This update is actually long overdue, and related to a project at Celestion I wasn't able to finish: a cone driver specifically designed for bass horns. It follows the ideas I outlined under My Approach to Bass Horn Design, and is a 12" driver designed for a compression ratio of about 1:2. As I wanted it to be easy for Celestion to put it into production, I used as many standard parts as possible, either directly or something that could be easily machined from standard parts. But I also added a feature that aren't easy to find in modern drivers: an underhung edge-wound voice coil.
Here are a some of the features of this driver:
Underhung 3" edge-wound voice coil
Copper sleve on pole piece
Focused magnetic gap
Low moving mass
Vented pole piece and back plate
250W power handling
Large ferrite magnet
Inverted dustcap to allow for phase plugs if desired
And for those who worry about the high power rating being detrimental to other qualities important for horn speakers, rest assured: this power rating was a result of the voice coil size and venting, not of "beefing up" the driver (which typically makes the moving assembly lighter) to take higher temperatures and forces.
Here's a side view of the driver, without the front segments. It's built on a Celestion FTR chassis.
The magnet system:
The gap flux is slightly above 1Tesla, which is quite good for a gap this size. It takes a substantial amount of magnet to produce that, especially when you lose gap width to a copper cap.
Comparisons with old DIY driver
Below are a couple of photos comparing the new driver to a DIY project I used to begin with, referred to as 12" DIY driver in the performance measurements. The DIY driver used the motor system from a pair of Celestion NTR08-2009D 8" woofers I found in the bin. They had the cones cut out, but the motor was salvageable (even the voice coil), and I used them to build a pair of 12" drivers using available parts. I used the lightest 12" by 2" cones I could find, and a fairly soft spider. They turned out to be quite good, with the BL^2/Re being a good match for my bass horns. But the new drivers are more robust, and also give a very good performance and produces very clean bass in the Big BenD horns.
Parameter
Old DIY driver
New driver
Re [Ohms]
4.46
5.8
Le [mH]
0.065
0.086
BL [N/A]
12.3
17.0
Mms [g]
39.2
64.5
Rms [Ns/m]
3.4
0.63
Cms [m/N]
4.36e-4
1.88e-4
Now I just hope Celestion will finialize this project and put the drivers into production, as I think this would be a good driver for bass and midbass horn use, especially for domestic use.
Finally, the last article about the Big BendD bass horn! This part will cover the installation and setup of the horn, with some comments on the subjective performance at the end.
Installation
Although the DIY 12" driver showed the most promise during outdoor testing, I still wanted to try the Altec 515-8G when setting up the horn. It was mounted in the rear chamber, the chamber was filled with wool, and the rear wall covered with pieces of an acoustic celing tile.
Then came the process of carrying all the parts into the living room, setting it up, adding gaskets, bolting it all together, installing the drivers, wiring it up and put the rest of the system back together. In the process I had help from my good friend Harry. This is really a two-person job, because carrying the big parts into the house isn't easy to do by oneself. The parts were all designed to fit through a standard door, but they are still a bit difficult to move around.
With two people working, the setup was done in a couple of hours. There are a lot of bolts, about 70 per horn, so or ratchet spanners got a real workout.
The complete setup is shown below. The white middle/throat sections blend in with the walls, making the bass horn less dominating in the room. It does work, some people have not recognised it as part of the bass horns.
The First Test: Where's the Bass?
Just setting up the horn and putting some music on, without any crossover or EQ, left us wondering: Where's the bass? Isn't this supposed to be a bass horn? There's a lot of lower midrange, but the sound was a bit thin.
The measurements made it obvious what the problem was: the output fell below 70Hz, and apart from a peak at 50Hz, the response was more like a midbass horn than a 30Hz bass horn. What was going on?
We swapped the Altec for the DIY 12", and that helped significantly; the better impedance match between the horn and driver was definitely an issue in the presence of room modes. Which turned out to be the real problem.
