Subwoofer calibrations.....I just dont get it.

Discussion in 'Speakers' started by Richard Moya, Apr 11, 2003.

  1. Richard Moya

    Richard Moya Stunt Coordinator

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    Ok here is my setup:

    Sony STRDA2ES receiver
    JBL NSP1
    Sony SAWM40 Sub

    Im using AVIA to calibrate speakers. Calibrated all to 75db (including Sub). This was easy. When I leave it like this and watch a movie, the bass is just not to my liking. I like I little deeper feel to my bass hits. That would require turning up the sub do get some more response.

    Im still a newb, and dont quite know what to look for when people refer to "holes" when listening to test tones. I watched the SPL meter and definitely see some fluctuations when doing the certain tone tests. Im assuming this is the "curve" everyone refers to. I also dont fully understand the whole subwoofer phase. I know what it means, but I dont get much of a difference in response from normal to phase.

    My question is this. How do I know what to look for? I get (at least I think I do) a clean transition from mains to sub. The only thing I see that I didnt like too much was the sub dropout at about 36hz. I assume this is to be expected from a budget sub.

    Running the freq drop test on Avia I get some serious rumbling bass (which I like).

    I need help understanding the whole subwoofer calibration issue and setting my crossover properly if I need to.

    thanks
     
  2. Richard Moya

    Richard Moya Stunt Coordinator

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    anyone? I have read alot of posts but am still looking for a laymans explanation....
     
  3. Mark Hedges

    Mark Hedges Second Unit

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    All I can say is that when I calibrated with Avia I had to turn my Sub down about 5 dB or so for everything to calibrate correctly. And after listening to it since then I think that was the right thing to do for music, but for movies it is anemic. I think a lot of people turn their sub up 5 dB or so from the calibrated level because they like a little more punch during movies.

    Mark
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    There are 2 effects that cause "holes":

    - Your receiver is sending anything below 120 hz to the sub, but the sub crossover is mistakenly set to ignore anything above... 90 hz. So you are not getting sounds between 90-120 hz because of the mis-match. Generally to turn the crossover WIDE OPEN and let the receiver handle all the crossover tasks.

    - You get a LOT of subwoofer sound by wall-reflection. And the walls reflect some frequencies a lot better than others. When the parallel walls combine with the direct sound at the listening position, this creates extra-loud volume for these frequencies. These are called "peaks". Sounds that are 10-20 hz different dont get as good wall-reflections so they are quieter and called "valley's". So by contrast they are holes.

    Some advice:

    - Use Avia to adjust your regular speakers to 75 db. But adjust the subwoofer to produce 83 db. (There is some past discussion on why, but thats a different topic).

    Now you need to plot your peaks/valleys:

    Get a copy of a Bass Test Tone cd (one that produces 20 seconds of single-frequency sounds from 10-100 hz.) or download a copy of the NCH Test Tone generator and burn your own test CD. Do octives (+8 hz) for each track starting with.. 16 hz - 120 hz.

    Put the SPL meter at your listening position and record the subwoofer volume at each frequency.

    The technique is NOT to increase the valley's, but reduce the peaks. Find your biggest peak and the next valley and calculate the difference.

    Now move the sub 1 foot along the wall and re-plot. For this new position, is the difference between the biggest peak and nearby valley LESS? If so, you have found a better position.

    Keep moving the sub, plotting the response. Stop when you have moved the sub 50% along the wall. Masking tape on the carpet is useful for recording the sub position. You can write the peak-valley difference on the tape.

    The best sub position is where you get the smallest peak-valley difference. A perfect room would give you the same volume at all frequencies.

    Hope this helps.
     
  5. Richard Moya

    Richard Moya Stunt Coordinator

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    You are a gentleman and a scholar.
    Very clear, consise and detailed.

    I appreciate the response.
     
  6. JakeMcM

    JakeMcM Stunt Coordinator

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    where can you download the test tones?
     
  7. Frank Carter

    Frank Carter Screenwriter

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    Here's a link to download the free NCH Tone Generator, it's free and easy to use.

