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Big BenD Bass Horn: Mouth Section

[Previous: Belts and Braces, part 3Main; Next: Belts and Braces, part 4]

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. 

MouthSection1

Then the lower braces are put in place and screwed to the throat flange and the lower mouth flange.

MouthSection2

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.

MouthSection3

Two more layers of 6mm plywood are laminated onto the first.

MouthSection4

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. 

MouthSection5

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. 

MouthSection5

Side braces and a top brace are added and fixed using wooden dowels. 

MouthSection7

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. MouthSection8

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.

MouthSection9

[Previous: Belts and Braces, part 3Main; Next: Belts and Braces, part 4]

Big BenD Bass Horn: Belts and Braces Part 4

[Previous: Mouth SectionMain; Next: Performance Measurements] 

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. 

BB4 MSide No Bracing

 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. 

BB4 MSide 57rst

 A vertical side brace, U, does not do much with the bell mode by itself, but it cleans up the upper modes. 

BB4 MSide 57rtu

 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...

BB4 MSide 57rvw

 Some slight changes by adding the vertical side brace:

BB4 MSide 57rwa2

 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. 

BB4 MTop NoBracing

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. 

BB4 MTop 44rwwb

Adding a rear lateral brace (WC) and two longitudinal braces (WD) helps somewhat, adding another four longitudinal braces helps even more. 

BB4 MTop 44rwdwf

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. 

BB4 MTop 44rwgwh

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. 

BB4 MXbrace bare

 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. 

BB4 MXbrace wiwh

The braces added to the outside so far are shown in the photo below:

BB4 MouthBracing

 

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).

BB4 MSide 57rwfwh

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. 

BB4 MSide 57rrare

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.

BB4 MSide 57rrf

BB4 MouthBracing4

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. 

BB4 MTop 44rwfwh

RA indicates the rim braces and cross brace, but now the outer braces (RD) makes some difference, especially around 100-200Hz. 

BB4 MTop 44rrard

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.

Appendix

[Previous: Mouth SectionMain; Next: Performance Measurements]

Big BenD Bass Horn: The Mouth Bend

[Previous: Belts and Braces, part 2Main; Next: Belts and Braces, part 3]

The third part of the horn air path is the mouth bend. This is the most complex part of the build, because all four sides of the bend are curved. In addition, the sides twist as they go around the bend. The best way to go about this is to do a similar approach to what we did for the midrange horn: laminating several layers of plywood on a jig.

The Jig

Then bend is 90 degrees,and the end surfaces are well defined. The inner bend radius is constant, but the outer radius varies. Additionally comes the twisted side walls. 

As a start, I set up two boards at right angles. To these boards, two other boards were fixed, these serve to hold the inner dimensions of the horn.

MouthBend Jig4

Two arms will hold spacers for shaping the side walls. These are fixed to the main jig with T-nuts. 

MouthBend Jig3

Here the side wall spacers are in place. They are important since the side wall twists as it goes around the bend. 

MouthBendJig

The flanges are used as part of the jig, to help push the plywood towards the inner panels. 

MouthBendJig2

 

Inner Bend

The inner bend is made from bendable plywood. The bracing is fixed to the flanges (the little rectangular 3mm plywood piece is used as a guide for the wooden dowel in each brace), and then works as the jig for the bend.

MouthBend inner

 Three layers of bendable plywood are laminated to make the inner wall. MouthBend inner2

Sides

The sides are cut using a jig. The jig is a piece of plywood marked from an "on-the-flat" drawing of the side panel. The on-the-flat dimensions are calculated from the computer model by a series of vector rotations etc. These pieces (cut a little oversize, as it's hard to get perfect accuracy) are laminated on the jig, and clamped tight against the jig spacers. 

MouthBend sides1

Here you can see how the side walls twist around the bend. The same phenomenon can be seen in the Western Electric horns, and is very clearly visible in the 12A horn.

MouthBend sides2

Outer Wall

The outer wall is made from a combination of 3mm hardwood plywood (inner and outer layers) and bendable plywood. I found it easiest to fix the layers with small nails as I put them on, rather than using clamps. Pieces of plywood are used to hold the layers in place against the flanges.

 MouthBend outer1

Final Touches

Or not really. The final thouches will of course be bracing, sanding and painting. But when laminating this many layers, and also pushing them against a flat surface while at the same time trying to align them correctly, put on clamps etc, it's hard to get a clean edge at the flanges. To improve this, I used the router to remove 3mm of the inner, outer and side walls, and fill these spaces with strips of 3mm plywood. This removes all the unevenness between the layers, and creates a flat, smooth surface for the flange.

MouthBend flange1

 

MouthBend flange2

The finished bend (except for bracing) looks like this.

MouthBend Complete

 [Previous: Belts and Braces, part 2Main; Next: Belts and Braces, part 3]

 

Big BenD Bass Horn: Belts and Braces part 3

[Previous: Belts and Braces, part 2Main; Next: The Mouth Section]

The mouth bend has larger surfaces that most of the other parts so far. They are curved, which does add some stiffness, but they are still large.

Outer Bend

The outer bend is the largest surface, and still resonates quite a bit despite the tension from curving. 

BB3 MB Outer NoBraces

With all measurements in a single plot, it's quite difficult to make out the individual curves. But it is clear that there is quite a bit of vibration from 200Hz and up, and also a few resonances around 80Hz which are probably the bell mode. 

If we take a look at point 33, which is in the middle, 600mm along the curve from the throat end,  we find clear peaks at 240Hz and 340Hz, with smaller peaks nearby. By adding a brace across the width of the horn 250mm from the mouth end, brace n, the lowest of these are reduced substantially, while those around 300-400Hz remain. Adding braces along the horn reduces the 240Hz modes by 5dB more, but does not help much above 300Hz. BB3 MB Outer 33knop

At point 35, which is at the center, 900mm from the throat, we find other modes. The cross brace now reduce the modes up to 400Hz at this point. 

