Firstly, I'm just a retired geologist, and a bass player with a long history of bass and hifi cab building - strictly as an amateur. With that said...
I think this really goes to cab voicing. The amount of damping and its placement depend upon the target voicing. There are no formulas allowing the calculation of the perfect amount and placement of acoustic damping material. A major problem to the calculation method is the need to describe a particular tone in a quantitative way. A numerical input would be required. Cab voicing is mostly done by ear. For non-voiced, hifi cabs, best engineering practices (crossover and cab modeling, and measurements of frequency response - on and off axis, distortion and phase - among others - are made. An engineer, who has a 'feel' for tone, designs and auditions prototypes having different drivers and crossover topologies. The best of these are sent to a few select players for tryout and provide feedback.
Unless a player simply enjoys fine tuning cab voicing, its best to account for voicing issues when purchasing/auditioning. The notion that a cab maker has not included damping material, or has not completely lined the interior in order to cut costs is, with the exception of low budget models, mistaken. If you find yourself wondering if your cab can be improved, you're probably best off doing some auditioning.
If a player wants to tweak a cab that may sound a little too aggressive or congested, adjusting the cab damping is an easy, reversible and potentially effective mod. For those who want to do a little tweaking here are a few guidelines. Placing some damping material on the area of the cab interior directly behind the exposed cone and ports has the most impact on voicing. Adding or changing damping on one of a pair of parallel interior walls pretty strongly impacts the tone. What can you expect to hear as a result of these changes. Only you can answer that. What I hear is the following: An undamped cab tends to have somewhat aggressive mids, sometimes even harshness, and can sound a little congested at higher (gig level) volumes - its hard to hear individual notes and overtones. Minimal damping on 3 (non-parallel) sides and behind the driver substantially reduces aggressiveness, harshness and congestion at gig volumes. A thin lining of all interior surfaces sounds much the same. Around 1 to 1 1/2 inches thickness (fully lined) the sound becomes hifi. At this point the audible effects of standing waves and reflections becomes difficult to hear. As we approach fully stuffed, the cab begins to sound lifeless, though a little less so for sealed cabs. The described effects strongly depend on the driver(s) open to the cab. The same damping amount and placement will yield different results with different drivers (and different cabs). Keep in mind that for some genres the harshness and congestion from an undamped cab may actually be desirable. There is nothing at all wrong with that. Also, that the changes to the tone made by different degrees of damping may be pretty hard to hear in a dense, high volume mix, especially if distortion/overdrive effects are being used. OTOH, I've heard some effects sound much better through a less congested cab - presumably, the sound of the effect was not masked by or buried in the congestion from the cab. You just have to try it and see what works for you. As a cab junkie, I have cabs for different desired tones. Just like with instruments. Want real Pbass tone - just use one.
