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    Damping factor myth corrected

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    bordonbert

    Posts : 815
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Damping factor myth corrected

    Post by bordonbert on Mon Jun 22, 2015 8:36 am

    I was of the opinion that I understood damping factor.  I've been dealing with it for decades and never come across a contradiction before. For those who have never looked into it, it's basically a measure of how successfully the movement of the cone is controlled and prevented from overshooting and ringing, particularly near the resonant frequency of the driver in its cabinet.  Mathematically it is:

    [ Speaker Coil Resistance ] / [ Amp O/P Impedance + Ancillary resistances (cables etc) ]

    The amp output impedance is usually considered to be resistive, while we know the speaker's is not but we only consider its resistive part.  So, as an example, a speaker with 6.3ohm coil resistance fed from an amplifier with 1ohm output impedance through cables with 0.05ohm resistance would see a damping factor of:

    Damping Factor (DF) = 6.3 / (1 + 0.05) = 6

    Enough numbers!  Higher damping factors mean tighter control.  That isn't always good!  Hifi, yes, damping factors in the 100s is generally considered a good thing and is reachable with ultra low impedance solid state amps.  Guitar rigs need less or they can sound dull and flat.  If you don't believe it, try running your guitar through a hifi amp into your usual speakers and hear how blaahhh it is.  Fortunately we have output transformers and the like to increase the output impedance or clever circuitry which deliberately raises solid state output impedance.

    Now I had always considered that the speaker parallel/series damping factor issue was intuitive.  Speakers in parallel can see the amp output impedance directly so basically don't affect each other much.  Speakers in series however see it through the voice coil of the other speaker.  It was "obvious" to me, (and to most other people out there including even people like JBL), that each speaker therefore lessened the DF of the other because they each saw the other as part of the source impedance feeding them.  I posted a question on another technical forum about matching transformers which included some of this in its info.  Waddya know, someone immediately popped up and politely but bluntly said I didn't understand DF too well.

    I was surprised and responded with explanations and he again insisted that it didn't work like this.  He maintained that speakers in series do not affect each others' DFs.  Effectively the DF was the same for both speakers, (assuming they are identical), and was the same for each as if there were only one.  So I swallowed my surprise, (pride), and dug into it.  At first all I could find was references which said the same as I believed including that JBL info sheet which clearly stated:
    ...connecting separate woofers in series reduces the damping factor to a value less than 1.  This will result in poor transient response.

    Then I found something which cleared it up in one.  It was a PDF written by a guy named Dick Pierce and it was a proper engineering analysis of the situation and a practical comparison between the predicted and "intuitive" belief.

    Damping Factor In Series Speakers

    For anyone who is interested in the technical side of guitar reproduction this is maybe an eye opener.  It appears that the two series speakers ARE independent of each other as far as DF is concerned.

    I went back and ate humble pie, (as I should), and thanked the guy for his time and for making me a little wiser. You're never too old to learn and never too clever to be wrong! (At least I'm not.)

      Current date/time is Fri Nov 24, 2017 9:51 am