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Thread: Xmax questions

  1. #1
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    Xmax questions

    Does JBL have a defined set of parameters they base their Xmax specs on? According to this AES paper, there are a number of different ways this can be measured: AES comments on Xmax . I think I've noticed a couple of JBL spec sheets that mention Xmax at 10% distortion, but what type of distortion isn't mentioned.

    Also, is Xmax a good indicator of maximum output at frequencies other than fs? For instance, if Xmax is .2" at 30hz fs, does it follow that an "equivalent" Xmax, (linear displacement at 10% distortion), at 60hz would be .1"?

    John

  2. #2
    John Warren
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    Xmax is determined by subtracting the axial length of the magnet gap from the axial length of the voice coil and then dividing the difference by 2.

    (Xmax x Cone area) / frequency squared = acoustic output.

  3. #3
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    Originally posted by John Warren
    Xmax is determined by subtracting the axial length of the magnet gap from the axial length of the voice coil and then dividing the difference by 2.

    (Xmax x Cone area) / frequency squared = acoustic output.
    While "theoretically" correct, I believe most manufacturers have incorporated variables, (such as distortion components), to provide a more meaningful specification. According to everything I've seen, stated Xmax is usually something less than the physical net difference you mentioned. If everyone used the purely physical specs, (no distortion variables, etc.), the specs might appear comparable from one manufacturer to another, but otherwise meaningless in how much usable output you can count on - what good is a huge Xmax if it's at 50% distortion?. Unfortunately, it appears no real standard is in place, (as is discussed in that AES paper), so everyone uses their own interpretation. That's why I wondered which method JBL uses.

    John

  4. #4
    HenryW
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    Re: Xmax questions

    Welcome John Warren

    Just in case John is new to you folks he will be a welcome engineer. Just remember - John tends to let the emperor know when he has no clothes on....

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    johnaec,

    It does appear that JBL also has a problem with which standard to use. For instance, it seems the 2242H spec is stated as having been measured via the distortion method whereas the 1500 SUB spec is figured according to the method posted by John Warren. I've always used the method posted by John Warren because it's easiest.
    Last edited by 4313B; 05-17-2004 at 07:34 AM.

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    Yeah, I was looking at the spec sheets for a couple 2012H speakers I have and Xmax is measured at 10% THD. But at least the theoretical method allows for quick calculations.

    John

  7. #7
    Paul Joppa
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    Whatever happened to the 15%?

    Just to confuse the issue ...

    The most difficulty shows up when the voice coil and gap are exactly equal. Simple xmax is zero, but the speaker still works and even makes a little bass. I think the older D130 and 130A (great-grandfather of the 2220) were built this way.

    In this case, there was once a claim that that the gap fringing allowed on average an extra xmax of 15% of the gap length. It was extended to say xmax = VC length - 0.85*gap length, at least for overhung coils. IIRC, this came from a JBL paper in the JAES.

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    Re: Whatever happened to the 15%?

    Hi Paul

    Good point! Let's find that paper.

    From an old post -

    The 2121 has a voice coil winding depth of 0.280"
    The 2122 has a voice coil winding depth of 0.180"
    The 2123 has a voice coil winding depth of 0.420"

    All three have a 0.280" top plate.
    All three currently have a moving mass of roughly 26 grams.

    The published xmax on these drivers:

    .060"
    .120"
    .100"

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