I want to know what Qms

jacksports Posts: 4
edited December 2013 in Custom Fabrication
I want to know what Qms, Vas and Qes means?
Post edited by jacksports on


  • mhardy6647
    mhardy6647 Posts: 31,284
    edited November 2013
    These are three of the parameters used to quantify the performance characteristics of a loudspeaker driver; three of the so-called "Thiele-Small" parameters. The T-S parameters are used to "fit" a driver to an enclosure (or, to think about it conversely, to "tune" an enclosure to a given driver). Arguably, the full gamut of T-S parameters is most important for woofers, but in practice the parameters may be determined for any driver and may be useful for MR and tweeters, too - if for no other reason than to select crossover frequencies.

    Here's a personal favorite primer on measurement of T-S parameters. It identifies the important parameters, even if the definitions given are a bit circular :-)


    Re Electrical [i.e., DC] resistance of voice coil
    Fs Resonant frequency of loudspeaker moving mass (in free air)
    Qes Electrical Q of loudspeaker ["Q" describes the shape of the peak of the impedance curve around Fs (see the figure below) - high "Q" = high, sharp "mountain peak"; low "Q" = broad, low "hill"]
    Qms Mechanical Q of loudspeaker
    Qts Total Q of loudspeaker [see the link to see how Qms and Qes relate to Qts]
    Vas Equivalent air volume of moving mass suspension [this is a measure of the compliance or, I ould say, the "floppyness" of the driver's suspension]


    Note the impedance peak around Fs - this is the "Q" curve. The impedance is fairly flat around 100 to 200 Hz; this is where the "nominal" impedance figure comes from. At higher frequencies, the impedance rises due to the "inductance" of the speaker's voice coil.

    There are some very good books on the subject of loudspeaker (enclosure) design - I like David Weems' books and Ray Alden's books. Vance Dickason's Loudspeaker Design Cookbook is another classic design reference.

    EDIT: Google turned up some more explanatory pages :-)
  • pitdogg2
    pitdogg2 Posts: 22,429
    edited November 2013
    bet this went zoom right over their head:)
  • mhardy6647
    mhardy6647 Posts: 31,284
    edited November 2013
    ahh, well... we do what we can. That "speakerplans dot com" has a pretty good nontechnical explanation of what the parameters mean.
  • MacLeod
    MacLeod Posts: 14,358
    edited November 2013
    Ive been in this hobby since 1988 and the sport as a competitor since 2005 and I STILL dont have a clue what all this T/S parameters is about. All I know is that you need a Qts of .40 or higher to run a speaker in infinite baffle and .71 Qts is considered perfect (flat). I kinda know what resonant frequency (Fs) means that basically it shows you how competent the driver is at lower frequencies like a tweeter with a Fs of 750 like the Scan Illuminator is going to work a lot better crossed over at 2.5KHz than a LPG 26NA with its 1500 Hz FS. But thats about it as far as my knowledge goes. I stick to the calculators like WinISD to do all the legwork.
    polkaudio sound quality competitor since 2005
    MECA SQ Rookie of the Year 06 ~ MECA State Champ 06,07,08,11 ~ MECA World Finals 2nd place 06,07,08,09
    08 Car Audio Nationals 1st ~ 07 N Georgia Nationals 1st ~ 06 Carl Casper Nationals 1st ~ USACi 05 Southeast AutumnFest 1st

    polkaudio SR6500 --- polkaudio MM1040 x2 -- Pioneer P99 -- Rockford Fosgate P1000X5D
  • Face
    Face Posts: 14,340
    edited November 2013
    Here are some much more advanced "calculators": http://audio.claub.net/software/jbabgy/jbagby.html

    Woofer Box and Circuit Designer makes WinISD look like a toy.
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
  • Speedskater
    Speedskater Posts: 495
    edited November 2013
    Many big libraries have loudspeaker books by (1)David Weems or (2)Vance Dickerson. Both authors start slowly.
  • mhardy6647
    mhardy6647 Posts: 31,284
    edited November 2013
    Vance Dickason (that'll make his books easier to find) :-)