Cotton traps: 1.5 pcf vs 3 pcf

[mobius.media]

I need to build some 4" cotton-based traps and I'm looking at 2 options for materials:

1) 1.5 pcf Bass Buster

• $44 CAD per 4"x2'x4' panel

Freq. 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz
NRC 4” 0.97 1.37 1.23 1.05 1.00 1.01 1.15

2) 3 pcf Echo Eliminator

• $52 CAD per 2"x2'x4' ($104 per 4"x2'x4' sandwiched pair)

Freq. 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz
NRC 2" .35 .94 1.32 1.22 1.06 1.03 1.15

So two things stand out for me. First, the the 3 pcf option will cost more than twice the 1.5 pcf option. Although I am rarely eager to spend more than I must, panels last a life time, so it's not something I'm eager to cheap out on, unless it's really better or no different for the result.

Second, I have no absorption data to judge the specs of 4" 3 pcf cotton. Additionally, looking at the chart for 1.5 pcf cotton, I am impressed by the high NRC, but somewhat wary about the 250 Hz bump and seemingly precipitous drop off below that. No data is supplied for sub-125 frequencies, but judging from the slope there I do not guess it would be good.

My only experience treating rooms is anecdotal. I have treated rooms with OFI-48 (703-type fiberglass) and Roxul RHT40 FSK. In my experience, I found the sound of the Roxul FSK far more favorable. It seemed to do a better job of tightening the low end without stifling the highs. However, I have no experience with cotton so I am not sure how the 250 Hz bump would sound in a room or what 3 pcf would sound like by comparison.

Currently, I am leaning toward using the cheaper, lighter 4" 1.5 pcf as cloud material as well as stuffing in behind the wells of QRD's. I may also make some triangular corner fills from this material to fit behind the straddled corner traps, cost allowing.

For the primary 4" corner traps though, I am still debating densities and don't really have much to go on. I imagine the 3 pcf would have a slightly reduced high frequency absorption, a lower bump (closer to ~125 centered, not 250) and better sub-125 absorption. But if I am doing the 1.5 pcf triangular corner infills, is this even worth worrying about? Am I better off saving a few hundred and sticking to 1.5 pcf throughout?

Any input is appreciated.

Thanks.

(Also, for anyone curious, in Canada, these two products are distributed by Technature in Toronto)