Tuning effect of damping material volume

[fbov]

I'm making a pair of ~70l NatP tower enclosures, and by chance have a large amount of Whispermat-like accoustic foam, enough to cover most of the interior of these enclosures. I understand that would be overkill, but it leads me to ask if I should subtract the volume of absorbent added when calculating an initial port length.

I understand that many absorbent materials are transparent at the low frequencies involved in port tuning, so they have no effect. I'm wondering if that's true of the heavy, high-end stuff, too.

thanks,
Frank

[Carl V]

Essentially no. Products like BH5 which have a thick & dense
layer will decrease the volume....maginally. Ultimately, you will
be adjusting port lengths after listening & measuing impedance
@ Fb.