So what is the answer to all these question about what to use to build treatment? Like so many question with math and science, the answer is, it depends. Like the question "solve for X", the more specific the details, the more specific the solution. If your question is really general, so too will be your answer. Like so many topics on the web, there are truths, half truths and outright falsehoods. It is no different than the myth of using egg cartons on the wall from years ago. It is not an outright lie, I mean you put them up and they do something. Probably something pretty hard to quantify but something. So when I answered the OP question, I stated, it'll do something. Unfortunately, that something is something you'll not be able to predict and
So lets start with the idea that there is no perfect absorber, no perfect reflective surface and there is no such thing as a perfect room. All studio design is compromise. One thing for another. Anechoic chamber, nope, not perfect. Funny thing, an anechoic chamber doesn't even need to be built with absorptive materials. Navy built one that is all stainless steel wedges and is anechoic via diffusion. Talk about a solve for X problem.
Density, what is better, more or less? Answer, it depends. What problem are you trying to solve is a far better question. 25 years ago, Owens Corning 703 & 705 had nearly a mythical place is studio building. It was the gold standard for building treatment and for simple fabric covered panels and a lot of commercial products, rigid fiberglass is still used. So what is the problem with it. Well, within its limits, nothing but either the original testing data was faulty or it has changed as the current test data shows it a lot less effective than the coefficient charts showed years ago. These sound absorption coefficient tables would show how effective they were for 1, 2 and 4 inches thick. It wasn't uncommon to extrapolate that out. You know, more is better. But is it? Data never said it was, people just thought it was. Truth is after 4 inches at low frequencies, rigid fiberglass becomes acoustically reflective and not absorptive. So only so much bang for the buck if you will.
The other irony is that the ratings only went down to 120Hz. Anything below that is in the pressure and not wave region of energy. Does the range for bass stop at 120Hz
Rigid mineral wool such as Safe n Sound is better going deeper than rigid fiberglass but it too plateaus' as it gets thicker eventually. Fluffy fiberglass starting at around 10 inches or so is about dead even with 2.5lb density or 10000GFR mineral wool. After that, fiberglass absorbs lower than rigid fiberglass or mineral wool. You can enter the data yourself as there are a few online calculators that will graph it out for you.
This graphic is a comparison of 2ft thick insulation choices between Safe N Sound mineral wool and R38 pink fiberglass. The blue line is pink fiberglass. If you are looking to build a bass trap, which of these would you choose? The mineral wool because of its density become reflective and starts to bottom out at 400hz yet the low density fiberglass goes nearly 10 times lower in frequency absorption.
I had a decent size problem down low so I went with 2ft deep bass trapping all along my back wall using R38. Had I used a higher density material, I wouldn't have hit my target. It would have trapped bass, just not low enough.
Don't need to take my word for it. There are plenty of ways to verify what I am saying using the tools that are readily available online.