Big BenD Bass Horn: Belts and Braces part 2
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.
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.
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.
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:
And here are the curves for the three measurement positions along the curved side wall, after bracing:
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:
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.
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.
Adding a second brace from top to bottom improved the vibration above 200Hz further.
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.
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.
Next up will be the construction and bracing of the mouth bend.
|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|