    The method Bob described placing your subwoofer is probably the best way to do it, however, it can be time consuming. If you don't want to go through that process, here's another commonly used method for finding an optimal location for your subwoofer:

    Crawling For Bass

    For the phase, I used the method described on Adire Audio's site:


    Bob,

    Could you please explain or point me to a link to the reason why one needs to set the subwoofer level to 83dB when the rest of the speakers are 75dB. I have S&V's Home Theater Tune Up(made by the same people who make Avia) and always found bass level to be extremely low.
     
  8. Frank Carter

    Frank Carter Screenwriter

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  9. Tekara

    Tekara Supporting Actor

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    on the NCH tone generator:
    I heard that it was bad to run pure waves through the subs, so wouldn't using this program for extended periods to plot graphs be considered harmful to the sub?

    I'm interested as come summertime I am going to calibrate EVERYTHING in my hometheater and spend a good chunk of time on getting everything nice and happy. I've messed with the tone generator a few times but though that running it for extended periods will cause problems so I haven't done much with it.
     
  10. Bob McElfresh

    Bob McElfresh Producer

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    Yes, you dont want to run the tones for extended times. It uses up a lot of power/heats up the sub. (but pure, mono-frequency sounds are actually LESS damaging to the sub because it's simpler).

    If you use the NCH Tone Generator, I suggest this:

    Track 1: 96 hz for 10-20 seconds (This gives you time to hit the main volume control and get volume somwhere in the 70-80 db range).
    Track 2: 88 Hz for 5-10 seconds

    (every track afterwards for 5 seconds so you can read the SPL meter, change the range and record the setting)

    Putting the SPL meter on a camera tripod at the primary listening position is a big help for this. This way you can glance/write, glance/write.

    This How Loud do you run your subwoofer thread talks a bit about running the sub a bit hotter.

    And in truth: calibrating the sub to match the mains, then bumping the sub output a bit more to enhance the effects is "seasoning to taste". It's your system. Use the adjustments/meters to get your system a common baseline, then tweek/enhance so it sounds good to you.

    Remember:

    A music system is about accuracy, but a HT system is about Impact.

    And that impact largely comes from the very artifical, enhanced, totally-non-existant-in-nature, subwoofer sounds on a movie.

    Radio Shack SPL Compensation

    Ok, the $40 SPL meter is not the most accurate/definitive SPL device. There is a compensation table that you can use on the readings for the different frequencies to adjust the meter readings so they match a much-more sensitive/accuate meter:

    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 is a thread on EQing the sub that is the source for much of my advice.

    House Curve
    They have found that people like the sound better if the subwoofer volume goes UP as the frequency goes DOWN. I think this is because our hearing is less-sensitive as the frequency goes down.) This is called a "House Curve".

    One common ammount of slope is to have the volume increase by 8 db as the frequency goes from 80 to 30 hz. (This is another reason for the 8 db bump in the SPL adjustment over the mains).

    Now to give your system a curve like this requires an equalizer. So we are getting into some advanced stuff here.

    (Hint: search the Speakers and subwoofer fourm for "House Curve" for lots of other subwoofer calibration discussion).
     
  11. JakeMcM

    JakeMcM Stunt Coordinator

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    so if you burn the mono tones and only play them a few seconds it should be ok? I was thinking of going from about 20hz up to 200hz, at that point it would be playing from the main speakers as well, will it be more likely to harm them, should I just do the sub?
     
  12. Bob McElfresh

    Bob McElfresh Producer

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    Five to ten seconds of each frequency should not harm anything so long as the volume stays 70-90db. Trying to do many seconds at 100+ db - different story.

    If you are really worried, do this:

    Get a wave file of 10-30 seconds of silence and program:

    Track 1: 100 hz for 15 seconds (lets you adjust the main volume to about 80db
    Track 2: Silence
    Track 3: 20 hz for 5 seconds
    Track 4: Silence
    Track 5: 28 hz for 5 seconds
    Track 6: Silence

    This way you are not continously driving the sub with sound. And the silence times lets you write things down, or change the range on the SPL meter. And if you miss getting the reading, just hit "Previous Track" on the remote to repeat.
     
  13. JakeMcM

    JakeMcM Stunt Coordinator

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    I see thanks
     

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