BB3 MB Outer 35kn

Two bends along the horn, near the middle, also reduce the vibration quite a bit. 

BB3 MB Outer 35ko

The two braces along the horn near the edges actually increase the vibration a bit, but they are required for the stand, so they have to stay.

BB3 MB Outer 35kp

I also experimented with a sash clamp across the mouth. Across the width of the mouth it cleaned up the 90-180Hz range a bit, across the height it cleans up the 180-300Hz range. 

Side Panels

the side panels mainly have modes in the 200-500Hz range. 

BB3 MB Side NoBraces

I added two braces to the side. The first was vertical (with the horn in the upright position), and reduced the vibration at position 36 by about 5dB.

BB3 MB Side 36kl

Adding a horizontl brace added another 5-10dB. Since I actually braced the side before the outer bend, I also include a measurement with the mouth bracing added, this also helped reduce the side vibration somewhat. Below 250Hz the level is now about 20dB from where it started. 

BB3 MB Side 36kmp

 

 

Inner Bend

The inner bend already has some braces, and the siginficant resonances are above 200Hz. 

BB3 MB Inner NoBraces

Some additional braces were added in between the original braces, but this was done rather late in the process. Here is a measurement of the inner bend with the mouth section added. This reduced the vibration enough to be insignificant compared to the vibration from other panels.

BB3 MB Inner 26wk

Final Bracing

The final bracing of the mouth bend is shown below, as is the two measurement positions 33 and 35 used for checking the mouth bend. 

BB3 BracingMp

[Previous: Belts and Braces, part 2Main; Next: The Mouth Section]

Legends

Bracing ID  Description 
 k) Unbraced mouth bend
l) Side brace, vertical
m) Side brace, horizontal
n) Width brace
o) Two inner braces along outer bend
p) Two outer braces along outer bend

 

 

Big BenD Bass Horn: Belts and Braces part 2

[Previous: Belts and Braces, part 1Main; Next: The Mouth Bend]

 As shown in the previous part, the wall vibrations in the throat bend are essentially out of band, thanks to the dimensions of the bend being rather small. Not so in the middle section (spire), though. Here the main culprit will of course be the front and back, but let's start looking at the sides first. 

In the figures below, the curve legends correspond to the file names. The numbers refer to the measurement position, and the letter after the number refer to the bracing that has been applied. A list is given at the end of this article. 

Side Walls

I measured the vibrations at the straight side wall in three places along the centre line: 200mm and 500mm from the top, and 200mm from the bottom. 

Straight side wall, no bracing

Similar with the curved side wall, here I also measured along the centre line, but at the centre of the three straight panels that make up the curve.

BB2 Spire CurvedSW NoBracing

There are some strong resonances in these panels, and both have strong peaks at around 125Hz. There are also troublesome peaks at about 200Hz and 300Hz too, these are in, or too close to the working range (up to about 300Hz) to leave them alone. 

The first thing I did was to add a flange to the mouth end of the spire. This is quite effective in moving the lowest (125Hz) mode, which is a bell mode. For the sides, I added braces along the length of the horn, this also reduced the vibration significantly. Most of the vibration is now moved above 200hz, and the leve is reduced by about 10dB. 

Here is a compraison between braced and unbraced panels for the straight sidewall:

BB2 Spire StrSW BracingG

And here are the curves for the three measurement positions along the curved side wall, after bracing:

BB2 Spire CurvedSW BracingC

Front Panel

This part is quite important, as it will face the listener. Unbraced, the responses at 7 different positions at the front wall look like this:

BB2 Spire Front NoBraces

There are some very strong modes inside the working range, and we need to kill those. For the following curves, I will only use measurement position 22, which is 800mm from the top (about 200mm from the mouth), and about 290mm from the straight side wall. 

Adding a brace close to point 22, running all the way along the front from top to bottom, in addition to the flange (unfortunately I don't have measurements of the effect of the flange alone at point 22), reduces the level below 300Hz a bit. There seems to be several modes located around 180Hz. 

BB2 Spire Front 22D

It is clear that for bracing to be really effective, the brace needs to be significantly stiffer than the panel it braces. For the braces applied so far, I used 18mm birch ply on end, about 50mm wide strips (apart from the brace for the curved side wall). While it helps, plywood has limited stiffness. I therefore tried adding a 40x40mm L angle iron, 4mm thick, to the D brace. This cleaned up the range above 200Hz significantly, but didn't suppress the 180Hz modes much. 

BB2 Spire Front 22E

Adding a second brace from top to bottom improved the vibration above 200Hz further.

BB2 Spire Front 22F

Finally I added corresponding braces on the back, a small brace on the front near the mouth on the curved side, and a small brace near the throat of the spire. This introduced some modes aroudn 700 and 1000Hz, which may come from the coupling between back and front. 

BB2 Spire Front 22IJ

In any case, the bracing has improved the wall vibrations a lot, what is left is the big bump around 180Hz, which is probably the bell mode shifted higher due to the mouth flange. 

Below is a photo of the applied bracing. The braces are labelled according to the legend.SpireBraces

Next up will be the construction and bracing of the mouth bend.

[Previous: Belts and Braces, part 1Main; Next: The Mouth Bend

Legends

Bracing ID  Description 
 b) Flange added to mouth of spire 
 c) Brace along curved side
d) First brace along front
e) 40x40mm angle iron along d)
f) Second brace along front
g) Brace along straight side
h) 2 braces along rear side opposide d/e/f
i) Small end front brace
j) Small internal brace about 540mm from mouth