Closed Back Cabinet Design
So, open back cabinet designs aren’t that great for bass – unless you are after an unusual sound. What about closed back designs then? Looking at them helps to understand a little more about why your bass cabinet is as it is. [By the way - exciting news of a new competition coming up very soon!]
Closed back (or infinite baffle) cabinets are a little heavier (not by much, just the weight of a fuller back and some other internals), and they produce a sound with much more bass than those early open backed cabinets. However, they do have their own set of challenges.
Cabinet Construction
As the air is trapped inside of the ‘box’, bracing – originally in the form of wooden struts – is required to stop the cabinet sides vibrating and distorting the sound. Similarly, damping – glueing padding to the inside of the cabinet or wadding inside the cabinet – is also required to stop undesirable resonances causing the sound to ‘ring on’ after you stop playing a note. If you have ever opened up a cabinet you were probably surprised at how much was actually inside it!
Supporting Bass Notes – How Low Can It Go?
How low a note the cabinet can reproduce is still dependent on the size of the cabinet – because of that trapped air. It is actually dependent on a combination of the design of the speaker and the size of the cabinet – with some pretty scary maths to work it all out. These days that is all done by computer.
The other design penalty is in the efficiency of the cabinet. That means how much power you have to put in to achieve the volume that you want. Something to keep in mind when choosing an amp and speaker combination. Your amp might be loud enough with one cabinet, but not with another. Differences in the efficiencies of cabinets is the reason.
There is one last tweak of the design which gets us to the design of most of today’s cabinets…
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Hi, Closed back is not infinite baffle. Open back would be infinite baffle. A sealed box is “acoustic suspension” certainly not infinite baffle.
Hi Alex,
Technically an infinite baffle would be a speaker mounted in a board that was infinitely wide and infinitely tall – not that practical
– so yes, a closed back / sealed box might be better described as a speaker in acoustic suspension.
An open back, in the way that a real bass cab is, is a finite baffle, since the surface distance the air pressure waves have to travel from the front of the speaker to the back is ‘finite’.