142.5l sub bottoming or clipping?

Discussion in 'Home Theater Projects' started by Dan_M, Jun 10, 2004.

  1. Dan_M

    Dan_M Agent

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    I just finished my first DIY sub after several months of lurking here for tips and hints. I used the adire 142.5l box plans and a 12" Dayton Titanic MK3 for the driver. The box is tuned to 18.1hz as specified by the plans, and powered by the PE 250W remote amp.

    After finishing it up I centered the amp gain and loaded up some bass heavy movies. The first thing I listened to was the shark scene in Finding Nemo. Right away when the shark chomps his jaws when they first meet the sub distorted. It wasn't really a loud pop or clang, just a loud farty noise that sounded horrible. Later when the mines detonate again I hear a farty mess. Does this sound like the driver is bottoming? Using unibox it looks like I should be fine as far as excursion goes, plus this amp is supposed to have a rumble filter that starts at 19.4hz. Someone on the PE tech board remarked that this driver would be starved with only 250W and would easily be pushed to clip. Could this be what I'm hearing?
     
  2. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    There's a decent chance this is the problem, especially depending on the volume you're listening at. It would probably be less of a problem if you calibrate the sub to the same level as the rest of your speakers (assuming they're calibrated to the same level as well) using an SPL meter since it should do around 110db (depending on distance from the sub, room, gain, etc) w/ that amount of power before clipping, especially since you're not feeding it constant sine waves or anything like that.
     
  3. GrahamT

    GrahamT Supporting Actor

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    My Tempest did the same thing and it was caused by an air leak. Try checking around the driver gasket or port gasket and amp. The 250 W is enough for the Shiva in a ported alignment IMO.
     
  4. Eric S

    Eric S Agent

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    Yup, just like real farts, its caused by an air leak [​IMG]
     
  5. Kyle Richardson

    Kyle Richardson Screenwriter

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    A bottoming driver makes more of a loud "clank" sound. It may possibly be clipping or an airleak.
     
  6. Dan_M

    Dan_M Agent

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    The distortion does go away if I set my reciever to attentuate the LFE signal 10db. I also don't hear the distortion when running test tones into it, only hard hits in movies. Another wierd thing I noticed is that the volume knob on the amp doesn't seem to be having an effect after it's turned more than 1/3 of the way up. Shouldn't the sub be getting louder throughout the entire sweep of the knob? I do plan on getting a SPL meter soon to properly calibrate, it's tough for me to judge how loud things should be with no experience with a sub this size.
     
  7. Chris Brock

    Chris Brock Second Unit

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    in that case then I woudl say that you definitly have the gain on the amp turned up to far. it sounds as though you are hitting the amps max output very early on and after that all that will happen is an increase in distortion and heat.
     
  8. Dan_M

    Dan_M Agent

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    I went over all my screws and wrenched them tighter, I'm pretty confident there's no air leaks. Since I bought the remote amp from PE I had to cut an extra hole in the box to mount the controls. I dressed the gasket on this control panel with silicone to be sure it doesn't leak. There are only 4 screws holding it in and the plastic flexes as you crank down on them. The driver is sealed with the gasket material PE ships drivers with, and the port is sealed with some rope caulk.

    I guess I'll just leave it attentuated -10db until I can get a SPL meter and take some real measurements.
     
  9. Stephen Hopkins

    Stephen Hopkins HW Reviewer
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    I would reccomend a nice fat beed of caulk on the inside at every joint just to make sure.
     
  10. ColinM

    ColinM Cinematographer

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    Centering the volume control on that amp is putting it up there pretty high - In my case, it's never over 2/5...9 o'clock or so.
     

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