Big BenD Bass Horn: Belts and Braces part 3
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.
The outer bend is the largest surface, and still resonates quite a bit despite the tension from curving.
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.
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.
Two bends along the horn, near the middle, also reduce the vibration quite a bit.
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.
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.
the side panels mainly have modes in the 200-500Hz range.
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.
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.
The inner bend already has some braces, and the siginficant resonances are above 200Hz.
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.
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.
|k)||Unbraced mouth bend|
|l)||Side brace, vertical|
|m)||Side brace, horizontal|
|o)||Two inner braces along outer bend|
|p)||Two outer braces along outer bend|