Fine-tuning a sub-woofer

Discussion in 'Speakers & Subwoofers' started by Simon Ngan, Feb 22, 2004.

  1. Simon Ngan

    Simon Ngan Stunt Coordinator

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    I'm trying to fine tune my MartinLogan Descent sub to blend in with the system:

    Aeon as fronts
    Cinema as center
    Scripts as surround
    And of course a Descent
    Primare SP31.7 Processor

    I have a radio shack analog meter and I've set up the levels through the processor's pink noise and they all match. I didn't use the compensation chart for the SPL meter. I just set the crossover at 80Hz for all channels and make sure they all read 75db on the meter.

    Then, I use a program called NCH Tone Generator to generate pink noise in various frequencies and I've made a chart from 20Hz to 140Hz. Here's the results:

    20Hz - 82db
    30Hz - 89db
    40Hz - 88db
    50Hz - 77db
    60Hz - 73db
    70Hz - 74db
    80Hz - 73db
    90Hz - 78db
    100Hz - 73db
    110Hz - 70db
    120Hz - 67db
    130Hz - 77db
    140Hz - 78db

    What I'm trying to get is what frequency is coming from the processor's internal test tone? Above numbers are straight from the meter and I seem to have some valleys and hills. I don't know what else I can do to make it better? There is a 25Hz level on the Descent? Should I play around with that?

    Any help would be much appreciated.

    Simon
     
  2. Nathan Stohler

    Nathan Stohler Second Unit

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    Pink noise is not a single frequency. Your test tones are generating frequencies over much of the spectrum, with an equal amount of energy for each octave of the spectrum.

    In other words, you hearing all frequencies simultaneously.

    As for flattening out your subwoofer response, you can:
    a) use an equalizer
    b) try placing your sub in different locations
    c) adjusting the polarity on your sub

    Although it is not too hard to use an equalizer to get a flat response curve in your sitting position, it can be very difficult to get a flat frequency response for all listening positions, especially for bass frequencies.

    I see that your frequency response between 20 and 40 Hz is quite high compared to the rest of your test tones, especially when you factor in the correction factors. I'm not sure what the 25Hz level on your processor does, but if it is some kind of equalizer at that frequency, I would try turning it down.

    --Nathan
     
  3. Wayne A. Pflughaupt

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    Simon,

    I’ve had the best success using 1/6-octave sine wave test tones rather than the filtered (i.e., frequency-specific) pink noise signal you’re using. The sine waves are steady tones, whereas the pink noise is somewhat random. This results in more fluctuation in the readings, even with filtered pink noise.

    However, it’s not impossible to get good readings with pink noise, it just takes more effort.

    If your program can do sine waves or filtered pink noise in 1/6-octave spacing let me know and I’ll post the frequency values you’ll want to use.

    In the meantime, do a search on “BFD” or “Behinger Feedback Destyroyer” on this forum and the DIY forum to learn more about using an equalizer to tame your sub’s response.

    Regards,
    Wayne A. Pflughaupt
     
  4. Rob Michaw

    Rob Michaw Stunt Coordinator

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    I have the Feedback Destroyer Pro. I have it set up, and unfortunately I do not have the individual test tones. I just have a sweep. My question is, how do I set it up with test tones I download and burn to a CD? When I play the CD, it will play either a left channel or right. How do I get the disc channeled to the sub? On the Avia disc, I can do an isolated sweep on the sub, but that is it.

    Any ideas?
     
  5. Simon Ngan

    Simon Ngan Stunt Coordinator

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    Hi guys,

    Thanks for your information, much appreciated. I'm checking out the 25Hz control and I hope this may fix the issue where I have too much around 20Hz-40Hz.

    Wayne, as far as the test tones I used for the chart, I was actually using SINE WAVE TONE. From the drop down menu of the NCH program, I can select:

    Sine
    Square Trianglar
    Saw Tooth
    Impulse
    White Noise
    Pink Noise

    And I picked SINE from the list and entered those frequencies in the chart. So I guess the program can do SINE WAVES?

