A speaker cabinet, also known as an enclosure or housing, plays a pivotal role in enhancing the overall audio quality and ensuring accurate sound reproduction from a speaker driver.
The speaker cabinets job is multifaceted. It holds the speaker drivers in place while providing a rigid coupling. The second job is to provide a volume or air within the cabinet which acts as an air spring and allowing the natural frequency of the box be tuned for deep bass output — a fundamental factor for a full audio spectrum.
And the last key job of the speaker cabinet is to absorb all the backward rearward soundwaves and prevent them from reaching the listener. Hearing the forward and rearward soundwaves does exist in some designs (i.e. Open baffles), but for truly accurate sound reproduction we need to only allow the forward waves to radiate into the listening space.
But here is the thing, absorbing the rearward soundwaves is no easy task. These soundwaves carry immense amounts of energy that need to be absorbed and dissipated; if not, our speaker will sound muddy and unrealistic.
So how do we absorb these soundwaves you ask? Well, we rely on acoustic damping materials, which slow the waves movement by converting the acoustic energy into heat and as a result, attenuating these rearward waves that are bouncing around inside the speaker cabinet.
But, still, the acoustic damping material can only do so much. Residual vibrations are carried into the speaker cabinet. You may have felt these vibrations by putting your hand on the outside of s speaker. At loud volumes these vibrations can be quite extraordinary.
Speaker companies will do all sorts of things to control cabinet vibrations. They sometimes go as far as making speakers from solid aluminum, which weigh 100's of kilograms and cost 10's of thousands of dollars to manufacture.
In DIY audio, we rely on bracing and thick cabinet walls to achieve similar results. For instance, our Trohet speaker design have been designed from dual layer cabinet materials and proprietary bracing, which all reduce cabinet vibrations.
But there are additional things we can do that are affordable and easy to achieve. My favorite is to line the speaker cabinets with a flexible damping material. Either rubber or lead sheeting works very well.
But what type of rubber?
Either use mass loaded vinyl, which is a dense rubbery material. Stores like Jaycar and Radioshack sell this material. Not exactly cheap either.
A cheaper alternative is to use 5mm thick rubber sheet sourced from your local hardware store. If you in Australia, then Bunnings sells rubber sheet for around $40/m2.
To glue it down use contact cement on both the rubber and internal cabinet surfaces.
I will say that doing this to a prebuilt set of speakers is very fiddley and is far easier to do before the speaker cabinet is glued up. If you plan on building a pair of the Copenhagen's or Trohet's, then you will receive details on how to apply the rubber before the glue up stage.
But one question lingers. Why don't all big-name audio brands do this? The answer is cost and weight. Adding rubber sheet adds additional manufacturing costs which reduces profits, and it also adds weight, increasing the cost of logistics.
As part of our course, we recommend that everyone applies this cabinet damping trick, its cheap, easy and the effects are huge.
Take it one step further and instead of using rubber, use lead! Info on applying lead sheeting is included in our course notes if you're interested.