Mikewen - I was in your exact situation some years ago. Here's some advice I gave someone else that I found to work for my man-cave listening room:
First, please read Dr. Floyd Tooles latest book as hes an audio engineer GOD and highly respected scientist and not affiliated with any acoustical treatment manufacturers. Therefore, hes unbiased and not afraid to call a spade a spade.
Second, the manufacturers of acoustic treatment all have informative websites so I'd recommend you study websites from: RPG, RealTraps, GIK, Auralex, Acoustic Sciences Corp (ASC) and probably others but these are the main players from what I can tell.
Before you begin any acoustical treatment project you need to know what the problems are so you can develop a strategy to deal with them. Too often people go crazy adding fiberglass absorbers and create dead sounding rooms because they used too many or used them in the wrong locations or ineffectively and not at their maximum potential.
Id suggest you get a sound pressure level meter or easy to use acoustical testing kit from Dayton Audio OmniMic or XTZ and test the room's frequency response for starters. Id recommend downloading the test tones from RealTraps which spans 10-300Hz I think. You can save a lot of time by using the following link to only test those tones that correspond to the musical scale of western music http://www.soundoctor.com/freq.htm I dont see much merit in measuring fractions of semi-tones that arent likely to be played by an instrument or seldomly played.
Next treat the rooms bass region 300Hz. 300Hz is the approximate delineating point within most residential buildings where the bass modes become loosely spaced and problematic whereas above this transition zone or Schroeder frequency the tightly and densely packed waves exhibit different characteristics and challenges. So, roughly two zones with different approaches to dealing with each.
BASS FREQUENCY ZONE (<300Hz)
Its recommended that you first treat the bass zone to reduce the bass peaks and nulls which is likely to improve other areas of the frequency spectrum as well. Treating the bass region requires low-frequency absorption.
There are several types of bass traps which work differently from each other.
The most popular DIY bass trap is the "resistive" broadband type usually filled with fiberglass (or cotton etc) which should have a minimum thickness of 4" with the thicker the better. The resistive traps works best when the sound air particle SPEED is maximized (not its pressure) which occurs at one-quarter of the wavelength's distance out from the wall. For example, 100Hz has a wavelength of 11.3feet so one-quarter equals 34". Its very important to pull the resistive bass traps away from the wall boundaries to create an air space with the minimum being between 6.8 and 11 which corresponds to ¼ the wavelength of 500Hz and 300Hz respectively. Walls with doors in them or open areas will measure acoustically as having longer dimensions than the physical rooms dimensions due to bass leakage so experimentation and iterative testing and measuring is key. Remember to document every bass trap placement change with what the dB reading is pre- and post-change and a short blurb on the subjective change in the music youre hearing.
Another bass trap type is the "membrane" or "diaphragmatic" which works on the principle of absorbing sound most efficiently when the sound wave's PRESSURE is maximized, which is right at the point on impact with the wall - so diaphragmatic absorption traps hang on the wall and take up much less room foot print.
Since all small rooms suffer from bass modes you will need a combination of bass traps, multiple subwoofers, and parametric EQ to get the bass right. Typically the more bass traps you use the better the peaks and nulls become (i.e. lowering a peak and raising a null) while reducing ringing time and reverb time. BUT you will also need to turn up the bass volume. There are 2 other problems: (i) the bass traps don't magically stop working at say 300Hz which would be ideal, they keep on absorbing higher frequencies, albeit less effectively the higher up in freq you go, which only compounds the dead sounding room problem, and (ii) manufacturers don't always tell you how to properly set the bass traps up to maximize their effectiveness which may be self serving to get you to buy more of them.
To counter the bass traps absorbing higher frequencies, try putting something hard like a thin sheet of plywood in front of them so that mid-higher frequencies will reflect off of it while bass frequencies won't "see" the plywood. Curving the plywood will act as a mid/high freq diffuser.
MID/HIGH FREQUENCY ZONE (>300Hz)
After the bass region has been treated with absorption, turn your attention to the mid/high frequency zone by using reflections, diffusion, and absorption as the tools.
For the enjoyment of listening to music, treating 1st reflection points isn't really required (unless large slap echo exists between hard parallel surfaces) as Dr Floyd Toole points out in his latest book. The reflections actually add to the sense of listener envelopment and apparent sound source width and helps create a low "Inter Aural Cross-Correlation" which is a fancy scientific term that means that the left and right ears are hearing different things; A high IACC means the ears are hearing very similar or the same thing, which is bad and not as musically satisfying think mono vs stereo recordings. Toole goes on to say that treating 1st reflection points is a good idea IF you're a musician who is mastering their music or someone doing comparative stereo component evaluations (magazine reviewers?) who does not want to hear any impacts of the room on their work.
To keep the energy within the room and avoid making a dead sounding room, I'd recommend you use diffusion. Where you think you want to put diffusion will dictate whether it should be 1Dimensional or 2Dimensional. 1D is recommended for scattering sound latterly (horizontally) and should be placed on the front wall and side walls in front of the listening seat to widen the apparent sound stage width. 2D should be placed on the rear parts of the side walls and back wall behind the listener to create a sense of listener envelopment.