    If you can let me know what frequencies I should use to measure, that'd be great. I'm also using MONO output, is this the right thing to do? Or I should use STEREO? This is selectable from the program.

    Rob, if my program can generate right test tones, I can save them for you and you can burn the WAVs to a CD and test them yourself.

    Thanks,

    Simon
     
  6. Simon Ngan

    Simon Ngan Stunt Coordinator

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    Rob, just realized you're from Canada too. Did you end up paying a lot for your feedback destroyer pro due to duties and taxes? Or did you get it from Canada?

    There was one for sale here but it was gone the minute it was posted [​IMG]

    Thanks,

    Simon
     
  7. Rob Michaw

    Rob Michaw Stunt Coordinator

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    I didn't mean to infer that I cannot get the test tones. I meant, that I just have them on a CD which doesn' specifically direct the tone to the sub. Doesn't that cause a problem?
     
  8. Nathan Stohler

    Nathan Stohler Second Unit

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    If the bass management on your receiver is set to send low frequency material to your sub, then the CD will automatically send low frequency test tones to your sub. Tones near the crossover frequency will be sent to the sub and your main speakers. Tones above the crossover frequency will be sent to only your main speakers.

    I prefer to measure the combination of my main speakers and subwoofer, because that is what will be heard normally. Others prefer to measure the main speaker response and sub response separately.

    Simon, here are the typical frequencies (in 1/6 octave steps) that you would use to measure freq. response, along with the Radio Shack SPL correction factors:

    16Hz +11.5
    18Hz +8.0
    20Hz +7.5
    22Hz +6.5
    25Hz +5.0
    28Hz +4.0
    31.5Hz +3.0
    36Hz +2.5
    40Hz +2.5
    45Hz +2.0
    50Hz +1.5
    56Hz +1.5
    63Hz +1.5
    71Hz +1.5
    80Hz +1.5
    89Hz +1.5
    100Hz +2.0
    111Hz +1.0
    125Hz +0.5
    142.5Hz +0.5
    160Hz -0.5


    Here are the frequencies and correction factors for the entire range (in 1/3 octave increments):

    10Hz +20.5
    12.5Hz +16.5
    16Hz +11.5
    20Hz +7.5
    25Hz +5
    31.5Hz +3
    40Hz +2.5
    50Hz +1.5
    63Hz +1.5
    80Hz +1.5
    100Hz +2
    125Hz +0.5
    160Hz -0.5
    200Hz -0.5
    250Hz +0.5
    315Hz -0.5
    400Hz 0
    500Hz -0.5
    630Hz 0
    800Hz 0
    1KHz 0
    1.25Khz 0
    1.6KHz -0.5
    2Khz -1.5
    2.5Khz -1.5
    3.15Khz -1.5
    4KHz -2
    5KHz -2
    6.3KHz -2
    8KHz -2
    10Khz -1
    12.5KHz +0.5
    16KHz 0
    20KHz +1
     
  9. Lee Carbray

    Lee Carbray Second Unit

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    I got my BFD for a total cost of $120 CND. I got it off Ebay there are always some on there.
     
  10. PaulT

    PaulT Supporting Actor

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    Good price. New retail in Canada is 249CDN and Behringer has a rule with their dealers to only sell retail. I've gotten the same quote from 6 different Music Stores in 2 Provinces.
     
  11. JohnGil

    JohnGil Stunt Coordinator

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    $75.00 from a pawn shop was in like new condition, box and everything.
     
  12. Simon Ngan

    Simon Ngan Stunt Coordinator

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    Wow, I'm getting jealous about you guys [​IMG] Thanks for the information and I'll try again in the next few days and see if I can live without a BFD.

    Thanks,

    Simon
     
  13. Guy Kuo

    Guy Kuo Supporting Actor

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    http://www.etfacoustic.com/

    I use this for my own system. The degree of information and rapid feedback for your adjustments is incredible. This looks expensive, but if you are really serious about analysing your speakers and room for optimizing the sound, seriously consider getting the software and calibrated mic.
     

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