Please Help Inerpret Sub Freq Response Chart

Discussion in 'Speakers & Subwoofers' started by Brian Henchey, Jul 28, 2003.

  1. Brian Henchey

    Joined:
    Aug 23, 2000
    Messages:
    35
    Likes Received:
    0
    Trophy Points:
    0
    Taking my cue from the legions of other HTF members desiring that perfect subwoofer response curve, I endeavoured to first assess the damage being caused by my room and also determine what effect tuning my SVS 20-39PC+ to 16hz would have. Both subs are located in the same corner of a 21x29 room with listening position at about 12-13 ft. Crossover set to 70hz. Both subs at 180 phase. Test tones were burned to a CD and played through a Denon DVD-2900 DVD player digitally connected to my pre-pro. Master volume was set at -35, about 8 to 10 clicks below normal listening volume for DD material (with Avia 85db reference at about -10).

    I actually started with the subs tuned at 16hz. I was going for a qualitative analysis with different movies rather than a quantitative one. Both subs (at 16hz) were calibrated with Avia to be about 89 db. Tried different movies at -15 volume. AOTC "exploding ball" bottomed out one of the subs (probably shoudn't have calibrated hot, huh?), the first time that has ever happened (of course, that was the loudest I've ever turned it up, so there).

    Ran the sine wave test tones with 16hz (one port plugged), then at 20hz (no ports plugged). I did not recalibrate with Avia--both runs had identical master volume and subwoofer level settings. On the chart, pink line is 16hz and blue line is 20hz. Please excuse my misspelling of the word "dual"--no pun was intended [​IMG].



    Any assistance in interpreting these curves would be most appreciated. Would it be worth it to purchase a BFD?

    Thanks in advanceFrequency Response Curve
     
  2. DavidES

    DavidES Stunt Coordinator

    Joined:
    Jun 23, 2003
    Messages:
    116
    Likes Received:
    0
    Trophy Points:
    0
    Assuming that the measurements were done at your sweet spot and that you have applied any appropriate microphone compensation:

    IMO, I would use the pink one because it is flatter and more natural-sounding for my tastes plus the extra output at 16Hz. The blue would be boomy and one-notey for material at the 19-26Hz blue set of tones.

    Can you re-arrange seating and maybe sub placement a little (a few inches, don't sit in the exact center of width) to help bring up the 26-35Hz 8dB pink dip, which to me, looks like a room suckout mode? I think you'll notice that pink dip somewhat, more if you used the blue line, that one would be very noticeable on music with double-bass guitars and deep drums.

    You might need eq for the +4dB 56Hz bump, that one looks like a small room boom/mode, but I'd work on the big dip first because the re-arranging might help correct the 56Hz bump also.

    You may want to check responses at other seats, if you have them, to tweak in the response there also. May have to compromise some.

    You definitely have done a great job there to start out. I wish I had your response at my place.

    Hope this helps.
     
  3. Edward J M

    Edward J M Cinematographer

    Joined:
    Sep 22, 2002
    Messages:
    2,031
    Likes Received:
    0
    Trophy Points:
    0
    I ran a 20-39PC+ for a year and I always ran it with all three ports open (unless I was experimenting). While opinions will vary, IMO the Plus subs need a lot of vent to breath properly and feel slightly "stuffy" with a port plugged. Yes, they will still deliver the goods in the SPL department with a port plugged, but not as effortlessly.

    Your graph does not really go deep enough, since you will see the 16 Hz tune hangs on well into the low teens (if you set the SS filter to 12 Hz). With that said, I never felt like the native tuning of the 20-39 enclosure lacked extension. It provides strong response to below 20 Hz with all ports open. And I never considered the room gain hump in the 22 Hz region (mine exhibited this too) to provide anything but a thrilling oomph for HT.

    I agree you might be running them a bit hot compared to the 100-125 Hz region, and you could probably benefit from a BFD or other parametric EQ.

    FWIW, here is my FR sweeps of the 20-39PC+ in both tunes in my room:

    [​IMG]
     
  4. Brian Henchey

    Joined:
    Aug 23, 2000
    Messages:
    35
    Likes Received:
    0
    Trophy Points:
    0
    Thanks for all the responses! Later this evening, I will try some slightly altered placements for the sub and a run with one PC+ at 16hz and one at 20hz (should be interesting).

    In addition, the HT is located in a room above a detached garage with lots of floor movement/rumbling in the average action movie. I wonder if the response would clean up if I built a sand-filled sand to prevent the subs from directly vibrating the floor (fundamental frequency vibration is OK).
     
  5. Brian Henchey

    Joined:
    Aug 23, 2000
    Messages:
    35
    Likes Received:
    0
    Trophy Points:
    0


    Did you run with all ports open, but with the SS filter set to something other than 20hz? If so, any problems with bottoming in this config?
     
  6. Brian Henchey

    Joined:
    Aug 23, 2000
    Messages:
    35
    Likes Received:
    0
    Trophy Points:
    0
    OK, I've got some new data from last night.

    First, I left all ports open on each sub, set the SS filter to 16hz and played the opening of AOTC (Cavalcade and Ship Explosion) at -20 (with the subs about 5dbs hot) [-11 is 85db reference in Avia, highest listening volume is -25]. No ill effects (port noise, bottoming, etc.), but the psychological stress from worrying about the increased driver excursion in any bass-heavy sequence was significantly higher. Flipping the SS filter to 20hz dramatically increased the SPL output and "rumble effect" and made for a much more fun experience when the ship landed and exploded.

    Next, I ran three sets of test tones (calibrated volume of 100hz tone to 80db (in stereo) and ran through the test tones with the main amp off): (1) baseline with SS filter (yellow); (2) one at 20hz and one tuned to 16hz (with one port plugged) (blue); and (3) both subs at 20hz tune, but in a slightly different positions (pink).

    The last one sounds *really* good in AOTC. After recalibrating flat to 85db's (uncorrected) with Avia, I discovered that my Sunfire TGIII's subwoofer test tone now tracks with Avia (before it had been significantly lower).

    I also learned a valuable lesson about focusing less on the graphed response and more on what sounds good from the couch. A BFD may be a future addition and I am still seriously considering isolating the subs from the floor with a 3" slate or granite slab, but I think I have found my final positions.

    Thanks for listening,
    Brian

    New Subwoofer Data

    PS--What's the best way to attach pictures to the text?
     
  7. Edward J M

    Edward J M Cinematographer

    Joined:
    Sep 22, 2002
    Messages:
    2,031
    Likes Received:
    0
    Trophy Points:
    0
     

Share This Page