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Speaker Failure article

Discussion in 'Electronic Repair' started by Phil Allison, Jun 5, 2012.

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  1. Phil Allison

    Phil Allison Guest

    Hi all,

    this short article, by yours truly, sets out to debunk the many absurd myths
    surrounding sudden failures of loudspeakers - particularly woofers used in
    live music and disco sound systems.

    http://sound.westhost.com/articles/speaker-failure.html

    The article is rather tightly written, so you may need to read it a bit at a
    time and cogitate.

    Take a careful look at the links provided at the end of the article too.

    And before anyone asks, the 40mm voice coil and magnet gap are from a 10
    inch woofer used in a very old AR2a.

    The pics shown in the article were taken by me with a pocket size Cannon
    A430.



    ..... Phil
     
  2. The article is rather tightly written, so you may need
    Actually, it's terse and to the point. It needs a bit of editing here and
    there, but everything does. I'm currently editing articles for "Electronic
    Design". If every one were of this quality, I wouldn't have a job.

    I'm reminded of Robert Stroud, the Birdman of Alcatraz.

    Talk about Freudian slips...
     
  3. Phil Allison

    Phil Allison Guest

    "William Sommerwerck"

    ** That's the nicest thing Bill has ever said about me.

    Tears .........


    ** Because I misspelled "Canon" ??



    ...... Phil
     
  4. The pics shown in the article were taken by me
    Well, you are rather a loud, loose cannon.
     
  5. Phil Allison

    Phil Allison Guest

    "William Sommerwerck"

    ** No way - I am a real straight shooter, every time.

    As us Aussies would say, Sommerwanker is the one with:

    " A few roos loose in the top paddock "



    ..... Phil
     
  6. ** No way -- I am a real straight shooter, every time.

    The following is meant seriously and sincerely...

    You're not aware of your Jekyll-and-Hyde behavior? If you are, do you think
    it's normal or justifiable?
     
  7. Phil Allison

    Phil Allison Guest

    "William Sommerwanker"

    ** No such thing for me or anyone else to aware of.

    But I have long ( something like 8 years long ) been very aware that YOU
    are merely another half witted, autistic, mental defective.

    Usenet is flooded with hundreds of similar, defective individuals, attracted
    just like moths are attracted to lights at night.

    So is the entire academic world, the public service and all forms of
    bureaucracy.

    Human garbage and the root cause of all trouble and misery on this planet.



    .... Phil
     
  8. Wond

    Wond Guest

    Great summary, Phil, thanks for posting. And, thanks Rod, for the
    site!
     
  9. You're just the sort of person I enjoy fluttering around.
    So this is all serious? It's not some out-of-control joke?

    Why would anyone want you as a friend, let alone be around you?
     
  10. Phil Allison

    Phil Allison Guest

    "spamtrap1888"
    "Phil Allison"
    --------------------------------------------------------------------------
    Pretty good! The write up does raise a few issues I would like to
    address:

    1. Back in the day we found that the pole piece vent is useless for
    cooling. Without the vent, when the woofer compresses the air in the
    enclosure, the dustcap (assume solid) pushes air between the voice
    coil former and the pole piece. This causes air to flow out of the
    space between the magnet and the pole piece, over the wires of the
    voice coil and out the gap. On the expansion stroke, air flows back
    over the voice coil and then between the VC former and the pole piece.
    --------------------------------------------------------------------------------

    ** With most speakers, there is a sealed volume behind the magnetic gap so
    very little air flow goes on.
    Hot air adjacent to the voice coil circulates with cooler air and this
    serves to heat the magnet structure, along with conduction and radiation of
    heat from the voice coil.

    The main reason for having a hollow pole piece is to relieve pressure on the
    cone during large excursions, but it also allows moving air to cool the pole
    piece too.

    ---------------------------------------------------------------------------
    With the vented pole piece, most of the air simply goes in and out the
    vent, and not past the voice coil. The forced air movement past the
    voice coil caused by the unvented pole piece should cool it better
    than simple conduction (plus some convection) to the pole and to the
    top plate when the pole piece is vented.
    ----------------------------------------------------

    ** JBL designed a way to force cool air over at least part of the voice coil
    in operation - provided the cone is moving significantly at low frequencies
    of course. Their idea, called " Vented Gap Cooling " takes advantage of the
    AES 50Hz to 500Hz testing method to get around double the previous published
    power ratings.

    http://www.jblpro.com/catalog/support/getfile.aspx?doctype=3&docid=297

    See the final two paras on the first page.

