Does a subwoofer "bottoming out" make a noise or don't you hear anything?

Discussion in 'Archived Threads 2001-2004' started by Robert Sun, Nov 18, 2002.

  1. Robert Sun

    Robert Sun Agent

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    In LOTR when the finger is cut off at the beginning my subwoofer makes a terrible screeching noise. Is this it bottoming out?
     
  2. Patrick Sun

    Patrick Sun Moderator
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    That sounds more like the amp is clipping the LFE input signal. This means you get some funky higher harmonics being sent to the sub, and it makes the funky, scary sounding noises when sent to the subwoofer. This happens when you have the sub playing at too high a level, and the amp can't supply the necessary current to drive the subwoofer driver for a normal low frequency waveform. Solution, calibrate the subwoofer with a calibration disc, and back off the subwoofer level (LOTR is notorious for having over-cooked/loud bass).

    Bottoming usually sounds like a big fart/pop as the sub is asked to play too low a frequency at too high a volume level.
     
  3. Robert Sun

    Robert Sun Agent

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    Patrick,

    Thanks for your answer. But when you say the amp can't provide the current to drive the subwoofer are you referring to the amp that my speakers/sub are connected to or the amp of the subwoofer itself?

    The sound is defintely not like a fart/pop!
     
  4. Mike Sloan

    Mike Sloan Second Unit

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    The Plate amp on you subwoofer may be clipping. What type of sub due you have, room dimmensions, how loud were you playing the LOTR in relation to Ref. Calibration, how hot were you running the sub output from receiver, what do you have the LFE set to in setup menu. Sounds like alot...but it helps to track down problem. I got some weird noises from my sub..I worked on placement and calibrations...and I actually increased the output with less distortion! If this is the theatrical release of LOTR...it is cut really loud and I listen to it at minus 14 under ref.
     
  5. Edward J M

    Edward J M Cinematographer

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    Hard bottoming can make a metallic "CLACK" type sound, too. Does this better describe what you heard? I've never heard it described as a screech sound, though.

    Ed
     
  6. DanielN

    DanielN Stunt Coordinator

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  7. Ryan_Garcia

    Ryan_Garcia Extra

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    when bottoming occurs, does this damaged youre subwoofer already? i recently played the LOTR and heard my sub played funny tunes, i cant tell if its the result of the amp clipping the sound or its bottomin already. it sound like "tack,tack,tack,tack" like youre adjusting a knob of an old telivision but louder. im using a vtf-2. im playing 4knotches above reference level, 9 o'clock in the sub volume. when i watched other movies, no problem are being encountered just this one with the LOTR. try to look at it. on the main menu of LOTR, select scene index, the bass sound there is really tight and thats where i encountered that funny sound.
     
  8. Guy Usher

    Guy Usher Supporting Actor

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    Yes you are bottoming out your sub, but it doesnt make any difference you are driving it into distress, you must turn it down to a point below where this occurs. You most likely have both things going on clipping and bottoming, at 4 clicks above reference you are pushing it too hard, if you need that much volume then get another sub, probaly a larger more powerful one and maybe you will still need more than one.
    If you are lucky there may be no damage but it is certain that it dont like it. Voice coils on big woofers are tuffer than the ones on tweeters but are not indestructable. at a low volume level put your ear up to it and listen very closly for any scratchy sounds as the woofer moves. You can hear slight damage before the woofer quits all together but if you hear any scratchyness(sp) you have done in the woofers voice coil. Most likely can be fixed, if you hear any noise dont play it anymore untill you get it looked at.
    There is more to it than I described, maybe one of our DIYers can explain it better.
     
  9. Brett DiMichele

    Brett DiMichele Producer

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    When you bottom out a woofer the voice coil former comes
    into contact with the T-Yoke (Backing Plate of the Motor
    Structure) Voice Coil Formers are usually made of stiff
    Paper or Kapton and then the Voice Coil wire is wrapped
    around the Former.

    When you strike the Former against the Backing Plate this
    can cause the Former to be bent and warped and this will
    deform the Voice Coil. When this happens the Voice Coil
    will usually begin rubbing the Magnet Assembly and then
    the Shelack will be worn off of the Voice Coil wire and
    it will create a dead short. When this happened *Poof* It's
    over baby!

    Turn the sub off, unplug it.. And gently push the driver's
    cone inwards evenly from both sides of the cone (don't push
    hard if it's a metal cone you could dent it easily) when
    you move it inwards and let it come back out, put your ear
    close to the driver and listen for a rubbing sound... You
    may also be able to feel it rubbing as you push it..

    If it's rubbing then the driver will need rebuilt or replaced.
    Make sure you push evenly though, if you push one side more
    than the other you will cause the voice coil to rub because
    it's not centered in the magnetic gap. So push straight
    down, gently...Exerting no more pressure on one side than
    the other.. If it rubs it's almost toast.. Get it looked
    at.. If it doesn't run you are probably okay..

    A sub will take a little bottoming without damage but do
    not let it happen often.. Back the Gain down and calibrate
    it a little colder.. It's not worth damaging the driver
    to hear more bass, if this is the case then experiment with
    room placement and/or get a larger sub.

    The Tack Tack Tack sound is when it barely bottoms out on
    a rapid succession of notes.. When it really bottoms out
    hard it will go *CLACK* and the *POP* noise is the surround
    traveling farther than it's suposed to (this usually happens
    on outward excursion)
     
  10. Tim Morton

    Tim Morton Stunt Coordinator

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    Just watch LOTR last night for the first time, and the first test of my new DIY NHT 1259 subwoofer mounted under my floor (fireing up thru many holes)...I was playing it VERY loud....amazing sound track(and movie)...the sub never seemed to struggle at all, whether it was slamming me in the gut in the scene dropping the ring...or shaking the foundation in the countless scenes whule i sat in my sofa enjoying the effects. The family thought i was nuts...but what a thrill.
     

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