Best way to decrease the internal volume of a sealed speaker?

I have a very fine sealed 0.75 cu foot cabinet that I would like to reduce the internal volume to about to about 0.45-0.65 cu feet. There is limited space to add things like bricks, pavers etc inside.
I am thinking of using some plastic containers with lids glued to the inside cabinet. Should they be filled with sand?


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Perhaps a little more info is needed.

This is for my DIY speaker project. I purchased high gloss cabinets that were assembled and sized for a particular woofer, but I have decided to experiment further and change the woofer.
The new woofer specs indicate that it would work better with a smaller sealed cabinet.
So, I would appreciate some creative suggestions to make the internal volume smaller.

How about using some Black Hole? It's pretty thick, so you may not have to use it on all surfaces, and it's very beneficial in controlling resonance anyway.
Add bracing---dowels attached to opposing walls. It will not only decrease the enclosure's internal volume, but make it more non-resonant (or at least raise the frequency at which it resonates, thus making the resonance less audible).

Thanks. but what is Black Hole? Is it a spray?


Thanks but how will the bracing be attached? The internals of this cabinet is pretty tight. See link below.


ozzy, Black Hole 5 is a 1-3/8" thick cabinet damping pad with adhesive on one side. You attach it to the cabinet's interior walls to damp panel resonance and absorb internal reflections. GR Research sells an even better panel damping material named NoRez.

Bracing dowels can be secured to interior cabinet walls with wood glue or epoxy.

Cascade V bloc..Thick purple paint..sucked into the mdf..seals the contents of the wall material from the energy robbing glue and saw dust that kills the dynamics. This product drys hard like concrete.. You could add a quart of micro bearing steel to increase the amount of base solid content. Dont use sand, kitty litter, rice, or lead..Tom
Black hole is very effective. I use it in my Klipsch Epics and the improvement was very noticeable. Glad to hear about the GR Research No Rez. I will look into it.
For experimental purposes (trying to find the correct internal volume reduction for a given sound) you could cut off various lengths of appropriate sized blocks of wood. Easy to do and to dial in to your preferred sound. Once the amount of volume reduction is known, you can choose a more permanent final solution, if you like.
I second Black Hole 5 - I used it on a DIY speaker project.  It is meant to go only behind your drivers for rear wall dampening.  If you put too much (like the entire internal speaker) you will rob the speaker of bass.

As Bdp24 mentions, it comes with a very strong adhesive backing.
Wow! Thank you all for some really creative suggestions.

Questions though:
It sounds like the Black Hole 5 would be similar to adding wool, fiberglass, acousta stuff etc. material. These materials are used to absorb and to compensate for a smaller box, not reduce for a larger box. But, I will investigate.
How is the Cascade V applied?

Adding wood bracing inside is the best way to go, but the inside of the cabinet is curved with very little room to use tools inside.

How about PVC pipe sealed at both ends?
Or a cardboard box sealed on the outside of the box with that Black sealant spray that I see on TV?
Can the spray expandable foam be used? Or will that absorb? Perhaps after it is applied I could spray the Black sealant over it? But that would be messy and stinky for a while.

The foam will work, but it expands a lot. If you go that route, use the window and door foam. It is minimally expanding. But it does make a mess! Clean any up that gets on the outside of the cabinet immediately. You’ll need paint thinner for that. It should be acoustically inert, as long as it doesn’t break free..

What about cutting pieces of MDF that equal the volume reduction you require, gluing and screwing them together, then gluing them onto the bottom of the cabinet? Wood glue will be stronger than the mdf. You could also use dynamat.. I put some in my speakers to increase bass clarity and reduce sidewall resonance, but in a small speaker like that, it will definitely decrease the internal volume a relatively significant amount.
In most cases there is way too much stuffing, fiberglass or whatever, in speakers. The best results will be obtained by removing most or all of the stuffing. Let the speakers breathe. Any high frequency ringing, etc. internally can be handled with a small amount of hollow fiber wool, about the size of a grapefruit. 🍊The worst thing I found with bass drivers was the dust cap ringing. That can be addressed with a small damper such as the Acoustic Disc from Golden Sound.