Common 1D types include QRD, hemi-cylindrical, binary-amplitude (and others?) while 2D would include Skylines and derivations thereof.
Secondly, you want the diffuser to work as broad range as possible so as not to act as a low pass filter and throw off your spectral balance that you paid dearly for in your selection of speakers. High frequency effectiveness for:
* Skylines is determined by the size of the block (not the depth) with small sizes allowing for higher freq diffusion effectiveness. I could only find 1.5" *1.5" blocks of wood for my Skyline which will work up to and likely beyond 4.5KHz which is higher than the highest note on a piano. Even George Massenberg's famous 'Studio C' uses 1"*1" sized blocks which will take it up to 6.75KHz or so.
* Hemi-cylindrical diffusers (often called Poly's) is a function of how hard the outer material is (so as not to absorb mid/high frequencies) and the angle of incidence; the max diffusion occurs head-on (meaning perpendicular) to the centre of the hemi diffuser.
* QRD is determined by the width of the wells with thinner being better.
The diffuser should work down to your room's transition (or Schroeder) frequency which is about 300Hz. Low frequeny effectiveness for:
* Skyline is determined by the max cell depth which should be a substantial % of the 300Hz freq wavelength. 50% would mean a depth of 22.6". Of course you could opt for 25% (11.3") or even 14% (6.5") but it would mean less and less effectiveness at 300Hz .
* Hemi-Cylindrical is determined by the length of the radius or strictly speaking the 'Sagita' which is that part of the radius that sticks out from the wall into the room when the hemi's arc is less than 180 degrees. The farther out into the room the radius extends the deeper the depth it diffuses to. The same calculations would apply as above meaning a radius of 6.5" will only be about 14% of the 300Hz freq wavelength and be less effective than a 11.3" or 22.6" radius.
* QRD well depths determine the LF limits. I've only seen one manufacturer which makes a 15" well depth. Good on them!
Lastly, the diffusion and scattering coefficients are useful metrics for evaluation purposes except that they haven't been standardized nor widely accepted or mandated for inclusion as part of a company's stock marketing collateral for their respective diffusers lines . . .
If you're planning on putting diffusion on the back wall behind the listening chair, then ponder the following considerations:
1.) LOW FREQUENCY OPERATING RANGE - as diffusers need to handle long wavelengths and should work down to your room's Schroeder or Transition frequency (typcially 300Hz), pick a design with the deepest cell/well depth as you can find or build yourself, with the limitation being distance from diffuser to listening chair (see point 2. below).
2.) DISTANCE BETWEEN BACK WALL & LISTENING CHAIR - a general rule of thumb is that you should be 3 wavelengths away from the lowest frequency that the diffuser will perform well at, so that the sound coalesces before it reaches your ears and troublesome lobbing isn't heard. If you're considering the more popular QRD- or Skyline-types of diffusers, then a conservative calculation of the lowest frequency wavelength will be to multiply the deepest cell depth in inches by 2 to get the entire wavelength of the lowest freq that the diffuser will work well at and then express it in feet by dividing it by 12 and then divide it into the speed of sound of 1130. For example, a Skyline of max cell depth of 7.5" will perform well down to 904Hz [ 1130/((7.5*2)/12) ]. The diffuser will continue to perform to lower frequencies but with diminishing ability. Now calculate 3 wavelengths of 904Hz as follows: 1130/904 * 12 * 3 = 45inches. So you'd need to sit at least 45inches away from the rear wall with a diffuser of max depth equal to 7.5 inches.
3.) DIFFUSER PLACEMENT - be sure they are centered at ear level and should cover 2ft below and above your ear level so a minimum dimension of 4ft.
4.) WALL COVERAGE AREA - Prof Trevor Cox a diffusion expert, told me that some general diffusion principles exist, most relevant is the less periodicity the more diffuse the scattering. Dr. Floyd Toole concurs saying "these designs get better as they get larger." So, to improve scattering efficiency over your desired wall area, choose a Skyline diffuser with the largest prime number or QRD diffuser with the largest number of wells to cover the desired area, and in both cases a max cell/well depth as calculated above. So it's better to have one large diffuser than several smaller ones put together. For a hemi-cylindrical diffuser, pick one with at least a 12" radius however when combined with other hemi-cylindrical diffusers it loses its ability to diffuse low frequencies, so space them apart, change angles, and change their radius depths too if possible.
he front wall is likely the least important wall to treat (if you're not using bi/dipole speakers) within the context of forward firing two channel stereo (not HT) speakers. The reason being is that a number of factors combine to make the sound from the front wall much less audible than say 1st reflection points off side walls, ceiling, and floor. From Toole's book, these factors include:
* sound energy propagation loss or decline in steady-state sound level as a function of distance through air at about -6dB per double distance
* reduced sound energy upon impact with surfaces that are absorbing or diffusing before traveling to the front wall
* precedence effect caused our attention to focus on the first arrival/direct sound and we simply are not aware of the reflections as spatially separate events, or much less so.
I know this is a lot of info to absorb (pun intended).