Standing waves in my subwoofer box

[Thephantompsychic]

My box I'm testing rn is 75L. It is braced (as well as I could I think) and has CLD green glue damping for most of the walls (because eh, why not?) I calculated a standing wave at 140hz with VituixCad's standing waves calculator. I also have quite a bit of resonance at around 190hz too.

My question is this, I know that acoustic foam or acoustic insulation is really good for deadening standing waves in a speaker box, but are standing waves affected by damping at all? Like is my CLD damping supposed to have affected the standing wave? Or is that fixed by adding acoustic foam/insulation?

[billfitzmaurice]

You don't have standing waves of consequence in a box that small. CLD does nothing to tame what you do have, which is internal reflections. That's the job of damping, with foam, polyfill, felt carpet underlayment, Type 700 fiberglass, etc.

[bassman_soundking]

The best I can suggest is no equal length walls, or double of another wall.
An example would be 15"x19"x21" external to get around 75L net after driver and bracing.
Probably a dumb shape, but just saying a cube can be a bad shape.

[Thephantompsychic]

Bill,
Seeing as I haven't added any polyfill or the like, this makes sense to me and these internal reflections are very audible. Thank you for confirming this.

Bassman,
As it stands, my box is 15"W×28"H×13"D. It is very much not a cube as it stands. I am however, bad at bracing.

If either of you could give some bracing tips, that'd be appreciated. I use the "dowel every 8 inches" method but I don't know if there's a "scientifically best" method or anything.

[billfitzmaurice]

Dowels every 8 inches should suffice.

[fpitas]

At times I've found that, in addition to wall-to-opposite-wall bracing, an additional brace from there to a box corner makes a big difference in the knock test.

[bassman_soundking]

You can do dowels or window braces.
If you look at Parts Express Knock-down sub enclosures. they have an elaborate bracing scheme.
Im not up to the task of reproducing those with my current tool though.

[djg]

IMO, trying to emulate CNC bracing with hand tools is noble but misguided. But, that's just my opinion.

[bassman_soundking]

Yeah it would be nice to do it, but I havent the skill.
Double walls and regular bracing are as far as I have gone

[fpitas]

Staggering the brace spacing a bit is best.

[bassman_soundking]

Non-equidistant spacing sounds like a good idea for the same reason not to build a cube shaped enclosure!

[Serenitynow]

This is why I cut EVERYTHING on the CNC

[bassman_soundking]

This is why I cut EVERYTHING on the CNC

If only I had a CNC.
I would be bombarding this forum with builds.🤔

[Serenitynow]

If only I had a CNC.
I would be bombarding this forum with builds.🤔

I have the capability to make cabinents, but all of the internal electronic components are way over my head unfortunately .

I'm really looking for a good stereo pair to build, but finding a proven design with all the information that fits my needs is daunting to say the least.

[bassman_soundking]

I have the capability to make cabinents, but all of the internal electronic components are way over my head unfortunately .

I'm really looking for a good stereo pair to build, but finding a proven design with all the information that fits my needs is daunting to say the least.

Im planning to build several sets of speakers.
Like you my issue is one of crossover design.
Learning cant be done overnight, and the price of the parts isnt low.
Starting my projects off with prefab XO networks is what Ill do on most.
Later Ill go back and work on the crossovers with help from the forum hopefully.

[mobius]

Cubes are generally fine for subs because of the reason billfitzmaurice stated above - standing waves are of no consequence in a small sub box.

Because in order to generate a standing wave, you need that frequency to actually be produced in the box. Even without bracing, the standing waves inherent to small boxes are going to be above the frequencies a sub is producing so it doesn't matter if the standing waves between all opposite surfaces in a cube are all the same frequency, they won't be activated.

More important is to brace the sub so the panels don't deflect due to the changes in internal air pressure from the sub's cone excursion and to a slightly lesser extent, to also keep the natural resonant frequencies of the subs panels above the frequencies the sub is producing.

[fpitas]

Yes, it's obvious why the box walls shouldn't weeze in and out with the sub excursion. The sub panels can and will resonate at harmonics of the sub frequencies.

[mobius]

I think maybe what you mean to say here is that the harmonics of any subwoofer note can activate the natural resonant frequencies of the box walls when they are the same, which is true if there is enough energy to the harmonics. But if you are rolling the sub frequencies off 2nd order LR at 80Hz, well then there really isn't going to be that much SPL/energy to any harmonic frequencies above about 160Hz maybe even lower. The fundamental at 160Hz with the 80Hz xo is going to be down about 12dB or so to start with and the 160Hz 1st harmonic of 80Hz is going to be much lower in SPL than that to start off with. It'll be down even more if the receiver happens to use a 4th order LP filter. Or you set your sub xo lower than 80Hz.

So just make sure that the sub wall natural resonances are above about 200-300Hz and there shouldn't be a problem.

[fpitas]

Yes, in general you have a point, although some people roll subs as high as 120Hz.

[devnull]

Decent bracing doesn't have to be really fancy. I use simple stick built plywood window frame bracing. A double layer of plywood with offset joints glued and brad nailed together. Doesn't take that long to put together, a lot less waste than one piece braces.

If you want a quick and dirty test for panel vibrations throw a couple of bags of sand or cement on top of the subwoofer. That adds mass to the top horizontal panel and puts the vertical panels under compression making it harder for the panels to vibrate. The sound shouldn't change.

Just a couple of thoughts while I have my morning coffee.....

[fpitas]

That is true, it doesn't need to be fancy. I've used window braces as well as scrap pieces of 3/4" plywood, 1" wide. The window braces do look cool.

[billfitzmaurice]

If you look at Parts Express Knock-down sub enclosures. they have an elaborate bracing scheme.
.

They look impressive, but rail bracing is far less effective than panel to panel bracing. The same applies to window bracing, where the sections that connect the opposing faces do all the work, while the rails just add excess weight. The last place that requires bracing are the cabinet corners, being the strongest part of the box. Anything within six inches of the corners is wasted material and unnecessary weight.

[bassman_soundking]

I always felt the most important panel to brace was with drivers on it,
I have either under braced or over braced most enclosures 😂