craptacular frequency response

Discussion in 'Speakers' started by Oupei, Mar 21, 2005.

  1. Oupei

    Oupei Agent

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    edit: updated plot. old plot can be found here
    [​IMG]

    I made this plot using the RS SPL Meter with corrections and 16bit 44kHz mono wav files burned on a cdr played on mains and sub from my Atlantic Technology 170 system.

    I know I can BFD the bass, but everything above 90hz is played by the mains and I can't do anything about that. That -9dB null at 200hz is absolutely horrendous and completely ruins what would otherwise be +/- 3.5 dB in the mains. Where did I go wrong?

    edit: new pics of my ht setup:
    pic3
    pic4
    pic5
    old pics:
    pic1
    pic2
    it's a bit more funished now than when I took the pics, but the major things are still the same.
     
  2. Ilkka R

    Ilkka R Second Unit

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    That tiny sub is playing flat to 14Hz...?? What is it and are you sure you measured correctly?
     
  3. John S

    John S Producer

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    Looks like some room issues perhaps? I would try some different placements, to see if the 200hz null can be improved. I take it your not using anything like dual centers or anything like that.
     
  4. John Garcia

    John Garcia Executive Producer

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    I'd check how you measured as well. That response is saying that sub is good to 80dB at 13Hz, and still has decent output at 10Hz...

    Your room looks relatively square with no treatments at all. Not the best thing for good response.
     
  5. John S

    John S Producer

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    Hmm, I didn't even look at the link showing the room.

    The 200 dip can often be from slap back off an opposing wall causing the null. For whatever reason 200 around the frequency that likes to be nulled by an opposing wall.


    Hard to say. I am still a little taken back by the sub performance. Not saying it's wrong, but my 15" Velo really drops off below 20hz.

    Just not really sure. Maybe try towing out the fronts a little and see if the 200hz improves.
     
  6. Jeremy_R

    Jeremy_R Stunt Coordinator

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    It's hard to tell from your pic, but it looks like you need to pull your sub out away from that corner by about 6-12 inches. That will help smooth out the bass response a little bit.

    It looks like you would be a good candidate for some room treatments.
     
  7. Oupei

    Oupei Agent

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    ikka,
    the measurements were taken quite a while back, I just never got around to plotting them so I dunno if maybe the results are inaccurate. The sub is an atlantic technology 172 bpm, a 12incher that claims 20-150 +/-3dB.

    John S,
    not using dual centers, this was measured using L/R and sub. Might L/R cause the same interference as dual centers?

    John G,
    yeah, the pics are a bit old too. I'd take a few new ones but I left my digital camera at school. I now have coffee tables in the 2 back corners and at either side of the front row loveseat. I also have curtains over the rear doors and left rear windows that aren't show in the picture. I'll see if I can get some new pics up before the end of my break.
    The room is exactly 15ft left to right, just under 20ft front to back, and 8.5ft high (~7ft around the edges with the vaulted ceiling).

    John S again,
    heh, lots of people doubting my sub. ok, i went back over my handwritten measurements and i noticed that I set the receiver at -10dB for 10-40hz and -20dB for the rest. However, I don't think it would make sense for me to lower the measurements from 10-40 by 10dB... I'll measure it again.

    Jeremy,
    The sub is 8in from the front wall and 7.5in from the right wall.

    ok guys, thanks for the inputs. It looks like I have some more work cut out for me. I'll get to getting some new pictures and taking new measurements.
     
  8. John S

    John S Producer

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    I'd try towing the fronts out a little.. That slap back off of the back wall, could certainly generate a null at around 200hz, I have seen it plenty of times. You would be way surprised at what the difference a little tow in here or a little tow out there can have on the meter.
     
  9. Oupei

    Oupei Agent

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    ok, i redid the plot...

    I'm pretty sure there's some interference going on in the upper frequencies though. When playing frequencies over 100hz, moving over a seat or so moves me from a null to a peak. I think the front left and right mounted on the wall like that creates a comb filter effect.

    The low frequencies are really crappy too. There's a huge peak right next to a huge null.

    as for feedback, the peak at 56hz is when the wavelength is 20ft, exactly the length of the room. the null at 200hz is when the wavelength is 5.75ft. At that wavelength, the room measures 2.5x3.5x1.5 wavelengths. I'm guessing this is no coincidence, so what can I do about it?
     
