[DanDan]

I am of the opinion that more is generally better. Therefore thicker is better. Unfortunately I have never performed comparitive tests. I will when a suitable opportunity presents itself.
For now I can only say:-
1 My intuition says more has to be better, in the case of depth of Bass Trapping.
(within reasonable Gas Flow limits)
2 I have achieved remarkable results at 34Hz using 34 inch wide Studiotips Superchunks.
3 This may be heresy but I am going there for the sake of teasing this out.
Fox- I am not at all convinced that consideration of wave velocity alone gives a full picture. By the way are we talking about wave velocity here, or particle velocity, or particle displacement?
Let's look at this from an observational, intuitive point of view. For example, generate a sine wave at a room mode frequency. As one moves towards a boundary the sound level increases dramatically. Surely material placed closer and closer to that boundary will affect the sound increasingly. It is hard for me to imagine that it wouldn't.
Another way of looking at it- Let's say the first axial mode, lenghtwise, of a typical room. Say 40Hz. Generate a sine wave at this frequency. At the front and back wall the sound level will be extremely high, with velocity at minimum. Conversely, at the room centre there will be virtually no sound, but velocity must be at it's peak.
If it were otherwise possible, would one put a Bass Trap there? Again, hard for me to imagine that it would work best there.

DD