    ----------------------------------------------------------------------------

    2. While the point is well taken that attempting to eliminate clipping
    by using higher powered amps merely increases power supplied to the
    speaker, for a given amplifier, tweeters are significantly more likely
    to be destroyed when the amplifier clips, because amplifier clipping
    sharply increases the amount of high frequency signal, and tweeters'
    small excursions produce less cooling.

    --------------------------------------------------------------------------------

    ** Tweeters burn out for the exact same reason woofers do when the ampler
    clips - cos the *average power level* has gone up !!

    With normal unclipped programme, a tweeter may receive 5% of the applied
    power or 5 watts out of 100. With 6 dB of clipping, that same tweeter will
    now receive 20 watts, a direct result of turning up the gain by 6dB.

    Others have done tests to show that the increase in high frequency energy
    due to peak clipping music programme is small in comparison to the above.

    ------------------------------------------------------------------------------

    3. Saying that the minimum impedance is purely resistive grates a bit,
    because the voice coil always has some inductive reactance, however
    small, and there is some intrawinding capacitance as well.

    -----------------------------------------------------------------------------

    ** Real speakers (woofers) test purely resistive at some frequency in the
    200Hz to 500Hz range and this condition corresponds with the impedance
    minimum.

    If the impedance minimum for a particular driver is at 250Hz, then below
    that frequency the impedance is capacitive and above that frequency it is
    inductive. ( There is usually about an octave range where the impedance
    varies by only 10%. )

    If you sweep test a driver using a dual trace scope, one channel for voltage
    and one showing current, you can observe the two traces coinciding in phase
    at any impedance minimum or maximum.


    The minimum
    impedance is just the spot where the high electrical impedance of the
    mechanical resonance rolls off and before the inductive reactance
    takes hold.

    ** Correct.

    If you were to jam the voice coil tight in the cap, then all you have is the
    R of the coil plus some ( lossy) inductance.


    ..... Phil
     
  11. Admittedly my AC theory is a bit rusty, as am I,
    Mr Allison is quick to call me and others autistic idiots. Well, this
    autistic idiot will be the first to give the correct answer.

    The reactive component of the voice-coil impedance -- or any impedance --
    doesn't dissipate energy. It's as simple as that.
     
  12. Phil Allison

    Phil Allison Guest

    "klem kedidelhopper"

    This is a very well written article Phil however I have a question. In
    "voice coil facts" you state:

    "When a current is passed through a voice coil, heat is generated. The
    amount of heat in watts is given by the very simple formula:

    P = I² * R - where I is the RMS current and R is the actual
    resistance of the wire, at any temperature"

    Admittedly my AC theory is a bit rusty, as am I but since a voice coil
    has an inductive component to it shouldn't the formula incorporate
    the J factor, (R+JX)? Or is this just a theoretical example of perhaps
    a DC current being passed through the voice coil? Lenny


    ** Sometimes the facts are SOOOO simple folk refuse to believe them.

    In any complex, reactive circuit ONLY the resistive components dissipate
    actual heat.

    Inductive or capacitive components in series with resistive ones only act to
    REDUCE the current flow.

    So, with a speaker voice coil, one only has to know the RMS current flow and
    the actual R value to calculate the HEAT in watts.

    R varies with temp, so watch out.


    .... Phil
     
  13. Phil Allison

    Phil Allison Guest

    <>

    Actually thst airflow in the voice coile is negligible in an infinite baffle
    system. And the smaller the cabinet, the more negligible it is.

    I built a system once the was so totally short on cubic, but so totally tall
    on watts that if I would have left it that way the cabinet would have
    exploded.

    They were some type of thruster type speakers, the foam was all out. I put
    woofers in for the 10" passive radiators as well as the 8" actual woofers.
    The one set was four ohms which made the system 2.6 ohms.

    Boomy as hell, but I have a Soundcraftsman PE 2217. Every DB of boost at 31
    and every one of cut at 120 did most of it, and they sounded good.(I used
    them for surround at the time) Really, this is what the system needed.
    Nobody bitched when Bose did it. (remember the EQs for the 901s)

    But those cones barely moved. Without a port fighting them they didn't have
    to move as much, and their response was alot more predictable.

    Just so you know, I am better than Dr. Bose. I can get you good sound out of
    cardboard boxes. If you doubt that, come on down and challenge me. You know
    what else that will blow your fucking mind ? I can do it all with Radio
    Shack shit !

    You think I'm kidding ? Well I am right here. Sound is a matter of taste,
    like cooking. Even Radio Shack tweeters can be set up to sound good, if you
    know what you are doing.

    I can make a crossover that makes a dead frog sound like an ESS Heil. Well
    close.


    ** And besides all that - you are such a modest chap.

    Plus you spelling is just wonderful .....



    .... Phil
     
  14. JW

    JW Guest

    Please tell me that that was intentional...
     
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