  10. John Garcia

    John Garcia Executive Producer

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    Bass traps in two or more corners might help. Are the curtains pretty heavy duty (dense)? If not, try some that are. Throw a soft blanket over those leather couches. Try hanging a piece of fabric or thin blanket on the walls at the first reflection point (which looks like your door or just past it on the right side) of the main speakers to see if it helps.

    Is this a 7.1 or 5.1 setup? If it's 5.1, can you move the surrounds to the side of the couches?

    One thing I would definitely want to do is get those main speakers off the wall (Yes, I can see that they look like on-wall speakers though).
     
  11. Oupei

    Oupei Agent

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    alright, i got some up to date pics of my ht up (links at the first post).

    John G,
    yup, it's 7.1. Would it help if I lined the insides of the coffee tables that are in the back corners and next to the loveseat with rigid fiberglass? Yeah, i know it doesn't cover much area, but that would space the fiberglass about a foot from the wall. I know about first reflection point, but I'd prefer not putting anything there...
    I'm in the process of making some speaker stands for the PS2. When I'm done, I'll borrow them for the HT and see what that does.

    Thanks again to everybody for the input.
     
  12. John S

    John S Producer

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    I still say a slight toe out on the front sound stage could solve some of the issues. The sub will be tougher for sure.

    I'll be curious as to how good is good enough for you on this?????
     
  13. John Garcia

    John Garcia Executive Producer

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    Toeing the mains in will move the first reflection point, and could help.

    Also, pull that center all the way to the leading edge of the table or put a piece of cloth in front of it. Right now it is reflecting directly off the table.
     
  14. Wayne A. Pflughaupt

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    Nice looking room, Oupei. [​IMG]

    I’m going to put my money on the sub itself as the problem. The ragged response above 56 Hz is not unnatural. However, the significant lack of output below that point is. A good 12” sub should shake the walls in a room that size and hit 20-25 Hz without breaking a sweat.

    Oupei, you have any friends you could borrow another sub from for the sake of comparision?

    Regards,
    Wayne A. Pflughaupt
     
  15. Oupei

    Oupei Agent

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    heh heh, do I sense someone getting sick of me draining them for advice? eh, don't sweat it, I'm pretty much ready to give up on this. all this measuring and photoing and nuthin to show for it. well except solid evidence that my HT is not what I hoped it would be. How good is good enough? well, before I started I wanted +/-3dB 20Hz-20kHz. Then of course I slacked to +/-3.5... and then +/-4 from 50-20k... and well, I guess I can't really do anything about it now.

    I'll try some toe, but I can't do much with them on the wall. no problem moving center up to the front of the table though [​IMG] and I can always be happy with it just looking nice. don't have any friends with real subs though, so I can't borrow one for comparison. but that just makes me the one with the best sub on the block.
     
  16. Jeremy_R

    Jeremy_R Stunt Coordinator

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    If you are really looking for +/-4 then you should hire someone to acoustically treat your room for you. It's going to be almost impossible to get that nice of a response without some good treatments.

    http://www.realtraps.com./rfz.htm
     
  17. John S

    John S Producer

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    Agree'd Jeremy_R.. Heck you could spend 5 digits just attempting to get +/- 3db at 1 meter 20hz to 20Khz..... [​IMG]

    The tow out / in on the fronts, should be pretty easy with a little creative thinking.

    I'd bank on your numbers improving just from this. I have seen excellent numbers produced in perfectly rectangular rooms, just adding decent heavy drapes along the walls by the way.


    +/- 4db might be possible If that was done. Not cheap, try purchasing fabrick and sweet talking somebody with a nice sewing machine to help you with it.
     
  18. Oupei

    Oupei Agent

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    wow, I had no idea a flat frequency response is so difficult and costly to achieve. I thought pretty much every HT on this forum would have like +/- 4.5dB at least.

    I can toe the fronts a degree or two by adding extra rubber feet behind them, and I'm still working on making speaker stands.

    thanks.
     
  19. John S

    John S Producer

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    Well most dedicated theaters here probably do. But not without a lot of thought and some effort being put into treating the room is all.

    As said, the drapes thing goes a long way, and is probably the easiest.

    If you ever measured / plotted this in living room environments.. heck they are all over the place. [​IMG]
     

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