TLDR: We got started with very large speakers. They weighed too much and were difficult to use. We now build smaller speakers with lots of bracing that are (relatively) light and easy to use.
When designing new speaker cabinets, we employ three engineering constraints for all of our boxes:
1) All cabinets must weigh less than 100 pounds
2) Two dimensions must be no more than 22.5 by 82 inches
3) All parts are cut from sheets of plywood
The first constraint is so that one person with a hand truck can move the boxes around. The second is so the finished units fit through a standard door. And the third is so the designs can be easily reproducible.
When we first started designing custom cabinets, we made them with 3/4″ ply because that is what was in our original (super heavy!) industry-standard boxes. Most designers used 3/4 ply because it can be cut using standard wood tools, it is strong, and… the pieces fit together nicely without requiring high precision. Thicker wood allows for more error. The main drawback is that thicker plywood weighs substantially more than thinner plywood.
We now, exclusively, use 1/2″ plywood. The main drawback of using thinner plywood is that it requires far more bracing.
And that is what this post is all about.
Below are some photos of our old, 300+ pound monster speakers that were originally built to fly (hang using rigging) at the Shoreline Amphitheater.
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Time to dive deep…
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We use 1/2″ (.47 inches actually) birch 4×8′ sheets of plywood for our speakers. Most people still choose to use thicker plywood: 3/4″ or even 1″! Thicker plywood is stronger (more ply sheets and more glue).
With speakers, you want all of the energy to be channeled in the direction of cone (we call them drivers). The more flex your cabinets have, the more wasted energy. Wasted energy means less sound volume because the force that otherwise would be moving air (and creating sound) is absorbed into the sides of the box. In extreme cases, this movement within a cabinet can even create a noticeable change in sound/tone. The less your boxes vibrate (expand and compress), the louder and more pure the sound.
It’s simple physics. A rigid box will produce the sound coming out of the driver. A flexible box will absorb a lot of the energy and produce muffled sound.
While this pertains to what we hear, there is another aspect of stronger plywood that most people do not think about. Our company is called Little Bass Heads because we obsess over bass. Bass is created by quickly moving large cones extreme distances (sometimes multiple inches). A low bass sound is 30 Hz (Hz = cycles per second). Our bass speakers are normally 18″ so for a low bass note at high volume, the 18″ cone can be moving 3 inches 30 times every second. That’s a whole lotta shakin’ going on. We regularly blow apart our prototype bass boxes!
We prevent disaster by using bracing. Lots of bracing. Bracing everywhere you look. We design our bass boxes to have no more than 7″ of unbraced plywood. 1/2” plywood with bracing is an inch thick – which more than doubles its strength.
In the images below, we use 4 triangular ports (in black) to tune the box below 30Hz. All bracing (in red) is placed for unobstructed airflow.
1) For the sections running down the middle of plywood faces, we add a perpendicular-to-the-face, 2″ brace to add the incredible “resistance to bending” of an I Beam.
2) We brace the handles specifically where the cabinet design uses an inset to allow for flush-mount external handle hardware. (Without this bracing the plywood would be an unacceptably thin 1/4″!)
3) We rabbet all of our walls which make the joints stronger but weakens the individual pieces of joined wood. The braces in the corners are designed to add strength exactly where it is needed most by creating corners that are 1″ thick.
In Summary: By designing with 1/2″ plywood and creative bracing, we have reduced our fully-loaded 18″ ported bass cabinets from 90 pounds to just over 60!
24-07-15 – Middle PB18 (Fusion Link)
24-07-15 – Middle PB18 (File Download)