Room Modes
My living room is about 7m long and 3.5-4m wide. This means that the second mode in the length direction and the first mode in the width direction are both at about 50Hz, which is also clear from the measurement above. Between these two modes, energy transfer in the room is quite limited. A wider room would definitely be an improvement, but you have to work with what you've got.
In the modal region in a room, the response can vary quite a lot from position to position. Some listening positions have a smoother response, while others place the listener at peaks or nulls of modes. To find the best starting point for EQing the response, I measured the response in the room at a 2.5 by 3m grid, every 0.5m. The results for all positions are shown below.
It turned out that the response was quite smooth about 3m from the horns mouths, somewhat behind the middle of the room. The responses at this line are shown below. Annoyingly, the response drops quite sharply below 45Hz, due to the lack of modes between 25Hz and 50Hz, but apart from rebuilding the room (which I'm sure the landlord wouldn't like), there's not much to be done.
I have applied some EQ to lift the response a bit, but one should be very careful with EQing dips caused by room modes. And I'm not going to spend the rest of my life in this house, so hopefully my nest home will have a more beneficial distribution of modes int eh listening room!
Listening Tests
So, after all this work, how does it sound? The lack of output below 45Hz is only noticable on music where you know there should be something down there. Apart from that, the response in the listening position is quite smooth, and it has the traits of bass horns that I have been missing for so long: proper impact - even at low volumes -, and responsive, detailed and tight bass. Low frequency details in the recordings are quite easy to hear, and there's no overhang or resonance. It sounds effortless even at very high volumes too. The horn integrates well with the fast and detailed midrange of the Axi2050, making it a good combination. I also tried a delay-derived subtractive crossover, as described in the Horn Book, and it made a worthwile improvement to the coherence and naturalness in the lower midrange.
All in all, I'm very satisfied with the performance.
The final (and largest) part of the horn is the mouth section with its bracing. The mouth section was designed to consist of mainly straight panels, with only the lower panel being curved.
Since the mouth section is quite big, I figured the best way to build it was by using the bracing and flanges as a jig. Below is the lower flange and the support for the rear of the mouth section. The two are spaced apart according to the design, and held in place temporarily by a couple of scrap pieces.
Then the lower braces are put in place and screwed to the throat flange and the lower mouth flange.
The extra support can now be removed, and the first layer of the lower wall can be fixed to the braces by nails and glue.
Two more layers of 6mm plywood are laminated onto the first.
Note the routed slit in the lower mouth flange. This is to make a proper and good looking termination for the laminated lower panel. By doing this carefully, there is no gap between the panel and the flange.
Side and top panels in place. Since the side panels flare outward, they have to be cut in a way that makes the top and bottom panels flush with the cut. See the Midrange Horn for one way to do it using a band saw. For this horn, I made a small jig to tilt the jig saw the right amount (equal to the angle of the side walls) when doing the cuts.
Side braces and a top brace are added and fixed using wooden dowels.
With the mouth section done, a support frame for the mouth bend was made. It is bolted to the outer braces of the mouth bend.
And finally the horn can be assembled. Here is the first mock-up without using any bolts. The next stage now is to add braces to the mouth section, and the acoustic performance of the horn can be checked.
With one horn finished, I took it outside to do some frequency response measurements. First thing to do is to measure the horn under conditions similar conditions to the simulations. This is a very important part of designing speakers, if you want to use simulation tools in the process. You need to verify that your simulation is correct, and if not, in what way. It is especially important if you are writing your own simulation software. I think people are getting better at it, but there have been many cases on DIYaudio of people complaining about their simulations being wrong, when the actual problem is that they have not simulated what they have actually built.
Corner
The actual condition of a horn built into an "infinite corner" (3 infinite baffles perpendicular to each other) isn't easy to achieve in practice. The best I could do was to use the walls of my house and garage:
And then put some extra sheets of plywood between the buildings to try to close the gap.
So, finally the first measurements, with and without baffles. The difference isn't huge, This may be because the garage is still close enough in terms of wavelengths to contribute to the baffle effect. (I'm not sure if the levels are actually correct, trying to do the calibration in ARTA gave some confusing results.)