The Cascade can be applied by a heavy brush or even trowel like device. I have used this product on several cabinets. I kept track of the volume installed by using a platic cup filled to volume. You want the same = amount in each box. Apply more to all joint seams and corners as this will further stiffen these areas. Drys to a concrete like substance and grainular in feel. The micro bearing fill added in the gallan pale will add more mass and will be even more dispersive to resonance. I prefer long hair sheeps wool as fill material. You could even add non ferrous rods installed near each internal corner but this may alter the cabinet cosmetics while improving the sound. Dont add any right angle shapes to the cabinet..More you can do with the driver and mounting...good luck. Tom
Use Styrofoam blocks - easy to install and easy to remove too

Best of Luck


Will the expandable foam absorb sound like the other products such as wool, fiberglass etc.? If so I think that would be the opposite of reducing the internal box volume. That's why I thought of sealing the exposed area with the Flex Seal spray.
Cutting the pieces of mdf wood and installing them as you suggested might work. I guess I worry that the glued wood may come loose and then rattle or move around.


I will look into the Casacde V sounds interesting.


I agree and I was intending on limiting the amount of fill used.


Will the Styrofoam blocks absorb? Should I coat them with something?



I agree with Geoff limit the amount of damping material. The Cascade reduces the drag of the glue and saw dust and increases the dispersive area of its solids. MDF which is probably in use here..sounds like a dark muffled congested cough..the Cascade seals out that character. The sheeps wool acts like a comb filter rather than damper of character. Tom
If you use Flex Seal, it may have petroleum distillates in it, as the more expensive rubber sprays at Wally World (10-12 bucks) will have. However, the cheaper stuff (2-4 bucks) doesn't have pd's. Pd's are rather corrosive to electronic circuitry.

Automotive undercoating spray (same thing as Flex Seal really and is what you'd be looking for at Walmart or your hardware store) will dry overnight to roughly half the thickness at which it was applied. However, after several days, the depth of the coating will continue to shrink and then be no thicker really than a coat of paint, so it might require simply too many coats and too much drying time to be of practical use as a way to reduce volume on its own...but good as a sealant, though. 

Flex Seal could possibly be thicker, but I have no idea if it contains distillates.

Styrofoam, I would think, would not absorb too much, although something that low mass may be excited to resonate a bit maybe, coating it with the rubber spray may help somewhat. 

Any object left inside the cabinet must Not be hollow, fill it with sand, if nothing else!
+1 Audiotweak

Best cutting edge advise so far
The last thing you want is flexible materials. They will increase your internal volume.

If you want to permanently increase it, and don't have room for the usual brick / wood block ideas I suggest a 2-part mixable epoxy that dries hard.

You may also experiment less permanently with buck shot. Placed in bags it's relatively easy to arrange and move.

Styrofoam will do exactly what you need it will  not absorb no need to coat it and best of all you can infinitely tweak by cutting off material  until you have the correct volume before you make anything permanent. 

Best of luck


Thank you for that info. I was thinking of just using the flex seal over the Styrofoam IF the Styrofoam alone would act as an absorbent. But it sounds like it does not.


I will order some of the Cascade to be used as a inside lining.


Wouldn't a mixable 2 part epoxy require putting it into some type of mold? What type of thickness do you suggest?


Not sure what type of Styrofoam thickness I should be using. I was thinking 2-4", does that sound about right? I'll try Hobby Lobby first.

Best way to decrease the internal volume of a sealed speaker?
Liquid Nail a few bricks at the bottom, will make them feel heavy and expensive.

Cheers George
So using the Styrofoam...

Please correct me if my math is incorrect or if I am on the wrong track.

First my assumptions.
1 cu foot is equal to 1728 inches.
My present speaker cabinet is 0.75 cu feet or 1296 inches.
The woofers I am getting are said to perform best in a 0.45-0.65 sealed cabinet.

