Thinnest practical enclosure walls?

[Mr Evil]

I'm getting closer to building my next project now, so practical considerations are coming to the fore. I want to keep weight and size as low as possible, so I was wondering what the thinnest cabinet panels I could practically get away with would be without the sound suffering too much.

The cabinet will be made of MDF and be about 6l in volume with internal dimensions of approximately 29cm * 11cm * 18cm (that's about 11.5" * 4.5" * 7.25" for the metric-impaired ), a standard cuboid shape. It wouldn't be expected to reproduce anything below about 125Hz, that being sent to a separate subwoofer for which panel thickness is not an issue.

6mm MDF is the thinnest I could buy. It seems to me as though that might be sufficient, as the small dimensions should still give reasonable rigidity, but having never made speakers this size with thin panels I cannot really be sure. The next step up is 12mm (I am plagued by lack of choice ).

Is there any way to calculate what the resonant frequency of a sheet of MDF might be expected to be?

I could add some bracing and attach damping material to the panels, but I can't do anything too extreme because the cabinets also need to contain an amp and accompanying PSU. Perhaps use a thicker panel just for the front baffle?

Your thoughts are appreciated.

[gozierdt]

The stiffness of a panel is proportional to the cube of the thickness. So if we take the stiffness of your proposed 6mm panel as 1, a 12mm panel will be 8x as stiff, and a 24mm panel will be 64x as stiff. So you can see your fighting a tough battle. If you used many small internal stiffeners, you could improve the situation, but at the expense of a lot of build complexity. That's why honeycomb panels are so strong. The honeycomb structure doesn't add much weight, but it supports the outside surfaces far apart. I'd go with a minimum of 12mm walls, and still add some stiffening ribs.

[johninCR]

Just give up the 24mm in each dimension and use 18mm minimum thickness. Manufactures often use 12mm, but that's why cheap speakers are cheap sounding. If you're going through all the trouble of building your own, do it right. While the cab won't be subject to significant pressures of a sub, the resonant frequencies of those sized panels will be within the frequency range of the speaker. Even with 18mm a shelf brace cutting that longest dimension in half is usually recommended. 6mm mdf just isn't going to cut it. Well braced 12mm might work ok, but I wouldn't.

[Volenti]

Use marine, or void free ply, it's far stiffer than mdf of the same thickness, it costs more but your enclosures aren't very large so the difference won't be much, shelf bracing and good damping material will still be needed though, you could get away with using 9mm ply, though 12mm would be preferable.

[Colin]

If you want to stick with your existing material, you could damp the 6mm panels with insulation board (old BBC trick) or, for mass, layer with plasterboard. But you must have a decent thickness on the front panel, pref 18mm, to mount the drive unit.

The other BBC trick is to lower the resonant frequency of thin panels by loading with Bitumin pads. I'm not sure how well this would work with a small enclosure but if you want to experiment, you can get these in the UK from Wilmslow Audio or Falcon Acoustics (they're both on the web if you do a Google UK search).

Colin

[BillFitzmaurice]

1/2 inch (13 mm). Thinner than that and you have joinery issues in addition to vibration problems. You'll need full shelf style panel to panel bracing, with no more than 6 inches (15cm) unbraced panel dimension on one plane. Cut holes in the shelves to reduce weight and also minimize reflective surfaces inside the cabinet. I design pro-sound cabinets that have to stand up to internal pressures ten times higher than hi-fi / HT boxes and still be portable and I never use thicker materials than 1/2 inch.

www.billfitzmaurice.com

[markp]

Get a hold of some Corian scraps from a kitchen counter place. It comes in 3/8" and would make a nice thin-walled box if it is that small. Remember to coat the inside with some sound damping goop.

[angel]

For joining 6mm panels, if you have a very patient person doing the cutting, you can interlock the plates battlement-style (that is, cut 1cm down, cut 1cm right, cut 1cm up, skip 1cm right, cut 1cm down, etc..), or some other, similarly difficult join.

Heed Colin's advice. Adding mass to a panel reduces its resonance. And you will probably want something like wool felt on the outside.

In order to increase the stiffness, you may want to use a matrix construction inside, like in a muffler. B&W used to do this for a while, but they stopped. Probably too much work (i.e. money).

To make a matrix, you are looking to divide the enclosure into cubes of equal size, with 6 holes in each cube (except the ones by the walls, of course), and stuffing material in each cube. Any joins should normally be done with an elastic glue, but I guess in your case you might want a harder glue for stiffness. Try to keep the total number of joins to a minimum; although making sure that you don't end up with all parallell surfaces and all equal lengths is a good thing. Using different kinds of wood is also good. The cubes adjacent to the walls may be subdivided into 8 smaller cubes, if you have the patience for it. This approach has been successfully used by at least one Danish enthusiast with great results, but realizing its maximum potential requires effort. Lots.

Make sure you make some shallow cuts in the walls so that the matrix is locked in place.

In any case, please double the front baffle thickness, and preferrably also the top baffle thickness. This is probably best accomplished by layering two 6mm panels of different woods, rather than using a single 12mm panel.

And if you don't use any of the other tricks, at least use a full-height vertical brace that you glue into some shallow cuts in the front/back/top/bottom baffles.

Hope this helps.

[wunhuanglo]

From your dimensions I calculated the following:

1.9 ft^2 surface area

6mm panels - 0.04 ft^3

12mm panels - 0.08 ft^3

18mm panels (3/4") - 0.12 ft^3

MDF is approximately 40 lb/ft^3

Veneer core plywood is approximately 32 lb/ft^3

So the thinnest MDF panels would make a 1.6 lb cabinet, the thickest at 4.8 lb cabinet.

Are three pounds so much of a difference you can't use 3/4" - 18mm material?

[CeramicMan]

How about curved fibreglass? You could sculpt the shape that you want out of stacked layers of styrofoam, and then glue the fibre-glass onto the outside. Styrofoam arguably has excellent acoustic properties because it's just like stuffing material except that it's suspended in a solid instead of air. You could also cut the layers of styrofoam to form an intricate sound-absorbing matrix on the inside of each "box".

The outer shell of fibreglass would be very strong even if it's thin, as long as the sides are curved (pref. spherically, not just cylindrically).

Possible problems: time-consuming and messy. A lightweight speaker-box always "reacts" to the "action" on the cone of a speaker. Therefore if the box is too light, its vibrations could make it rattle against hard furniture, or wherever you intend to put it. Those vibrations shouldn't result in any sound loss though, as long as the fibreglass is moving without flexing.