Subwoofer and Poor Room Acoustics

Discussion in 'Archived Threads 2001-2004' started by Michael_V, Jan 26, 2003.

  1. Michael_V

    Michael_V Second Unit

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    This was part of another recent thread but the problem remains so here goes:

    We recently moved into overseas military housing built of concrete blocks and designed for one purpose: to withstand typhoon force winds. With smooth, cement walls and hard, tile floors, it is an acoustical nightmare!

    My wife and I put moderately heavy window treatments up and carpet on about 2/3 of the floor in the 12 x 25 foot living room where we have our HT, but the acoustics are still terrible. Through creative speaker placement and much tweaking of my Anthem pre-pro, the midrange and highs are passable but the bass (through my Monitor 7.2 towers and/or my SVS subwoofer) remains boomy and poorly defined.

    Is there anything I can do, short of covering my walls and ceiling with acoustic foam, that will help control and refine my bass frequencies?

    Thanks!
     
  2. Richard Greene

    Richard Greene Stunt Coordinator

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    Cement surfaces cause very irregulat bass response due to standing waves. This plasterboard is more flexible so the standing waves between opposing surfaces are less extreme.

    Parametric EQ:
    You can reduce bass peaks at one seating position: http://www.musiciansfriend.com/srs7/...back+destroyer[/url]


    To fix the bass frequency response at a variety of seating positions requires bass traps:
    http://ic.net/~jtgale/diy2.htm[/url]


    An alternative would be to build a simple dipole subwoofer
    that makes bass much more directional = less interaction with the room:
    http://www.geocities.com/kreskovs/basscannon.html[/url]
     
  3. Michael_V

    Michael_V Second Unit

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    Wow! Thanks, Richard! Great links. It looks like I will be doing a good deal of research! It also looks like I will probably need to invest in a bass trap . . . or do you usually require more than one for a 12 x 25 foot room?

    Also, how does the feedback destroyer work? What is required to use this device?

    Thanks!
     
  4. Richard Greene

    Richard Greene Stunt Coordinator

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    I feel sorry for you. My dorm room at the Rochester Institute of Technology about 30 years ago was all concrete and the sound was terrible.

    Standing waves cause a very uneven distribution of bass frequencies in a listening room.
    Peaks and nulls.
    Or "Hot Spots" and "Cold Spots".

    The best location for a subwoofer in a room with strong standing waves may be right next to your seat and well away from all walls and corners. A time delay for the subwoofer is very useful for this near-field subwoofer position. Unfortunately, the bass will be a lot weaker than using a room corner for your subwoofer (a corner is the maximum output position in a rectangular room) and you may still hear an annoying floor to ceiling standing wave (which will be fully excited whenever a subwoofer is located on the floor).

    If you place your ears away from all bass peaks and nulls, the bass frequency response will be smoother.
    Of course you have to give first priority to placing your ears where the front speakers sound best ... which may not be the best location for bass.
    The hot and cold spots are easy to predict in a rectangular
    room with stiff walls: Avoid sitting near walls and corners and also avoid sitting at even fractions, such as 1/2, 1/4 and 1/8, of the distance between opposing reflecting surfaces:
    (1) Front wall to back wall
    (2) Side wall to side wall
    (3) Floor to ceiling (probably nothing much you can do about this standing wave, because you'd havew to raise or lower your seat to move your ears = you'd be too high or too low for the main speakers)

    For example:
    If your room was 16' wide, it would be best to avoid sitting 4' from the left wall, half way between the two walls, or 4' from the right wall.


    You'd need a lot of bass traps for a concrete room.
    I'd start with with one large diameter trap in each room corner = four.