So, if I use 0.45 cu feet as the goal that would equate to about 778 total inches.
I have found available a 8x8x8 styrofoam cube and that would math out to about 512 in total inches.
So, if I subtract the 512 from my present 1296 it would leave the internal volume to be 784 inches or a little more than 0.45 cu feet internally.

Does this sound right???

Why not use the higher number as the goal? Actually you're almost there without doing anything. By the way, that should be cubic inches, not inches. 
Post removed 

So, what is the negative of having a slightly larger internal volume than optimum size? Lack of bass definition? Low efficiency?


Can you provide a source to obtain the Cascade V?  I see something called Cascade listed at Parts Express but it is a spray. Is that it?

BYW, The woofers I have ordered are the ScanSpeak Revelator 18w/8532G-007

Oz, FlexSeal does contain 'aliphatic petroleum solvent' & 'liquified petroleum gas' save it for your roof or 'cutting/repairing boats' ( right, sure...)....;)

Black Hole does sound like a win...I'd coat the interior with a white latex first, for the seal & dust issue and it'll ensure the BH will stick 'n stay...

Your calcs are correct...*G*  I had a thought as to how you might 'dial' the cabs in for your SSRs', since the difference between 173 & 519 is rather large.  It would entail a bit of swapping the face panel on & off, but the cab does have the hardware that would make it less painless...

A typical 4x4 from Home Despot...nominal 3 5/8" sq. = 13.14

173 / 13.14 = 13 1/8"  for .65 cu.ft.
519 / 13.14 = 39 1/2"  for .45 cu.ft.

One 4x4 @ 8' ought to be enough for both.

Divide those lengths to fit within, add/subtract until you like what you hear.  Duplicate for cab #2.  Glue down, paint, apply BH, final assembly.

Crack a cold one and enjoy the fruit of labor, etc.

Ought to beat mucking about with foam...'interior aesthetics' are up to you. ;)  That interior bracing panel shown in the PE pics makes doing something on the back panel to break standing waves front/rear (one of my pet peeves..) difficult to address....but some of my projects go off the deep end with such...

Good luck...*S*


Thank you so much for your information. I am now leaning towards the higher volume 0.65 internals. This means I only need to reduce the inside by 173.

I guess the only issue I have with using 4x4’s is that they would probably need to be just glued at the top inside. And then the  cabinet with the 4X4’s glued would be placed on its back to finish the speaker / front baffle install until it can be uprighted again to rest on the stand.
So, I am not sure how well a glued 4x4(s) would stay stationary during that movement. Does liquid nails work that great?

Oz, I'm not a big fan of Liquid Nails...I'd suggest a tube of 'fast set' silicone caulk, applied 'bout the diameter of toothpaste... Somewhat 'stinky' (acetic acid, a.k.a. vinegar...*L* Fades away soon enough...) but not toxic to you or electronics....

Sets up in about an hour...since the 4x4 pieces are small, there shouldn't be any issues.  You could cut the 4x4 into smaller pieces (2, 3, etc.) to make it less of a 'hunk', and distribute them within the cab.  Smaller will be less likely to come loose, regardless of how loud you push them... ;)

BTW, re the 4x4....try to select one that's as 'dry' as possible.  If they're all 'wet', let it sit for awhile either in the sun or indoors in a warm space.  4x4s' are typically pressure treated for exterior use and can be moist.  It's a copper-based solution; again, safe to you and gadgets.  If the wood's dry(er), the silicone will adhere better....

Have fun, mind your fingers around the saws....;)


1 gallon displaces 241 cubic inches. So a gallon of Cascade when cured should shrink down to the volume you would need a gallon for each box..They would be well sealed and more rigid. Tom 
Why not simply get a quantum device from Geoff or an air compliance device from Bybee?

Why bother fiddling with foam, glue and other physical stuff and having to open up the speaker.

Simply employ one of these quantum mechanical devces to warp the space time dimensions of your speakers or change the actual behaviour of air compliance of your entire room to have the same desired effect.

Engineering by length, volume and acoustic properties is so old school.

