The bass reflex "part" of the cabinet needs and uses the airborne sound inside the cabinet. Ideally the sound which has the bandwidth of the bassreflex port shouldn't be lowered by absorbing material. The standing waves however are not desired because they cause peaks with a high amplitude and a small bandwidth aka resonances. Resonances are one of the worst distortions you can get. From that perspective an ideal bass reflex cabinet should have standing waves which are in the higher frequencies. Ideally the bass driver should already be faded out completely by the crossover so that the standing waves aren't exited. But in reality you have to deal with compromises.I take that absorbing internal sound transmission of the cabinet is beneficial or desired, but isn't a loudspeaker primarily designed to benefit from the cabinet internal amplification?
All cabinet walls can be seen as membranes where a bitumen foil adds mass which lowers the resonance frequency aka the first eigenmode. This eigenmode typically transmits the most energy into the cabinet and therefore also to the outside air since it is coupled by all outside surface movement of the cabinet walls.Can you explain a bit about normal modes and how can bitumen shift them to lower frequencies due to the higher mass? Isn't a thin bitumen adhesive pad irrelevant to the speaker's total weight? I have also read that bitumen doesn't absorb bass, it actually reflects it, but it absorbs the wall's resonance. It might pair well with felt and/or polyester sheet on it, so it absorbs some of it aswell. It would absorb air pressure for the bass port, though.
The bitumen can absorb surfaces movements to some extent but it is frequency dependent and higher frequencies are absorbed much more efficiently.
Since the first normal mode is relatively low with an unbraced wall of a typical box bitumen can't do much in these lower frequencies.
But bracing can. Ideally it prevents the movement of the part of the wall with the highest amplitude, which is the middle of the cabinet wall. This shifts the normal mode of the cabinet wall way up. Ideally so much up that material like bitumen or material which absorbs airborne sound works effectively.
You can mount two or more wood parts through the bass opening of the cabinet and glue them together and against the cabinet walls.How can you add bracing to a finished cabinet such as the DBR62?
Focus on the parts of the cabinet walls which have the largest unbraced area. A support for the magnet of the driver can also be beneficial since it can also move and cause structure borne sound, which travels through the cabinet. Have a look at the enclosure of the e.g. a KEF LS 50 they put in a lot of effort.
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