Acoustic Egg Crate Foam tests:
In order to test the sonic effects of lining speaker enclosures internally with 2.5inch base-to-peak acoustic "egg crate foam" which Parts Express sells as #260-315, I set up the Clio microphone at a height of 15 cm above the plane of the chosen driver surround (a closeout widerange driver made by Vifa/ScanSpeak, 5" #299-246) which was placed in a cardboard baffle (55cm x 59cm) resting on a 50cm stack of the same foam all on a thick carpet.  The 15cm gap from driver surrount to microphone allowed the insertion of 2 pieces of the foam oriented back to back, that is, peaks away from each other.  I reason that sound waves within the speaker enclosure must pass twice through the wall-lining foam, thus the two layers.  To keep the foam from actually touching the driver surrounds, 1.5cm stacks of magazines were placed on the baffle adjacent to the driver supporting the inserted egg crate foam.  The setup is shown below without added foam on left and with foam on right.
There are two different batches of the same acoustic foam, easily distinguished by their shade of gray.
I thought that the positioning of the foam relative to foam peaks and valleys might alter the sonic filtration, but as is shown in both the MLS and Sinewave frequency responses below, such is not the case.  Below 500 Hz, the insertion of this foam increases measured bass response.  I suspect this results from an acoustic lens effect, since severely off axis bass waves will travel through less foam (the valleys) and will travel faster causing the wave to bend around toward the microphone.  This anomaly shouldn't apply to the effects of foam lining the inside walls of a loudspeaker enclosure.  That the foam does decrease the speed of the sound waves is apparent in the waterfall decay plots shown at the bottom of this page.  Beginning at about 800 Hz and above, the foam also attenuates the sound energy.  The higher the frequency, the greater this attenuation.  Clearly, this effect is desirable so that mid frequencies (and higher) are less likely to cause wall vibration or escape back through the cone or any ports into the listening environment.  Because the wave velocity is reduced by the foam, it is clear that lining the inside of a loudspeaker enclosure will increase the box apparent volume.  I'm not clever enough to see from these measurements by how much the box volume would appear larger.  Therefore, I shall next compare the results in the same setup interposing other damping materials whose effects on apparent box volume are better understood.  (R19 fiberglass, for example)
More damping measurements!