corner braces or reflectors.

Testing Details:
Dell Laptop, Windows XP
Woofer Tester 2 (Impedance and Phase)
E-Mu 0202 USB External Sound Card
Radio Shack 33-2050 modified per Wallin, Panasonic WM61 Capsule on a remote wand for a Mic and Preamp
RoomEQ Wizard (SPL measurements)

Amplifier used was an Audiosource Amp100, which is flat in our area of interest.

The WooferTester 2 was calibrated and is accurate.

The SPL equipment is not absolutely calibrated, but reads within 1 dB at 100 Hz of a calibrated meter. No calibration files were used for the mic or sound card. Comparisons between my 3 meters were within 1 dB. The plots presented are the data as measured, no smoothing or correction factors were applied.

Due to the rain, I was a bit rushed, so the reference voltage was not as exact as I'd have liked. I think I ran the sweeps at 2.81 V rather than 2.83, I just wanted to get the box in out of the rain.

I think the dip at 72 Hz is the result of the fundamental resonance of the single fold. It is off a couple of Hz from what is predicted by the math, but that seems to be the easiest explanation. It may also have been a measurement artifact. It was not as evident in the close-mic response.

As soon as I can find a more capable amp (I have one, I just don't know where I put it...), I plan to put some power to it and see if I can hear the woofers complain during home theater use. There are a few scenes in Iron Man that have made nearly every sub that I have tested thus far complain (though the toys I play with are nothing like the equipment some of the folks here use). I also want to get an idea of how this tapped horn sounds with music.

I also have some other 8-inch woofers that I'd like to try out in the enclosure, a pair of Tang Band W8-740s and some old OEM drivers that I've had sitting around. While neither of these drivers is a good fit, I hopefully will be able to learn something from the effort.

Ultimately, the plan is to take what I have learned here and incorporate that into a more refined fully folded design, so this is by no means a final point in this project.

Got any software suggestions? I am currently using REW, primarily because the capabilities are hard to beat for the price.

If you've got any more measurement suggestions, I'm all ears. I've got pretty good equipment and I am a bit of a measurement geek.

The phase plot is interesting. It compares, but not as well as the impedance and SPL. Will get that polished up and posted as soon as I can. Should I plot it with impedance?