Room treatments where to start?
You ask great questions, many Ive wrestled with myself. The approach I took was to
(1) Educate myself about small room acoustics.
(2) Measure the room before you do anything to establish a baseline to determine if and where problems exist and to compare against the addition of treatments and placement options.
(3) Create frequency range (i.e. low VS middle/high frequencies) dependant strategies for dealing with offending modes
(4) Determine if buying and/or building (DIY) treatments is best for you, after considering using regular household items like furniture, carpets, bookcases etc.
Education - Educating yourself will be an investment that will (i) save you money by buying or making (i.e. DIY) the right products, (ii) save you time from conducting infinite trial and error placement options of the treatments, and (iii) allow you to critically evaluate products offered by manufacturers, most of whom exagerate their products performance specs for marketing purposes. Id highly recommend you read Floyd Tooles latest book http://www.amazon.com/Sound-Reproduction-Acoustics-Psychoacoustics-Loudspeakers/dp/0240520092/ref=sr_1_1?s=books&ie=UTF8&qid=1284840260&sr=1-1 and also Alton Everests book http://www.amazon.com/Master-Handbook-Acoustics-Alton-Everest/dp/0071603328/ref=pd_sim_b_1. Harman has many good White Papers found here: http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White papers
Tools at the very least get a SPL meter which Radio Shack sells and download test tones from RealTraps to help with analyzing bass frequency modal peaks and nulls. You will also need to calculate your rooms modes or standing waves so this tool will do the job: http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/Calculators.aspx?CategoryID=Calculators. You will also benefit from using a frequency wavelength chart for calculating 25% and 50% wavelength distances for absorption placement and diffusion depth respectively. I like this one: http://www.soundoctor.com/freq.htm Like anything, if you cant measure it then you cant manage it.
Strategy Having a game plan is important and will help to determine what kind of treatment goes where and in which sequence things should be addressed. Its likely a good idea to establish a baseline to determine if you have acoustical problems, and from which to compare the addition of treatments and placements thereof. Deal with the offending reflections and leave the others alone. I use an Excel spreadsheet to record the SPL for a broad range of frequencies before any room treatments which became my baseline. Use the same Excel sheet to record the SPLs again for each subsequent Test of adding treatments or different placement combinations to note the difference from your baseline.
Frequency-dependant treatments: generally speaking, you should treat the low and mid/high frequency regions separately; use about 300Hz as the transition zone frequency to differentiate the treatment strategies. Bass frequencies ( 300Hz).
Personally, I chose to tackle the bass region first because modal peaks in this region can reach 20dB which drowns out the mid and high frequencies, so clearing up the bass should also introduce more perceived clarity into the upper frequencies too.
20Hz to +-50Hz -> use EQ to tame these very low peaks.
50Hz to +-100Hz -> use Helmholtz or Diaphragmatic type bass traps for this next lowest region. Placement effectiveness for diaphragmatic bass traps is where air particle pressure is maximized which is at the wall, so these traps are hung right on the wall with no air gap behind them.
100Hz 300Hz -> use resistive type bass traps (e.g. Fiberglass filled) for this bass region. Placement effectiveness is maximized when air particle velocity is maximized which is at the 25% wavelength mark, so from 34 to 11.5 out from the wall for 100Hz and 300Hz respectively. A one or two inch air space wont cut it try 7 to 11 air space instead and extend the fiberglass bass trap as far into the room as you can (34 to get to 100Hz). 34 less 11 equals a bass trap of 23 thickness, so buying 2, 4 or even 6 thick traps wont likely work well, unless you layer them to extend the total thickness.
If you can swing it, put bass traps in all four room corners from floor to ceiling, and at the mid point of your front or back wall in the floor/wall corner.
For the mid/high frequencies, you can start off using normal possessions like furniture, bookcases etc, but you will need to know where things should go and if they should be diffusive or absorptive in nature.
Of the 6 room surfaces, what should you tackle in which sequence? Heres one approach:
(1) Since first reflection points are the strongest reflections, deal with the floor and ceiling ones first via absorption and diffusion respectively. Research indicates that lateral side wall first reflections add positive contributions to your rooms sonics by widing the apparent sound stage and creating listener envelopment.
(2) Try adding diffusion to the back wall to keep the energy in the room but attenuate the direct reflections back to the listening position
(3) The front wall can be either absorption or diffusion based on personal taste and type of speakers
Another strategy worth considering is Buy VS Build. Do you want to buy finished products and pay top dollar (as used acoustical treatments are seldom available) or invest your time and energy into building something that will save you money and whose specs can outperform mass produced items?
After saying all of the above here are some of my personal experiences:
Wall to wall carpet absorbs much more high frequencies than I first expected so to prevent a dead sounding room, I focused on using diffusion rather than more absorption for mid/high frequencies which works like a charm. I also put a hard diffusive/reflective front face to many of my bass traps to again prevent further absorption of mid/high frequencies.
Two parallel side walls introduced a very audible flutter echo which was eliminated with diffusion
Walls with openings (e.g. doors, windows) extend the acoustical length of the room dimension. For example, in my room placing bass trap absorption out from the front wall (which has a door that was shut) at the 25% point of 80Hz didnt have the same impact as when the absorbers were moved to the back wall (with no door/windows) and placed at the exact same point.
Experiment with the depth of the air space behind your resistive type bass traps. Real audible differences can be heard by pulling them out further and further from the corners.