So how well does this fit with the simulations? Actually pretty good, see below. The Response below 200Hz is almost spot on, the response at higher frequencies deviates, probably because of the simple model used for simulating the curving, and because I used a different driver than in the measurements.
What I'm quite happy with is that the curving approach worked as intended: there are no sharp dips and peaks or suckouts in the response, and it doesn't roll off until about 500Hz. This creates a nice overlap with the midrange horn. At the lower end, the response starts to fall off rapidly below 30Hz, which was the intended lower limit. The response is a bit uneven, but we'll see that this changes with a different driver.
Wall
The second test condition is in front of a wall. Neither of the two are fully representative of the operation conditions in actual use, but it will show the effect of placing the horn in front of, instead of flush with, a wall.
The effect of one missing side wall and the increased distance to the back wall is evident: a loss of level at low frequencies that was predicted by both the simulations and scale model measurements.
Driver Tests
The next measurements were done with REW as I found it easier to do a level calibration there than when using ARTA. Four different drivers were tested:
Altec 515-8G, a 15" driver built in a Celestion FTR-3070 chassis, A 12" guitar speaker, and a 12" high efficiency woofer built using various parts available. The results are shown below.
There is a clear difference between the drivers; the guitar speaker clearly fails and has a very peaky response (not all musical instrument drivers are suitable for bass horn use, even if Dr. Bruce Edgar had good results with EVM-12). The 15" drivers perform about the same. The best results comes from the 12" driver I built. This isn't actually very surprising, since it has the best impedance match with the horn of all the drivers.
The dips in the response at 483Hz and 870Hz are from standing waves in the empty rear chamber.
Distortion
Following is some distortion meausrements of the drivers tested. I measured at 95, 105 and 115dB SPL at a 2m distance. Only the results for 115dB (114dB for the 12" prototype) are shown.
The 15" drivers are quite similar in response, but the Altec clearly has lower distortion. The guitar driver has very high distortion (not surprisingly, since it has a stiff paper surround and only 1mm overhang on the voice coil), and is best left to what it was designed to do: create distortion for electric guitars.
The 12" DIY driver has the smoothest response, and while the distortion is slightly higher than the Altec, they are both quite at "sane" listening levels.
In this final article on bracing, we will look at the mouth section braces, and also compare the measurements of the final horn with some of the original, unbraced measurements. This article will be quite long, as there is a lot of ground to cover, and many experiments. As before, I can't show all the measurements, as there is just too much data too look at. But I will include measurements at strategic positions, which I used to evaluate the performance during application.
At the bottom of the article is a photo of the braces with letters to identify them (the letter R indicates the mouth without bracing). These letters correspond with the letters in the curve legends.
The bottom wall was fairly well braced from the start, as the braces were used as a jig. But additional braces were added later. We will take a look at the bottom wall when looking at the final results.
Mouth Side Wall
The first set of measurements show the vibration at the unbraced side wall at all the measurement positions. While quite messy, we see some strong modes at 112, 185 and 350Hz, the lowest is the bell mode of the mouth and is very hard to get completely rid of.
Adding the centre horizontal brace on the side wall, S, shifts the bell mode down somewhat, and cleans up another weaker mode around 58Hz. The range above the bell mode is also cleaner, but the upper modes are not much affected. Adding the T braces (upper and lower horizontal braces) helps in the range above 300Hz.
A vertical side brace, U, does not do much with the bell mode by itself, but it cleans up the upper modes.
At this stage I was starting to think I may have to apply some more drastic measures to the mouth to kill the bell mode. So I got some more angle iron, and applied around the mouth. Below are the measurements of these reinforcements by themselves, V indicates side angle iron only, W indicates side and top angle iron. It's actually not much of an improvement by itself, apart from at the 58Hz mode. Well, at least that indicates that something is right...
Some slight changes by adding the vertical side brace:
We'll revisit the side when more bracing is in place.
Mouth Top
Without bracing, two things are clear: The absolute level of the peaks is lower (for the most part) than for the side, and the actual bell mode is the mode at 58Hz.
