Info on Acoustic Room Treatment

Discussion in 'Home Theater Projects' started by MikeTz, Mar 7, 2004.

  1. MikeTz

    MikeTz Stunt Coordinator

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    Before I upgrade my speakers I'd like to "get my room right" acoustically. I have some challenges like a vaulted ceiling and a wall of windows. Can anyone point me to information on determining the frequency response at the listening positions and acoustic treatment design to flatten resonances?

    Suggestions and comments from people who have tried acoustic treatments would be greatly appreciated.

    MT
     
  2. Chu Gai

    Chu Gai Lead Actor

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    I don't think there's any simple answer to this Mike. There are software programs which in conjuction with a good microphone and a sound card allow you to do RTA measurements. A book on the subject is "Master Handbook of Acoustics" by F. Alton Everest which you can pick up new or used on the web. In the meantime, the following link contains some information that you can take a look at based on the titles.

    A general first approach would be the control of first order reflection points whose location can be estimated by the use of a mirror. Maybe we can deal with those first and move on to other areas.

    Addendum...
    If you find that some of the links don't work, don't despair. If you go to www.archive.org you can paste the link there. They maintain an archive of older websites. Incredibly beneficial. For example, http://web.archive.org/web/200108160...july/room.html is a good article on room acoustics that no longer exists but in memory.
     
  3. Wayne A. Pflughaupt

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    Mike,

    Before you look at Chu’s links, it would help to know what you’re looking at. It seems like you’re confusing various audio concepts.

    A “resonance” is specific frequency that is exaggerated by a particular acoustic properties of a room.

    Acoustical treatment will not “flatten” resonances. It pretty much take an equalizer to do that. What acoustical treatments do is diffuse soundwaves or absorb them to some degree, typically to address the reflections that Chu mentioned.

    Determining the frequency response at the listening position is another issue all together, and has nothing to do with acoustical treatments. Plotting frequency response takes testing equipment like a real time analyzer, sine wave sweeps, etc.

    If your worse “problems” are windows and a vaulted ceiling, then you’re not doing bad at all. I’ve lived in houses with vaulted ceilings for the past 20 years and I’ve never found them to be a problem. They are high enough to render first order reflections a non-issue, and the angle helps diffuse the soundwaves. I personally like the “open” feel of the sound when the ceiling is not a reflective issue.

    Windows are easy – they can be addressed with heavy curtains.

    Regards,
    Wayne A. Pflughaupt
     
  4. Brian OK

    Brian OK Supporting Actor

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  5. Bob McElfresh

    Bob McElfresh Producer

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    The "Master handbook of Acoustics" is almost a text-book. One of Everest's other books is called "Sound Studio Construction on a Budget" is a bit more practical. Enough theory to tell you WHY things are done in small, medium and large rooms, and 12 chapters of practical examples. 2 of the chapters deal with home theaters.
     
  6. Ron Boster

    Ron Boster Screenwriter

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    To add to the already good responses:

    I would also visit www.realtraps.com Ethan is one of the owners and is great at educating HT hobbist on acoustics and bass.

    If you visit www.avsforum.com They have a forum on audio theory etc. that you could spend hours reading.


    Ron

    PS:I use four bass traps, one in each corner...I was amazed how it tamed the booming bass and the detailed that came out in the upper ranges (particularly from the rear surrounds).
     
  7. MikeTz

    MikeTz Stunt Coordinator

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    Thanks for the links guys. I've done some reading and it appears my room challenges may not be so bad after all. My vaulted ceiling is hexagonal (well half a hexagon) and the legs are all different lengths and angles. The lack of symmetry helps with reflections. The front wall height is lower than the rear and the dimensions of the room are 20' x 17'. Not optimum but not terrible either.

    Window shades and curtains act as absorbers and diffusors. Carpet helps and those LP cases and CD racks also act as diffusors. Even my wifes upholstered furniture is a plus.

    Most of the sites selling acoustic treatments want to sell me an average of $1500 worth of bass traps, absorbers, diffusors, and combo devices. Since the bass is a bit boomy I may try some bass traps in the corners if I can get my dealer to lend me the units. I'm not sure the rest of these devices will make that much of a positive difference, especially since the midrange and high frequencies sound natural already.

    MT
     
  8. Brian OK

    Brian OK Supporting Actor

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    Mike,

    Do not negate bass traps. They are more important in terms of taming the rooms response than the mids and HF.

    Taming 200hz and below is your target. The higher frequencies are easy.
    I recommend you go back to my link to Toole's white paper link above. You MUST access the "Axial Standing Room Calculator" spreadsheet and plot your room dimensions into the formula before you spend a dime. Learn where to place your speakers, sub, and listening chair/couch IN the room vs the standing waves present before you embark on treatments.

    You will need treatments, of course, but I recommend you plot your room first. That alone will tell you how your room behaves to sound pressure. My experience with the calculator was tremendous... and I have a room full of traps and diffusors. Honestly, I you want to avoid the mess and confusion of room modes and the hell they play in your room ,then calculate your axial standing wave plots ( by length, width, and height ) in your room before you proceed further.

    It is like avoiding land mines in your listening room. Just lay in your rooms dimensions and use it as a flashlight in the dark.
    If you are interested in DIY treatments or inexpensive diffusors then let me know. I use Jon Risch DIY formulas and Auralex diffusors.

    Good Luck,

    BOK

    typo edit
     
  9. MikeTz

    MikeTz Stunt Coordinator

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    Brian:

    Thanks for the link. I read Toole's white papers and have experimented with the excel spreadsheet. Lots of fun and a few surprises so far.

