Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*
I was looking for a link showing ""no appreciable absorption effect"" when used in subwoofers since that is what is being discussed here. I've searched and can't find anything. Since I have 2 retired Shiva sonotubes on hand, I may pull the foam out of one and see if there is any difference. According to what is being stated here they should perform the same."No appreciable absorption effect" is in reference to credible co-efficient testing of different materials - as can be found in many databases. You can research/look up this data, or you can refer to Ethan Winer's RealTraps web site, which has(or had) a database of lab test measurements of different materials, including two foam vendors(Auaralex and Foam By Mail). Rod Elliot's site(sound.westhost.com) also has some testing of various material as it relates to absorption in a small midbass application, which the general results match up with known general absorption properties established by other testing.
The only appreciable effect the foam can be expected to have in your subwoofer application, is that it can in some instances, reduce the port(or passive radiator) output due to physical air-coupling losses that can be induced in the system alignment. However, this seems unlikely based on the relatively thin foam you used based on the subwoofer cabinet size. For foam to be effective at say reducing the long dimension half-wave modal resonance at bass frequencies, or 1/4 wave null effect based on driver to nearest boundary point; you would have to use a high grade acoustic foam, such as Auralex, in 5-6" thickness. Alternatively, you can use 4" of 6-8lb/ft^3 mineral wool board for similar absorption properties. It must be clear, though, that this amount of material will certainly result in measurable loss in port efficiency/output.