Cavity resonances in an OB?

holdent · « **on:** 30 Sep 2022, 07:09 pm »

Frequently I come across a post that refers to avoiding "cavity resonances" in an OB with wings. Suggestions on doing this include keeping the wings short, ensuring they're asymmetrical, ensuring the wings aren't parallel, and/or offsetting the driver. I haven't come across anything that discusses how to calculate resonances. There's lots online about calculating resonances for open and closed cylinders and boxes but nothing that talks specifically to this issue.

I'm in the process of building a OB that's 20" W x 48" H with wings that are 8" wide and taper from about 2/3s of way up to be 3" wide. Two 15" woofers are at the bottom with a large 10" mid-woofer above them and a dome tweeter. The mid-woofer is surprisingly heavy and I'd like to provide additional support for them by rigidly mounting the wings perpendicular to the baffle. What kind of resonances are possible here both in frequency and magnitude?

Thanks!

richidoo · « **Reply #1 on:** 30 Sep 2022, 08:51 pm »

https://realtraps.com/modecalc.htm

Resonant freq = 1130/1.66*2 = 339Hz <--- this is the resonant freq between 20" parallel surfaces.

With the driver only inches from each wing, the resonance will be very loud and will drive you crazy.

Options:
>340Hz is high enough to be damped by 2" rigid fiberglass board installed to the inside of each wing.
>Angle the wings outward so resonance does not occur.
>Design a different bracing scheme using one vertical beam centered behind the drivers and connected to the baffle at multiple places between the drivers with horizontal struts. Rather than two parallel wings on the sides.
>Make the baffle strong and non-resonant enough to stand alone without external bracing, probably metal.

If you stick with the perpendicular side panels you will need to damp them from vibrating also. They will want to sing along in sympathy with front baffle and the 339 resonance. MDF has wide Q, so it will sing, and sing very ugly, no matter the part dimensions. Cross bracing between the wings will help. Batten bracing does nothing.

With two 15" woofers you don't need side wings to support bass extension, especially if you are using Q .7 or higher drivers. If you use normal Q .4 drivers and have separate amplifier for bass, you can add a little eq to contour the bass to match your room if needed.

No matter how heavy the 10" it will be held up just fine by plywood, or even MDF. Use through bolts and nuts rather than wood screws if you're concerned. The more important factor is the vibration of the baffle itself. It wants to sing along with the drivers that are bolted to it, so damping the baffle is a major part of dipole speaker design. I use rear strut bracing as described above, along with laminating layers of wood with green glue to good effect, but there are many baffle damping techniques. One I have wanted to try is sand filled. Hi Bob!

holdent · « **Reply #2 on:** 1 Oct 2022, 02:48 am »

Thanks for your reply! I really appreciate the ideas for alternatives to parallel wings.

My worry is that the cavity resonances aren't like room nodes because the cavity is not an enclosed or open ended box like a room - the cavity is open both to the back and top. Does this matter?

richidoo · « **Reply #3 on:** 1 Oct 2022, 05:08 pm »

Quote from: holdent on 1 Oct 2022, 02:48 am

My worry is that the cavity resonances aren't like room nodes because the cavity is not an enclosed or open ended box like a room - the cavity is open both to the back and top. Does this matter?

Yes, everything matters! Good question.
It's the same physics whether sound is bouncing between 2 wings or 6 walls. Sound will bounce between 2 parallel surfaces until the energy is dissipated.
Speed of sound is constant so the closer the parallel surfaces, the higher the resonant frequency of the reflections.
Does this answer your question?
The modecalc page linked in my previous post has more info about this. And a useful software app for acoustics.

The parallel wings are the only opportunity for acoustic resonance in your dipole design, since there's no top or rear to resonate with their opposites. Damping the inside surfaces of the wings with 1" OG703 should be enough.