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Plotting a frequency response??

Discussion in 'Subwoofers' started by lostprophet, Dec 12, 2004.

  1. lostprophet

    lostprophet
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    Really just wanted to check I'm doing this right. Running NCH from my pc into AV reciever, turned off my stereo amp so only the sub is running, crank it up to the correct volume (-22 for me, at least when setting up this is the volume at which the internal test tones were at 80db) then starting at 16Hz with NCH take a measurement with my spl from listening position and increase taking readings each time then plot in a graph and hey presto...right?

    I have my amp xover set to 100Hz (although normally 80Hz, just trying it for a bit), do I need to adjust this while testing the sub?

    Please please please can someone let me know the correct way to do this
     
  2. Dfour

    Dfour
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    No just do it as you have described. Some people say you should have your main stereo pair on so you can also check if the frequance responce and xover are working in tandem.
     
  3. eviljohn2

    eviljohn2
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    That's the right technique, but as DFours says, I'd consider doing it with your stereo speakers on too (and going through the amps crossover). Otherwise you're only considering a small piece of the whole picture.

    It's much better to start at a high frequency and go downwards rather than starting at 16Hz and working upwards. We don't want to inadvertently fry any components! :)
     
  4. lostprophet

    lostprophet
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    Ok thanks guys, I'll try that out
     
  5. bob1

    bob1
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    Are you playing at -22 on the sony display ,if so i would set the volume a lot lower ,you don't need to have the volume that high when plotting a graph.
     
  6. lostprophet

    lostprophet
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    thats the volume I get 80db from the test tones at, never been played with anything but the test tone at that volume though
     
  7. bob1

    bob1
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    lostprophet what i'm telling you is don't play the tones from the pc at -22 you might damage the sub.I'm shure the others don't relise that -22 on the sony display is above reference level as you messured to 80db and not 75db with the internal tones.
     
  8. eviljohn2

    eviljohn2
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    It's usually better to work just below reference level when doing frequency plots so you don't accidentally go too loud and fry something, even the most expensive of subs can break! Something like 5dB below would be fine, I tend to do -70dB myself which is still 5dB louder than I normally listen at. :)
     
  9. lostprophet

    lostprophet
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    ok, many thanks to both of you, I've read elsewhere on the forum that you should set up with 75 and other places 85 (although maybe thats with AVIA) so I chose 80 figuring it was a just a point to get the speakers at the same level, now I'll re-do it at 75db. Nothing has gone bang thankfully, now with your excellent advice I can set up my system with a bit more knowledge and a lot more caution!! Hadn't realised I was risking frying stuff!! Cheers peeps!
     
  10. eviljohn2

    eviljohn2
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    I wouldn't worry about frying stuff too much, a pure 16Hz sine wave at that sort of level may be cutting it a little fine though for something that's worth quite so much!

    The important thing is that all of the speakers are calibrated to the same level. Adjusting for the frequency response is a little more complicated, this is where things like crossover settings, phase controls and BFD's really come into their own.

    Using internal test tones it's usual to play them at 100dB and calibrate for 75dB although I'm not familiar with your Sony. It's a very large subject area but I'm sure you'll get on fine, just remember not to play anything too loud and to work down from higher frequencies so you can tell when your sub gives up the ghost (not going to be until around 20Hz or so with your big Paradigm!) :)
     
  11. lostprophet

    lostprophet
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    bob1, how have you set up your 1080? What volume etc? I'd be interested what you think of it as a reciever too as i haven't had mine for too long and I'm only just getting used to it and the huge array of adjustments available over my old reciever (it was rubbish!)
     
  12. bob1

    bob1
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    Hello by set up do you mean the speaker settings?
    I have all speakers set to small and crossover on all speakers at 80hz .
    I have had the amp 2 years now and think its good but lacks the power of the sony930 i used to have infact i was thinking of either a new amp or adding a power amp next year.
    If you need to know any details about the amp your not shure of please ask ,i might even know the answer.
     
