Having removed the drive units you can now take a look at the inside of the cabinet. You will probably be greeted with white fibre or grey foam, or perhaps a mixture of both. The white fibre is commonly known as BAF (Bonded Acetate Fibre) and is fairly good at absorbing midrange frequencies. The foam is normally used to line the walls of a speaker and acts in two ways. One is to absorb some of the midrange frequencies and stop them getting through the cabinet walls. The main reason for using it, though, is to 'damp' the bass resonances inside the enclosure.

There are two types of foam commonly used. The cheaper type feels stiff and rough to the touch. This is useless as it is too stiff to do its job. The other is soft and spongy to the touch and is fairly good at its job.

Again, note down how the fibre/foam is distributed inside the speaker before removing it. The speaker designer should have experimented with the placement of the foam/fibre inside the speaker, so you will need to replace it exactly as it was positioned in production.

Sometimes you find nothing, or not very much fibre/foam, inside a speaker. This may be because the designer was trying to encourage bass resonance or it may be that the manufacturer was a cheapskate! If you want to improve your speakers because the bass is lumpy or overblown in character then this is the first area to attack.

Improving the cabinet damping material can result in a reduction of bass boom, and a corresponding increase in bass clarity, and an improvement in midrange clarity and detailing by removing coloration caused by internal reflections.

Remember that the cabinet damping foam or fibre is there to do two things - act as an acoustic absorber for the sound swilling around inside the cabinet, and to damp low frequency resonances.

Panel damping

With all the internal foam/fibre removed we can now attend to resonances in the structure of the cabinet. If the manufacturer and designer have done their job well then the structure of the cabinet will be damped by either internal bracing or bitumen lining or both. You can easily check the effects of cabinet resonances by tapping the sides of the cabinet. If any of the panels produces a resounding woody 'boing' then these need treatment. Don't forget that the noise you produce from tapping an empty cabinet will sound 'hollow' due to the empty, coffin-like, box. What we are looking for is woody resonances in the panels, so ignore the hollow sounds emitting from the drive unit holes.

Any large, unbraced, parts of the cabinet structure are ripe for damping treatment. We use the Soniqs PDC - a 6mm thick, self adhesive, bitumen based mass damping medium - to 'kill' panel resonances. Alternatively you can try automotive shops to find panel damping sheets. Usually these are quite thin - they are designed for metal, not wood - so you may need to layer them to build up the mass.

This type of panel damping works by lowering the frequency of the resonance, to make it less audibly obvious, by increasing the mass of the panel and then damping it by resistive flexure in the material itself. Bitumen based pads have the advantage of high mass combined with resistive pliability. They do need to contact the wood panel securely, though, so remove any loose dust from the panel first and then, if not self-adhesive, glue down with an all-purpose glue and press firmly into place.

You will need to cover around two thirds of the central part of the area of any large, unsupported panel. The Soniqs PDC compound comes in a roll which is 150mm wide, so it is easy to use strips to cover any interior panel surface requiring treatment.

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