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Thread: measuring Thiele-Small parameters - what am I missing?

  1. #1
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,383
    I'm Andrew.

    Default measuring Thiele-Small parameters - what am I missing?

    Any loudspeaker experts out there? I know Vas is difficult to measure, and the "added weight" method is easier than building a test box, but what am I missing here?
    Here's the page I'm referring to:
    http://www.mh-audio.nl/Measuring_Loudspeakers.asp

    and here's the section of it that has me baffled (excuse the pun):

    Calculate Vas with Mass:
    With the speaker facing upward, place small weights evenly around the dust cap in the center. With the added mass on the speaker, find the shifted resonant frequency using the same method you used before to determine the free air resonant frequency.

    The formule is: Vas = 1.42 * 105 x Sd2 * Cms

    Sd = Cone area

    In our example, Mass = 30 grams, shifted resonant frequency = 13 Hz, VAS is 58.5 Liter.


    The formula for Vas uses only the cone area and Cms. Where has Cms come from?

  2. #2
    Join Date: Oct 2017

    Location: Coventry

    Posts: 70
    I'm Mick.

    Default

    Hi Found this..
    "Vas/Cms

    Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. Vas is one of the trickiest parameters to measure because air pressure changes relative to humidity and temperature — a precisely controlled lab environment is essential. Cms is measured in meters per Newton. Cms is the force exerted by the mechanical suspension of the speaker. It is simply a measurement of its stiffness. Considering stiffness (Cms), in conjunction with the Q parameters gives rise to the kind of subjective decisions made by car manufacturers when tuning cars between comfort to carry the president and precision to go racing. Think of the peaks and valleys of audio signals like a road surface then consider that the ideal speaker suspension is like car suspension that can traverse the rockiest terrain with race-car precision and sensitivity at the speed of a fighter plane. It’s quite a challenge because focusing on any one discipline tends to have a detrimental effect on the others."

    Here... https://www.eminence.com/support/und...dspeaker-data/


    And this ...

    "How to measure speaker compliance ?

    You should be quite comfortable with measuring thiele small parameters by now. Compliance (Cms) is easily calculated if you know, or measured, the parameters above.

    To calculate the compliance of the speaker use this formula :

    Cms = [(6.283 * fs)2 * Mms]-1

    Cms is measured in meters / newton. Mms is given, and measured in kilograms."

    Here...
    http://audiojudgement.com/measuring-...ameters-part1/

  3. #3
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,383
    I'm Andrew.

    Default

    Thanks for that Mick, but I know what Vas is already - I just don't get the bit I quoted in blue above. It tells you to add some mass and remeasure the resonance - then the formula which follows has no mention of mass or frequency! Instead there's a hitherto undefined parameter
    Read the page and tell me where Cms was mentioned before, I certainly can't see it.

  4. #4
    Join Date: Oct 2017

    Location: Coventry

    Posts: 70
    I'm Mick.

    Default

    Hi Andrew, It looks like they missed a step...

    Will this not solve your Cms value?


    "How to measure speaker compliance ?

    You should be quite comfortable with measuring thiele small parameters by now. Compliance (Cms) is easily calculated if you know, or measured, the parameters above.

    To calculate the compliance of the speaker use this formula :

    Cms = [(6.283 * fs)2 * Mms]-1

    Cms is measured in meters / newton. Mms is given, and measured in kilograms."

    Here...
    http://audiojudgement.com/measuring-...ameters-part1/

  5. #5
    Join Date: Oct 2017

    Location: Coventry

    Posts: 70
    I'm Mick.

    Default

    For this Seas drive unit the Cms is given

    Nominal Impedance: 8 Ohms
    Recommended Frequency Range: 20 - 1500 Hz
    Short Term Power Handling: 300W
    Long Term Power Handling: 80W
    Characteristic Sensitivity (2,83V, 1m): 89dB
    Voice Coil Diameter: 39mm
    Voice Coil Height: 14mm
    Air Gap Height: 6mm
    Linear Coil Travel (p-p): 8mm
    Maximum Coil Travel (p-p): 35mm
    Magnetic Gap Flux Density: 0.72 T
    Magnet Weight: 1.10kg
    Total Weight: 2.10kg
    Voice Coil Resistance: 6.3Ohms
    Voice Coil Inductance: 3.8mH
    Force Factor 9.7 N/A
    Free Air Resonance: 25 Hz
    Moving Mass: 37.4g
    Air Load Mass In IEC Baffle: 3.80g
    Suspension Compliance: 1.1mm/N
    Suspension Mechanical Resistance: 1.69Ns/m
    Effective Piston Area: 350cm2
    VAS: 169 Litres
    QMS: 3.84
    QES: 0.43
    QTS: 0.39

    From ...
    https://www.hificollective.co.uk/com...-h1411-08.html

  6. #6
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,383
    I'm Andrew.

    Default

    Thanks for the link Mick. In that method Mms is required to calculate Cms. How do you determine Mms? You need to add Mmd and Mmr. How do you determine Mmd? Like this:
    "We can calculate Mmd by placing the cone, the coil, half of the surround and half of the spider on a scale. To do this, you need to tear up your speaker, and that might be a deal-breaker."
    Yes, that's a deal-breaker!

    Anyway, I know there are ways to measure Vas. The formula in the original link (quoted in blue in the OP) looks simple enough at a glance but makes no sense when you look more carefully - at least to me.

  7. #7
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,383
    I'm Andrew.

    Default

    Quote Originally Posted by Mixc View Post
    For this Seas drive unit the Cms is given

    From ...
    https://www.hificollective.co.uk/com...-h1411-08.html
    Yes, some manufacturers supply full data with their drivers - most helpful
    Thanks again.

  8. #8
    Join Date: Oct 2017

    Location: Coventry

    Posts: 70
    I'm Mick.

    Default

    Hay if it was easy everyone would do it..
    I built a some enclosures many moons ago using a book called "Bullock on boxes", as far as i remember I had all the parameters i needed, the ones that weren't supplied with the drive unit could be measured. I also used the rule of thumb that i could reduce the internal volume easily with damping pads, so built them slightly larger then measured the response.
    These were the only speakers that i have ever seen deflect the phase cap with the room air pressure....
    Good luck with the build, What drive units are u using?

  9. #9
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 1,383
    I'm Andrew.

    Default

    Quote Originally Posted by Mixc View Post
    Good luck with the build, What drive units are u using?
    At the moment I'm just playing around with some measuring software (ARTA) and getting a feel for things. I have a selection of old drive units lying around and it would be nice to be able to do some reliable measurements on them. Measuring resonant frequencies and Q is relatively easy, and measuring Vas isn't impossible but building a test box just to take readings is a bit of a pain in the backside so I was tempted by the "added mass" method. Perhaps I shouldn't be so lazy

    Anyway, I'm just playing around. I'm not intending to go mad and build "the ultimate speakers" at the first attempt - I'm trying to learn more about the art of designing. Actually, I have made speakers in the past and used them quite happily, but I always felt like I was fudging my way through the process and just got lucky. Fancy computer measuring software etc. wasn't available to me then so I thought I might revisit things. Still, loudspeaker designing is more of a passing interest rather than something I'm dedicated to. Electronics is my bag - maybe I should stick to that

  10. #10
    Join Date: Apr 2012

    Location: Southall, West London

    Posts: 36,869
    I'm Geoff.

    Default

    Quote Originally Posted by RothwellAudio View Post
    I have a selection of old drive units lying around
    If they are drivers that are known to have been used in manufacturer's systems or supplied for use in particular cabinets, perhaps the box designs can be found. I could possibly identify where some may have been employed.

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