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Thread: What do all these specs mean?

  1. #1
    tjamz Guest

    What do all these specs mean?

    I'm sure we've all looked at spec sheets for speakers and thought "WTF do all these numbers mean?" I found this on a car audio forum and thought I'd throw them out for you guys as well. Thank you to Geolemon on www.caraudioforum.com for a great write up.

    Qms -

    A mechanical / pneumatic "quality" number that truthfully doesn't lend a great deal of insight to helping you choose a subwoofer for most applications.
    It is affected by physical properties that you can look at more accurately (IMO) through other specs, such as moving mass (look at Mms), suspension compliance (look at Vas, or a Cms curve), etc.

    Qes -
    An electrical "quality" number... think of it instead as a good indicator of motor strength. Do you need a subwoofer with a strong motor? You might be surprised to learn that high strength isn't always a good thing. (hint: Look at EBP... or check out Qts. )
    The smaller the number, the stronger the motor.
    (also, the smaller the number, the smaller the enclosure, quite often - but it is just one factor in that equation )

    Qts -
    A calculated number, using Qes and Qms - for a "total quality" number. I sort of downplayed Qms's importance earlier... and even in this calculation, Qes so far outweighs Qms in this calculation, that Qts actually ends up quite close in value to Qes - and small adjustments to Qes affect Qts much more than even large adjustments to Qms would have.
    Anyway - this is an important one. One of the "main 3" you need to calculate enclosure size/type/performance. (along with Fs and Vas).
    Qts mainly can tell you what sort of application your subwoofer is compatible in:
    High Qts value? Very compatible in a sealed enclosure.
    Low Qts value? Very compatible in a vented enclosure.
    Qts around 0.45 (middle of the road)? Design compromise to work OK in either, rather than outstanding in one and bad in the other.
    It's very helpful that way.

    Fs -
    Resonant frequency. This is the frequency the subwoofer most easily resonates at - and in free air, has the highest impedence (actual resistance with a subwoofer varies both with frequency and enclosure - it's not just "4 ohms" or whatever - it may rise up over 40 ohms, in fact, at points!) at that frequency.
    What it's useful for looking at is in judging how low a subwoofer can play.
    A low Fs means the subwoofer will be able to play lower...
    ...but means it won't be as efficient in the higher frequencies either.
    So maybe not always a good thing - or rather, something you need to compromise with realistically, to target your own personal tastes... loud? Or low? Or a little of both?

    EBP -
    EBP stands for "efficiency bandwidth product", and it's just a helpful calculation, to expand on what Qts tells you with regard to the "what box is it good with?" question.
    EBP = Fs / Qes. Simple calculation.
    It includes Fs in the equation, because in a sealed box, the subwoofer needs a low Fs in order to play low frequencies.
    In a vented box however, you can design the enclosure so that it helps extend the low frequencies farther than a sealed box, so Fs isn't as important for that.
    Qes is important because in a vented enclosure, you have higher pressure forces at work - so you need a stronger motor to contend with them. In a sealed box, that stronger motor will drive up the system resonance (a box/sub combination version of Fs), making for bad sealed performance.
    The actual EBP number tells you whether it's good in sealed or vented...
    EBP at 50 or below, sub is best suited to sealed.
    EBP higher than about 80, sub is best suited to vented boxes.
    EBP in between, sub is OK in either sealed or vented.

    Vas -
    Suspension compliance. This number represents the "volume of air" that provides the same "spring" force as the suspension system on the subwoofer.
    What it means to you, is that a smaller Vas makes for smaller enclosures.
    What it also means, is that a smaller Vas makes for lower efficiency, and a higher system resonance as well.

    Mms -
    Moving mass. The cone and coil, and surround and spider even have a mass associated with them. The higher the moving mass, the lower the efficiency of the speaker - but the lower the Fs as well. A designer adjusts these when deciding how thick of a cone (of course a thick cone may be rigid too, which is a good thing), or what cone material to use.
    This is one reason why "high output" SPL subs are actually much less efficient than cheapo subs. Cheapo subs can be louder than SPL subs? Yes, unless you have the wattage to make that SPL sub scream, and that cheapo sub melt.

    Xmax -
    Here's a good one... represents the linear excursion of the subwoofer, from the "at rest" position, moving in one direction, until the measurement limit is met.
    AT least that's how SOME manufacturers list Xmax.
    Others list it as "peak to peak" - which obviously doubles the value.
    Others list Xmax when they really mean Xmech (like Polk, for example).
    Some perform a calculation of voice coil winding length compared to the thickness of the top plate - that's the traditional method, and it's not horrible, it's just also not "apples to apples".
    So you have to know what you are looking at, when comparing.

    (and by "linear" - that means that the motor strength spec doesn't change, and suspension compliance spec doesn't change, as excursion changes. When it does begin to change, that marks the end of "linear" excursion)

    Xmech -
    This is essentially excursion without regard to how "linear" excursion is. This is how far the cone assembly can physically move, before it crashes into something and something breaks - coil smacks the back plate, cone crashes on the spider mount ring, tinsel leads rip out of the cone - needless to say, it's a limit you don't want to find the hard way!

    Xsus -
    This is the suspension component of "Xmax". Subwoofers may be "suspension limited", having a suspension that has a smaller range of linear excursion capability than the motor has. Xmax is always the smaller of the two values.

    Xmag -
    This is the motor strength component of "Xmax". Subwoofers may be "motor limited", having a motor that has a smaller range of linear excursion capability than the suspension has. Xmax is always the smaller of the two values.

    BL curve -
    This is an actual plot of motor strength with respect to excursion - simple plot really - motor strength on the Y-axis, excursion shown on the X-axis (and generally shows both positive and negative excursion - so traditional subwoofers with traditional architectures can look quite a bit like parabolas on these plots).
    Very helpful for understanding visually exactly HOW LINEAR a motor can be - and even how gradually it rolls off after that limit.

    Cms curve -
    The same thing as the BL curve, but shows suspension compliance with respect to excursion... so you can see exactly how linear the suspension is, and what it's limits are, where they are reached, and even how graduallly it rolls off after that limit.

    Re -
    This is just the DC resistance measurement of the coil. It's always a bit less than the "nominal impedence", and really doesn't tell you too much, other than "this is the lowest the impedence will ever dip to, on the impedence curve". (at some frequencies, impedence may rise up very high - a 4 ohm speaker might hit 50 ohms at Fs!)

    Le -
    Inductance is potentially an important thing to look at.
    a voice coil is a coil of wire - and so resembles an inductor.
    What does an inductor do? It resists changes in current direction and flow.
    What does a speaker do? It changes direction and frequency as quickly as the music throws those changes at it.
    So - basically you want your inductance to be as low as possible.
    Also... an inductor is a 6dB/octave low pass crossover - so at some point, inductance could be high enough that it prevents the speaker from playing frequencies higher than a certain point - the speaker itself prevents it - the lower the inductance, the higher the speaker is capable of playing.

  2. #2
    Join Date
    Mar 2004
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    huh

  3. #3
    Join Date
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    they mean nothing to most people on here, but some good info for ppl who are into car stereos and such.

  4. #4
    tjamz Guest
    Also great for anyone that ever wondered "Hey, what does this crap mean"

  5. #5
    Join Date
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    Nice, i see you know what you are talking about, good post!

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