  13. MikeRJ

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    Check the frequency response of the SPL meter as well, most of the reasonably priced ones don't go much lower than 30-40Hz.
     
  14. Nimby

    Nimby
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    In the case of the popular Radioshack SPL meter it does in fact go very low. But the built-in microphone sensitivity is not "flat" across the entire frequency range. The widely published correction figures are simply added to your actual C-slow meter readings.

    With a perfect subwoofer in a perfect room the corrected figures are then as close (as matters) to "flat" to as low as matters. Probably somewhere around 10Hz. :)

    There are other meters out there but I have no experience with them so cannot comment on them. The Radioshack is the popular choice worldwide for AV duties and has recenty had a "soft" facelift. Or face "slump" if you prefer. :D

    Nimby
     
  15. Ilkka R

    Ilkka R
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    Avia tones are recorded -30dBFS (below full scale), so 105dB-30dB=85dB. True reference level is 105dB, not 85dB.
    When you are plotting FR, you should do it atlest 70dB, I prefer 80dB.
    That has nothing to do with reference level. Tones are usually recorded 0dBFS or -10dbFS.
    80dB will not fry your coils, normal speaker sensitivity is 90dB/1W, now with 80dB, it's waaay below 1W. A good sub can handle hundreds of wats long time.

    Don't confuse measuring FR and calibrating you system. Two different things. :lesson:
     
  16. Nimby

    Nimby
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    No it can't. The high power of a sub amp is to accelerate the cone on heavy low frequency transients and then stop it from overshooting. If you feed continuous high power into a sub driver for any length of time you will fry the voice coil.

    There have been a number of posts on this very subject by the expert sub testers on other forums. I believe 30 seconds is considered a long test at high SPL. Shorter periods are more normal when testing maximum SPL at very low frequencies. Longer tests will produce thermal compression and make the test meaningless unless the destruction point is being researched. As the temperature of the voice coil rises its impedance increases. Reducing the amount of output power per watt.

    Fims and music do not provide the continuous high level demands on a subwoofer like test tones do. Test tone levels and periods do not really exist on DVD films. Most LF Effects are relatively brief and variable in frequency content and maximum level.

    Testing on continuous test tones should really be limited to around 10 seconds (Snapbug's tone length) and preferably below 80dB in-room output for response curve testing (and calibrating the sub with the speakers)

    Many subwoofers have some sort of limiting circuitry to avoid abuse at very low frequencies and very high levels.

    Nimby
     
  17. lostprophet

    lostprophet
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    So with correction values I can test down to 22hz (thats what Paradigm say the ps1200 goes down to)? Oh and Bob1 what volume is reference for you? I've found -29 gives me 75db with the test tones although I tend to watch films 10-15db below this and even then it scares me at times!! :zonked:
     
  18. Ilkka R

    Ilkka R
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    Yes, but than doesn't change the fact that a good subwoofer can handle say 300W continuous, it means forever. Short term power handling is different thing, it can be 500W, 1000W or even more. That is needed with those short transients.

    And as I said earlier, If you test your FR at 80dB, there is no danger at all. Power that voicecoil have to manage is less than one watt! Even at 90dB it's still near to one watt. Than 30s maximum test time is nonsence. With those levels, there is no limit. You can run that signal to your sub forever and it will be fine.

    If you are testing you subs MAXIMUM output, then you have to be careful. Then the power readings can be hundreds of watts and long continuous sinisignals CAN fry your subs voicecoil(s). But then we are talking about levels way over 100dB-110dB (atleats with SVS subs).

    Understand? :smashin:
     
  19. bob1

    bob1
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    Just tested it for you -20 on the display gives me 75db on the meter.
    Biased on that i listen to most dvd's at -25 ,i can't understand why anyone would want to listen at reference level. :eek:
     
  20. lostprophet

    lostprophet
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    agreed, watched lord of the rings tt last night and started it at reference and thwe start with gandalf fighting the flame demon thing was scary had to knock it down 10db to be able to stand it
     

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