Adding the angle iron (W) reduces the bell mode by 5dB, and adding the front lateral brace (WB) reduces it by another 5dB. But these braces do not do much for the rest of the modes.
Adding a rear lateral brace (WC) and two longitudinal braces (WD) helps somewhat, adding another four longitudinal braces helps even more.
Mouth Cross Brace
There was no way around it: The mouth needed a cross brace. A cross brace is of course very efficient in killing the bell mode, but it's not as good for killing the other higher modes of the panels. So both are needed. Adding the cross brace certainly reduces the panel vibration over a wide range, the bell mode is now down by 25dB, and the rest of the range up to 300Hz is much cleaner. Adding Mutestrip between the two braces, where they intersect, doesn't do much, but I left it there since very little of the material is used anyway.
But of course the mouth braces themselves will vibrate too! The horizontal brace has two distinct peaks at 67 and 70 Hz and a strong one around 300Hz, while the vertical brace has several peaks above 300Hz.
In order to combat the resonances of the horizontal brace, which were the most troublesome since they were quite strong in-band, I tried various things like stiffening with steel L-profiles. But the most efficient turned out to be mass loading the arms of the horizontal brace with pieces of flat steel. The result of this is shown below, it took the level of the peaks down by over 10dB.
The braces added to the outside so far are shown in the photo below:
Order of Application
When braces are glued onto the side walls, it's not easy to experiment with the order of application. But since I'm building two horns, I decided to test out the order of application at the mouth section.
Side Walls
For the first horn, here are a couple of measurements with no bracing (R), all outer braces (WF) and finally outer braces pluss cross brace (WH).
If the cross brace is added first (includes also the rim braces, I haven't measured without those), there isn't actually too much of a difference when the outer braces are added.
But what really killed the vibration, especially the lower modes (at this position) was the addition of a second layer of bracing at the mouth, that can be seen at the photo below. This was an idea suggested by my friend Torbjørn at the very end of the experiment, so it is not included in the previous mouth bracing experiments. It basically turns the rim braces and horn walls into 'C' beams.
Mouth Top
For the top wall, similar tests were done. Again, R is with no bracing, WF is with all outer braces, and WH is all braces including cross brace.
RA indicates the rim braces and cross brace, but now the outer braces (RD) makes some difference, especially around 100-200Hz.
Conclusion
The measurement of the efficiency of the bracing has been quite extensive. Over 300 measurements have been saved, but probably the same number have been taken in the process of testing things, finding good measurement points, and so on. The articles have presented only a select few of these. For those interested in a summary of "before" and "after" measurements for all measurement points, curves wiht a few comments are presented here.
For the most part, the bracing has been successful. Vibration has been reduced by at least 10dB most places, and in many cases a lot more. Several places the modes have been pushed out of band, other places they have been usefully attenuated. In a few cases, coupling has introduced vibrations in surfaces that weren't there to begin with.
Bracing and vibration are definitely complex topics, and this study has taught me a lot about practical bracing. The main points:
The braces must be stiffer than the panel it is meant to brace, and considerably so. Otherwise they will just add mass and lower the resonance frequency.
Raising the resonance frequency by partitioning the panel works, if the braces are stiff enough.
The bell mode is hard to kill by external bracing, and typically need cross bracing, although turning the rim braces into 'C' beams helps considerably.
Cross bracing can transmit vibration from one panel to another, and excite vibrations in the panels that were quiet. They are not perfect solutions and should be used with care.
The larger the panels, the more difficult it seemed to raise the mode frequencies by bracing. My initial goal of moving all modes out of band were not met for the larger parts of the horn, probably for a large part because the braces weren't stiff enough.
But all in all, the horn is a lot deader than in its unbraced state, and although it's hard to quantify the amount of vibration contributed by the horn walls, I think it's probably not much compared to the direct radiation from the horn mouth, as there are no definite dips or peaks in the response that could come from spurious resonances or vibration.
The measurements made it obvious what the problem was: the output fell below 70Hz, and apart from a peak at 50Hz, the response was more like a midbass horn than a 30Hz bass horn. What was going on? 
