Help, getting different SPL readings when moving around the room.

Discussion in 'Speakers & Subwoofers' started by DaveBarn, Feb 28, 2005.

  1. DaveBarn

    DaveBarn Extra

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    When trying to measure different SPL readings using 1/3 octave test tones I have noticed that the reading changes when I move around the room. I have the Rat Shack SPL meter on a tri pod at my main listening position. I understand that putting myself between the meter and the speakers could very the readings, but I’m behind the meter. Has anybody run into this problem or have a solution?
     
  2. ChrisWiggles

    ChrisWiggles Producer

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    This 'problem' is called acoustics. It's very normal. Treating your room with bass absorption can help minimize modal response problems, and lead to more uniform bass throughout the room. You should be measuring right in your listening position in any case.
     
  3. Max F

    Max F Second Unit

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    Hey, your learning through experiment - the best way to learn. Here try this:

    http://users.pandora.be/livingphysic...lf/ripple.html

    Its a simulated ripple tank. See how the ripples are nice and smooth. Now click on add border. This is how walls create standing waves. These waves stay in the same place for a particular frequency. If you change the frequency the locations of the waves also changes. This is why you get different readings for different locations. Kindof a simplistic view but pretty much correct. [​IMG]
     
  4. DaveBarn

    DaveBarn Extra

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    Chris,

    You mention treating my room with bass absorption. How does one go about doing that?
     
  5. ChrisWiggles

    ChrisWiggles Producer

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    By purchasing materials suitable for absorption, usually thick amounts of rigid fiberglass, usually put in corners. See the builder forum for more acoustical discussion, Ethan Winer's FAQ is a good place to start on the web, and also see F Alton Everest's: master handbook of acoustics.
     
  6. Jeff Gatie

    Jeff Gatie Lead Actor

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    Dave, even if you have an accoustically perfect room, you are still going to get higher readings in some parts of the room than others. The acoustically perfect room will eliminate the standing waves issue, but it will not solve a basic law of acoustics - that is the closer you move to a sound source, the louder it is. Ever notice that if you put your ear right next to a low sound source, it sounds pretty loud? Then when you back away, the sound gets softer? As the distance from the source gets larger, the change in volume from loud to soft is not as big, but you will still get larger readings right next to a speaker than you will from 6 feet away. This is why we calibrate to a "sweet spot". The "sweet spot" is basically the only spot where the volumes of all speakers are equal, as you move away from the sweet spot, the speakers you move closer to get louder and the ones you get farther from are softer.
     
  7. Kevin. W

    Kevin. W Screenwriter

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    Don't move[​IMG] Sit in the optimum spot of your setup and calibrate their. Then when you watch movies with others just make sure your sitting in that spot.

    Kevin
     
  8. DaveBarn

    DaveBarn Extra

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    Also, I have read that as a general rule the sub should be set higher (+3db) than the mains. How much higher is your sub vs. the mains? Using a SPL meter I have my mains all equal to 75db and the sub set to 78db using the receivers test tones. Would this or can this cause the bass to by more boomy?
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    Actually, the sub should be calibrated 2-3 dB *lower* than the mains. The reason why is that the meter is less sensitive to lower freqs. (You have to think about that for a second, might seem the opposite, but it isn't.) But also, use your ears. You shouldn't really ever hear the sub distinctly by itself. The blend with the mains should be seemless. The sub is there to *support* the mains.
     
  10. John S

    John S Producer

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    LOL.. Well I admit, even though I calibrate, I crank my sub, and lower it by ear by content / source. I run my sub way hot period. I know this and accept it as preference. [​IMG]
     
  11. DaveBarn

    DaveBarn Extra

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    When calibrating your sub to your mains with a SPL do you have sound coming out off all the speakers including the center or just the front left/rights?
     

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