Fantastic, thanks everybody!
I'm getting the idea about this and will read more here and at John Sayer's forum.
I have the power tools; don't see any problem doing this giving a bit of time. Need to get a handle on why the splayed walls are important and how much wall surface should be covered with panels, but I suppose that will become clear as I go. I also read somewhere that mixing panel types (703 + 705?) might help balance the absorbtion across the spectrum.
I not as concerned about actual sound-proofing (isolation from neighbors), more like deadening this space.
The mixing panel types... hmm.
I'll try and quickly explain that, if i can. In most cases, too much HF(high frequency) absorption is abtained, leaving the LF(low frequencies) to continue causing problems. This leads to even worse problems than no absorption at all.
I think that 705 is denser than 703, this allows better LF absorption and less HF absorption. This a good thing, but is not hugely necessary. You can find materials a lot cheaper than 703/5 that do the same job. I think as long as you get above around 40kg/m3(2.5pcf) then it should be fine. LF(well all) absorption depends on thickness.
To determine the(theoretical) limit of absorption, you must do some math. The speed of sound is around 340m/s. Hz tells us how many waves in a second. So with these two things, we can determine wavelength. So for 20Hz, 340/20=17m wavelength. For absorption, we work with quarter wavelegths, so to efficiently absorb down to 20Hz, we need 4.25m of absorbant material. This is no to say that thinner material cannot absorb well down to 20Hz, just that theoretically it tails off absove this.
If something is 4"(10cm) thick, then we do 340m/0.1m=3400/4(quarter wavelength)=850Hz. This is the theoretical limit of 4" absorbent material. Below this it will tail off, dependant on the type of material and it's density(and maybe other things).
We can increase the LF absorption by spacing the material from a reflective surface. So 4" material spaced 4" from a wall will absorb well down to 425Hz, and then tail off. Spacing can also reduce the effectiveness of the material though, so i recommended only spacing it by around the same depth as its thickness. People suggest placing traps in corners, because 1. bass builds up in(between) corners, and 2. it allows effective spacing without taking up too much room.
Hope this explains a bit, especially why carpet/curtains aren't good(very thin).