    The Behringer Feedback Destroyer can be used as a 12-band stereo digital parametric equalizer. You measure the frequency response with 1/6 ocatve sine waves from a test CD such as the $10 Stryke Basszone CD and a $40 Radio Shack analog sound meter:
    http://www.stryke.com/
    (click on "test CDs")

    Whatever bass frequencies are too loud, measured where your ears are located, can be reduced using a parametric EQ.
    While this can sound very good to your ears, the EQ settings would be different for ears more than a few feet away = you can't EQ the whole room, just one seat.
    Bass peaks are by far the most audible problem from standing waves. Nulls are much less audible. You can't do anything about the nulls with an EQ (nulls are bass troughs caused by standing waves) but reducing the peaks at your listening position makes a big difference. I have been building DIY subwoofers for over 20 years and have used equalzizers for every one of them. I currently use a Behringer 1124 "Feedback Destroyer" for my main system at home.

    Bass traps are different -- they reduce the bass reflections that cause the standing wave peaks and nulls ... along with the slow sound decay at room mode frequencies (these are ALL of the audible problems caused by standing waves). A parametric EQ plus a few corner bass traps is a good compromise (bass traps are big and ugly so few women would tolerate more than two in the front corners (where they do the most good).

    More reading:
    http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf
     
  5. Timber

    Timber Stunt Coordinator

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    Michael,
    I think Richard gave a lot of good advise. Something I remember from SGHT mag a few years ago was talk of a platform for the seating area. Basically a large flat box completely sealed that acts like a drum. I don't know if it would act the same as a bass trap, but it gives you a flexible area in an otherwise solid room. Plus it raises the seating area, possibly to allow two rows of seats. Of course, if this is in a typical Family/Living room, it may not work. But it would be less obtrusive than bass traps in the corners since the box would be under the sofa/seats. It could be covered in carpet also.

    Something else from SGHT to consider is seating position. You can find all the info plus an Excel spreadsheet for determining the your room modes here. Good luck.

    Tim
     
  6. Michael_V

    Michael_V Second Unit

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    Wow, again! Thanks for the great advice! Very detailed and helpful.

    Since my last post, I installed padding under our carpet, which helps minimally, and have been reading about DIY and premade bass traps. You are right, though, Richard: the WAF is extremely low for these monstrosities . . . unless I can find one that looks like furniture and not a water heater! Any recommendations from the expert?

    I may also invest in a feedback destroyer. Do you think the adjustments, if made from the center seat, would postively affect listeners three feet away on either side (I am thinking about my couch)?

    As far as furniture placement goes, I have no flexibility. My wife has already decorated and made our ugly little livingroom look reasonably nice, so there will be no changes made there! The subwoofer I can move for listening. Would putting this on some kind of stand help decrease the "excitement" of the room?

    Again, THANK YOU for the help!
     
  7. Timber

    Timber Stunt Coordinator

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    Michael,
    I have read that raising the sub can improve overall response. However, what the article specified was approx. half the height of the room. I don't know about you, but I doubt my wife(or I) would like a big box/tube elevated THAT much. But I would think you could experiment with some cinder blocks to hear if the sound improves. If not, just put the sub back on the floor. Of course, that is a lot of weight to be experimenting with.
    Another thing you could try, since you can't move the seating position - try moving the sub. You could use the method of placing the sub at the primary listening position, play some bass intensive music/movie, and then move around the room and listen for the smoothest bass response. Once you've found the best spot, put the sub there. It's not the most accurate, but it can help eliminate severe peaks and nulls due to room modes and standing waves. Which reminds me, I punched your room dimensions(assuming an 8' ceiling) into the freq. resp. spreadsheet and it shows some big peaks and nulls. Several below 158Hz with about 20-23db difference between the high and low end. That would definitely explain the poor bass response of that room. Hope this helps.

    Tim
     
  8. Richard Greene

    Richard Greene Stunt Coordinator

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    Since my last post, I installed padding under our carpet, which helps minimally,
    .
    .
    .
    *******RG:
    Carpet and padding absorb almost no bass
    .
    .
    .
    and have been reading about DIY and premade bass traps. You are right, though, Richard: the WAF is extremely low for these monstrosities . . . unless I can find one that looks like furniture and not a water heater! Any recommendations from the expert?