Alternatively your speaker wires can be useful in changing speaker response as effectively as any physical change. A change of 1 meter in cable length is just like a volume change of 1 liter.

Alternatively, your thanksgiving turkey is perfect for stuffing a speaker but may work even better if placed on your head.

Here is a DIY instruction video


I take that back ..3 coats would be enough..30 coats would take weeks to cure. Though the volume would be right. Tom 

Thanks. First I’m going to try the Styrofoam. I have a piece that should work out fine cubic inches wise (177.42).
I will be gluing this underneath of the inside top. I just hope it doesn’t come loose or rattle.


Too much holiday cheer?

Sorry for the late reply.. the expandable foam will dry with a smooth surface, unless you mess with it. It will be flexible, though, so take Erik's advice into account. I'm not sure how flexible materials would effectively increase the internal volume, but Erik doesn't tend to talk out his butt and has much experience building speakers.

That said, nothing wrong with experimenting.

If you do use 2x4's, considering using blue tack to hold them in place until you're happy with the sound. Then, if the cabinets are unpainted MDF on the inside, consider wood glue. It is very strong when gluing wood together (duh) and would not be flexible like silicone, which could add too much damping. Or maybe you need the extra damping of the silicone.. I suppose you could start with silicone and, if it's over damped, cut it out and use wood glue.
By the way, those cabinets look quality. Let us know how it all turns out

As I see it, you want to take out between 0.10 to 0.30 cu.ft.

Use a cu.ft. to calculator to convert. I would use 2x4s.

Start out with the smaller piece of wood then adding more blocks until you get your desired sound. Use double sided foam tape (3M @ Home Depot) that way you can add or remove the blocks.

I'v built quite a few speakers an using the calculator helps greatly in figuring out interior Cu.Ft. or Inches.

Hope this helps.


Seal the sawdust and glue with something...varnish the inside a couple of coats or so.Tom
toddverrone, cliffkhz, theaudiotweak,

Thank you for the help and the posts.
The woofer I will be using will be located very close to the bottom of the cabinet. Because of this it will be near impossible to place wood at the bottom of the cabinet.

The upper inside top appears to be the best place for any type of cabinet internal volume reduction.
So, at this point, I am first going to try the Styrofoam route. I think I can glue a chunk of it to the inside top. If this rattles or comes loose then the next plan would be to change the front baffle so that the woofer can be raised up and allow the installation of wood or bricks etc. at the bottom.
The front baffle is 1" thick and cutting openings again for the ribbon tweeter and the woofer is not an easy task. 


Well I've changed my mind about the Styrofoam.
It looks like I can install some 2x4's pieces inside. I have some 2x4 material made out of Trex that seems to be quite dense an much heavier than a regular 2x4.

I just installed the Trex 2x4's.
I installed (2) of them 18" long on each long side placed in the middle of the cabinet.
I figure with that wood my cabinets math out to be .62 cu feet. I already can tell the difference with tighter and deeper bass. The woofers will probably need some time to break in fully, but at this point I am very happy.

I really want to thank you all for the ideas and support. 

Good to hear! (See what I did there?)

Synthetic wood was probably a good idea.. denser and not affected by humidity. Well done
Trex....yeah, better than that pesky cellular stuff....good call. *G*

The problem with those " quantum mechanical devices" is, if it 'goes south' (or Whatever 'direction' one wants to call That...) is that parts are just insane to's not the $, it's the shipping and the 'back order wait state'....
I’ll second georgehifi’s suggestion of liquid nail and bricks - with a twist; use 4 bricks. Glue them as close as possible to the centres of the sides and back and the available space near the tweeter on the front. Help strengthen and damp the walls while you reduce volume. I believe most cabinets will benefit substantially from bracing. Let you think about it. Cheers. Pete
Google what is trex made of. All the same crap mdf is made of except trex has the added incredient of shredded plastic and probably more ga -luue as a binder for this compost pile. Tom
toddverrone,  asvjerry, ptss, theaudiotweak,

The speakers are breaking in quite nicely. I am very pleased with the results.
Thank you for your help.