    When you place speakers in a standing wave null that mode disappears. When one speaker is at a null and the other is at a different null do they both disappear? For instance if both speakers are equidistant from the back wall (i.e. in the same standing wave null) then that standing wave along the length of the room disappears. But if one speaker is in the fourth order null and the other is in the third along the width of the room do both disappear?

    MT
     
  10. Brian OK

    Brian OK Supporting Actor

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    Mike,

    It would appear to me that both speakers would 'disappear', but in the sense that they would be in different frequency nulls and both speakers would exibit drastically different bass response (not good) in the frequency in which they are plotted in your particular room- LWH. Lopsided soundfield and totally uneven.

    The two different nulls will present your speakers with two different frequency modes which will not be energized. He states "woofer location determines which of the room resonances is activated, and which is not".

    As you know, Toole concludes his presentation with room treatment solutions for good reason. He does say a few times that modes cannot be done away with, just attenuated by removing energy by damping reflective surfaces, etc.

    It is fun, but very challenging I agree. The trick is to attenuate/dampen the high pressure regions of the standing waves, and to elevate the low pressure regions.
    Instead of just throwing up treatment willy-nilly, this Toole piece at least give the student a fighting chance to smooth out his rooms response.

    Good Luck,

    BOK
     
  11. MikeTz

    MikeTz Stunt Coordinator

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    Brian:

    Thanks for the clarification. My movie screen is not on the middle of the front wall due to a window. So the front speakers will not be equidistant from the side walls (front speakers will be at the sides of the screen). Since the front speakers are not equidistant from the side walls they cannot be placed in the same standing wave null.

    I use Vandersteen speakers with a Vandersteen sub. The first order sub crossover is at 100Hz. That means the front speakers are responsible for 100Hz and higher frequencies. My length and width standing waves are all below 130Hz for the first four harmonics so the placement of the sub will have the biggest impact on bass in the room.

    My initial acoustic treatment plan will be bass traps in the front corners, absorbers behind the front speakers, and absorbers on the side walls at the primary reflection point. We'll see if this plan tames the room and tightens up the bass.

    MT
     
  12. Brian OK

    Brian OK Supporting Actor

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    Mike,

    I agree, your subs placement will really dictate your rooms bass response. This is a good thing if you have some flexibility in where you can place it. Also, with little play available for front speaker placement you may find that moving one or the other a few inches outside the null may boost bass response in a good way.
    Your room treatment plan is exactly what I have in place in my dedicated basement HT.

    I use Jon Risch's "Cheap & Dirty" recipe for corner bass traps and his side wall panel recipe-- but a bit thicker with fiberglass insulation (12" inches compressed to 6") wrapped tight with jute and long brass tacks to give that scallopped look. And front wall panels behind the speakers too.

    The real gem for me was using numerous foam-filled Auralex diffusors in the rear wall area and back section of the rooms walls. If you can pop up some diffusors it will impact your soundstage and depth in a big way.

    And keep in mind you may have some flexibility in where you can place your listening position. That will influence the modal energy you hear for sure.

    With 5 speakers in a room it comes down to doing the most you can to get a good smooth frequency response. We'll never get it perfect. Just can't happen.

    Let me know how it goes with your treatments.

    Brian
     
  13. Brian OK

    Brian OK Supporting Actor

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  14. MikeTz

    MikeTz Stunt Coordinator

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    Brian:

    Thanks. I just finished conducting a very non-scientific experiment in my room. I took an analog Radio Shack SPL meter and put it at my listening position. Using a Stereophile test CD with warble tones from 1000Hz down to 20Hz I mapped the response. The data suggests I have a standing wave null at 100Hz (down 8dB) and a peak at 30Hz (up 14dB).

    BTW I've read that there is some issue using the analog SPL meter with test CDs to map frequency in the lower octaves. Apparently you have to add some compensation table for the frequency response of the meter. Know anything about that?

    The question is if I keep my speakers and sub in the same location will acoustic treatments smooth the null and peak? Or will I be forced to use an equalizer and/or move the speakers and sub?

    MT
     
  15. Brian OK

    Brian OK Supporting Actor

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    Mike,

    an equalizer may be the option if the treatments don't smooth things out. Your call.

    try here for a nice acoustical discussion forum (keep in mind these guys are 2 channel based 90% of the time)

    you will get the scoop on the RS analog meter at below link. It needs an additional formula for accuracy, as is noted.

    http://www.audioasylum.com/forums/ri...sages/976.html

    ..... and here is a good thread about the RS meter from a guy with Vandersteen 2cs Signature speakers. Hope this all helps.

    http://www.audioasylum.com/forums/ri...sages/492.html
    BOK
     
  16. BruceD

    BruceD Screenwriter

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    Mike,

    Something to keep in mind, using warble tones to plot peaks and nulls, especially with only 1/3 octave steps is not a recommended methodolgy. Warble tones do not have a stable center frequency (it is constantly oscilating).

    A better setup is to use sine-wave test tones with at least 1/12 octave steps, keeping test times short (seconds) for each frequency to protect the speakers. If you do the testing at the 75-85dB test level, the "C" weighting response of the RS meter is actually pretty accurate in modeling the ear's perception of overall flat frequency response with respect to low frequencies.

    Most of the recommended RS SPL meter adjustments just change the "C" weighting scale response of the meter to a "U" weighting (i.e. no weighting) response which is not necessarily what you want.

    I would suggest you visit the ETF website and go through some of their demo-room tutorials. You could also try this software for free with your RS meter.
     

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