    ********RG:
    My system is in our living room and my DIY subwoofer already looks like a bass trap (48" tall by 18" diameter tube) so there was no hope (and no available corners)
    for real bass traps. But my parametric EQ works well.
    .
    .
    .
    I may also invest in a feedback destroyer. Do you think the adjustments, if made from the center seat, would postively affect listeners three feet away on either side (I am thinking about my couch)?

    *********RG:
    (1) For floor to ceiling room modes, the EQ should help all seating positions where the ears are roughly the same height off the floor
    (2) For front wall to back wall room modes the EQ should help all seating positions where the ears are roughly the same distance from the front and rear walls
    (3) For side wall to side wall room modes the EQ should help all seating positions where the ears are roughly the same distance from the side walls

    Using the above as a guide, whatever EQ setting works well for your seat should work well for a seat three feet to your side for (1) and (2), but not likely for (3) ...
    with one caveat: If one seat is against or near a wall and the other seat is at or near the middle of the room, the EQ that works well for the middle of the room seat is likely to provide excessive bass at the seat near the wall.
    .
    .
    .
    As far as furniture placement goes, I have no flexibility. My wife has already decorated and made our ugly little livingroom look reasonably nice, so there will be no changes made there! The subwoofer I can move for listening. Would putting this on some kind of stand help decrease the "excitement" of the room?

    ********RG:
    You can reduce the excitement of the floor to ceiling room modes by placing the sub driver (and/or your ears)
    half way between the floor and ceiling. That's not practical and also likely to happen if you are married and share the room with your wife ... of course we must be thankful we have wives to decorate our homes, because
    without them, our living rooms would probably look like the inside of a frat house or dorm room.

    Do consider placing your subwoofer right next to you
    (well away from walls and corners) in a "middle-of-the-room" near field position. Although there will be a large decline of bass output versus a corner location, you will
    excite the room modes far less ...which is likely to be a good trade-off.
     
  9. Michael_V

    Michael_V Second Unit

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  10. Frank Carter

    Frank Carter Screenwriter

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    Hey Micheal,

    Just wanted to say that the best thing I did for my HT was to add a quality subwoofer(SVS CS Ultra). The second best thing I've done is adding the Behringer Feedback Destroyer.
     
  11. Michael_V

    Michael_V Second Unit

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    Frank,

    Tell me more! How did you set the Behringer up and what were the results?
     
  12. Frank Carter

    Frank Carter Screenwriter

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    Well I had a huge peak from around 36-56hz and a smaller one smaller one around 70-80hz. I didn't realize how much this affected my reproduction of bass. After eqing, I'm now +/-2db from 15-90Hz, it did wonders for music. I can email you the excel sheet if you want to see how flat the response is.
     
  13. Michael_V

    Michael_V Second Unit

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    Well, I ordered the Feedback Destroyer last night. How did you analyze your peaks? Will I need additional equipment (other than an SPL) to use the Behringer? How far from your seating position do you notice an improvement?

    I also ordered blueprints for the CWAL bass trap. . . .

    Thanks!
     
  14. Frank Carter

    Frank Carter Screenwriter

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    All I needed was the Radio Shack SPL meter and a cd w/test tones. I bought Stryke's BassZone Test CD. If you have a CD burner, you can make your own tones for the cost of a CD-R by downloading the NCH tone generator. Just select the frequency/length of tone, save it as a .wav file then burn it.

    The Radio Shack meter isn't perfect for measuring tones down low so there are correction tones here.

    I'll email you the excel file I found on the net to plot your reponse. I modified it just a bit so that you enter the number straight off the SPL meter into the "raw spl" column and it automatically adds in the correction values to compensate for the meters inaccuracies.

    I'm not at my computer right now, but I have a page bookmarked on how to setup and use the BFD. Since I had no previous experience with eq'ing, I just had to guess at the filter's to get a feel for what did what. I'll give that link as well as send you the file when I get to my computer.
     
  15. Frank Carter

    Frank Carter Screenwriter

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  16. Michael_V

    Michael_V Second Unit

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    Great! I got the spread sheet! Thanks a million! I will have to study while I wait for my Behringer to arrive. . . .
     

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