Good luck.
You ask great questions, many Ive wrestled with myself. The approach I took was to
(1) Educate myself about small room acoustics.
(2) Measure the room before you do anything to establish a baseline to determine if and where problems exist and to compare against the addition of treatments and placement options.
(3) Create frequency range (i.e. low VS middle/high frequencies) dependant strategies for dealing with offending modes
(4) Determine if buying and/or building (DIY) treatments is best for you, after considering using regular household items like furniture, carpets, bookcases etc.
Education - Educating yourself will be an investment that will (i) save you money by buying or making (i.e. DIY) the right products, (ii) save you time from conducting infinite trial and error placement options of the treatments, and (iii) allow you to critically evaluate products offered by manufacturers, most of whom exagerate their products performance specs for marketing purposes. Id highly recommend you read Floyd Tooles latest book http://www.amazon.com/Sound-Reproduction-Acoustics-Psychoacoustics-Loudspeakers/dp/0240520092/ref=sr_1_1?s=books&ie=UTF8&qid=1284840260&sr=1-1 and also Alton Everests book http://www.amazon.com/Master-Handbook-Acoustics-Alton-Everest/dp/0071603328/ref=pd_sim_b_1. Harman has many good White Papers found here: http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White papers
Tools at the very least get a SPL meter which Radio Shack sells and download test tones from RealTraps to help with analyzing bass frequency modal peaks and nulls. You will also need to calculate your rooms modes or standing waves so this tool will do the job: http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/Calculators.aspx?CategoryID=Calculators. You will also benefit from using a frequency wavelength chart for calculating 25% and 50% wavelength distances for absorption placement and diffusion depth respectively. I like this one: http://www.soundoctor.com/freq.htm Like anything, if you cant measure it then you cant manage it.
Strategy Having a game plan is important and will help to determine what kind of treatment goes where and in which sequence things should be addressed. Its likely a good idea to establish a baseline to determine if you have acoustical problems, and from which to compare the addition of treatments and placements thereof. Deal with the offending reflections and leave the others alone. I use an Excel spreadsheet to record the SPL for a broad range of frequencies before any room treatments which became my baseline. Use the same Excel sheet to record the SPLs again for each subsequent Test of adding treatments or different placement combinations to note the difference from your baseline.
Frequency-dependant treatments: generally speaking, you should treat the low and mid/high frequency regions separately; use about 300Hz as the transition zone frequency to differentiate the treatment strategies. Bass frequencies ( 300Hz).
Personally, I chose to tackle the bass region first because modal peaks in this region can reach 20dB which drowns out the mid and high frequencies, so clearing up the bass should also introduce more perceived clarity into the upper frequencies too.
20Hz to +-50Hz -> use EQ to tame these very low peaks.
50Hz to +-100Hz -> use Helmholtz or Diaphragmatic type bass traps for this next lowest region. Placement effectiveness for diaphragmatic bass traps is where air particle pressure is maximized which is at the wall, so these traps are hung right on the wall with no air gap behind them.
100Hz 300Hz -> use resistive type bass traps (e.g. Fiberglass filled) for this bass region. Placement effectiveness is maximized when air particle velocity is maximized which is at the 25% wavelength mark, so from 34 to 11.5 out from the wall for 100Hz and 300Hz respectively. A one or two inch air space wont cut it try 7 to 11 air space instead and extend the fiberglass bass trap as far into the room as you can (34 to get to 100Hz). 34 less 11 equals a bass trap of 23 thickness, so buying 2, 4 or even 6 thick traps wont likely work well, unless you layer them to extend the total thickness.
If you can swing it, put bass traps in all four room corners from floor to ceiling, and at the mid point of your front or back wall in the floor/wall corner.
For the mid/high frequencies, you can start off using normal possessions like furniture, bookcases etc, but you will need to know where things should go and if they should be diffusive or absorptive in nature.
Of the 6 room surfaces, what should you tackle in which sequence? Heres one approach:
(1) Since first reflection points are the strongest reflections, deal with the floor and ceiling ones first via absorption and diffusion respectively. Research indicates that lateral side wall first reflections add positive contributions to your rooms sonics by widing the apparent sound stage and creating listener envelopment.
(2) Try adding diffusion to the back wall to keep the energy in the room but attenuate the direct reflections back to the listening position
(3) The front wall can be either absorption or diffusion based on personal taste and type of speakers
Another strategy worth considering is Buy VS Build. Do you want to buy finished products and pay top dollar (as used acoustical treatments are seldom available) or invest your time and energy into building something that will save you money and whose specs can outperform mass produced items?
After saying all of the above here are some of my personal experiences:
Wall to wall carpet absorbs much more high frequencies than I first expected so to prevent a dead sounding room, I focused on using diffusion rather than more absorption for mid/high frequencies which works like a charm. I also put a hard diffusive/reflective front face to many of my bass traps to again prevent further absorption of mid/high frequencies.
Two parallel side walls introduced a very audible flutter echo which was eliminated with diffusion
Walls with openings (e.g. doors, windows) extend the acoustical length of the room dimension. For example, in my room placing bass trap absorption out from the front wall (which has a door that was shut) at the 25% point of 80Hz didnt have the same impact as when the absorbers were moved to the back wall (with no door/windows) and placed at the exact same point.
Experiment with the depth of the air space behind your resistive type bass traps. Real audible differences can be heard by pulling them out further and further from the corners.
Good luck.