[Eric Swanson]

Several reasons why ply is better in this application. :

- Stability and differential movement. Solid wood moves A LOT along its width and thickness, very little along its length. Also is more likely to warp/twist/crack. Hence double bass tops/backs cracking when glued to long-grain ribs/blocking. The tops/backs shrink, the ribs don't move the same way. The dimensional instability of solid wood requires specific joinery methods for longevity. These tend to be more time consuming/expensive.

Also, when hit, (speaker cabs at work) solid wood may crack along its grain. Plywood just won't, because there is no grain to follow. A chunk of plywood wont break off when hit, like could happen with solid.

Also, because of the great movement of large widths of solid wood (averaging maybe 1/8" per foot), the fronts and backs of cabs would have to be plywood anyway. Otherwise, fabricators would have to use some ridiculously time-consuming engineering/joinery to keep them from ripping the boxes apart as they moved - the fronts and backs would have to "float" independent of the case sides.

- Joinery ease for the cases. In solid wood, the only meaningful joinery is long-grain to long-grain. To build a solid case side, the best joint is the dovetail (glue surface plus mechanical hold) or the box/finger joint (tons of glue surface). Both joints are relatively expensive to produce, compared to ply joinery options, with glue blocks in the corners.

In ply construction, even a butt joint provides some long-grain-to-long-grain glue surface. Add a dado, tongue and groove, glue blocks and staples and you are bomb-proof. Ply is inexpensive to join strongly.

It can also be joined to itself at any angle without thinking about movement. Backs/fronts can be slammed onto sides regardless of grain direction. Plywood has virtually no "grain direction" as far as movement goes, so parts can be glued together any old way.

- Economy. Material processing speed/number of steps, storage, purchasing, etc. Solid wood processing takes lots of toys and room. Ply processing is relatively simple/inexpensive.

Ply can be stacked in lifts, moved around by fork truck, and a ply sheet is ready to be made in parts. Solid wood needs to be milled, the dust/chips/rippings/off-cuts need disposal, and then you can mill your parts.

Sure, you can buy all the toys needed for a rough mill, but that's a lot of expensive stuff. You can buy milled stock, or have others custom mill, but that's an additional cost, compared to ply, per square foot of cab side.

Ply has great yield, especially for speaker companies nesting parts on a CNC router. Plus, if they are using solid and ply, they have to stock a bunch more types of material. If they are just using ply, everything comes from the same sheet.

Bottom line:
"Solid wood" is not always better, depending on the use. Folks think "solid is better." Not for heavy-duty boxes. It might sound better, but there would be a bunch more work involved. I can't talk about acoustics, but I can speak to box construction. Ply offers superior stability, fabrication ease, and economy.