Finding resonant frequency when you don't know the mass of your tonearm

Discussion in 'Turntables' started by doctor fuse, Apr 15, 2018.

  1. ripblade

    ripblade Super Member

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    Your numbers check out, but I doubt the arm's mass is that low.
     

     

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  2. doctor fuse

    doctor fuse AK Subscriber Subscriber

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    Not sure what I am looking at there, Rip...
     
  3. doctor fuse

    doctor fuse AK Subscriber Subscriber

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    I wonder if the resonant frequency is actually one of the mini peaks, at 8 Hz or 10 Hz?
     
  4. doctor fuse

    doctor fuse AK Subscriber Subscriber

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    If it is the 10 Hz mini-peak, the effective mass would be a more reasonable 7.
     
  5. ripblade

    ripblade Super Member

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    Just pointing out that the resonant peak is (or maybe it's just me..) is the highest measured peak.

    You could try Thomas' approach and measure music instead of a quiet groove. Or use something other than the mic in on the laptop.
     
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  6. tnsilver

    tnsilver AK Subscriber Subscriber

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    The formula is good but the 14Hz (calculated??? How? you need the eff. mass for this, and you don't have it to begin with) is just a guess, so if the math is right, the result is just a guess.
     

     

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  7. onwardjames

    onwardjames Hoardimus Maximus Subscriber

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    This is fascinating to me, but I cannot be bothered to do math where records are concerned, lol.

    Watching with great interest.
     
  8. doctor fuse

    doctor fuse AK Subscriber Subscriber

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    If I go by the spectrographs that I posted above (possibly erroneous due to being recorded by crappy laptop's DACs), I see peaks (indicating possible resonant frequency) at 14, 10 and 8 Hz. If I use these, I get effective mass results of app. 2.6, 7.5 and 12.5, respectively.
     
  9. Rockon1

    Rockon1 Well-Known Member

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    As am I but Im not sure why. ...The "why" is why is it important to know the RF? Not being snarky -just curious. Bob
     
  10. tnsilver

    tnsilver AK Subscriber Subscriber

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    Gotcha, so it's not calculated, it's just plugged in.

    The formula is:

    rf = 159 / sqrt ((eff. mass + cart weight + fastener weight) * (compliance))

    So it's basically:

    rf = 159 / sqrt ((X+ cart weight + fastener weight) * (compliance))

    and no further math is needed to see that whatever we do, we're still left with rf and X, so like I said earlier, X is a guess. Now I see it's an educated guess. In fact, it's a 1 in 3 guess.

    Assuming the math is right, 2.6g for eff. mass is too low. The black widow is 3g, and I doubt you can beat that, so it could be one of the other two or one of the other frequencies derivatives.
    who knows... Honestly, that spectrograph beats me and makes no sense to me at all.
     
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  11. gusten

    gusten Addicted Member

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    It´s a bit tricky as You can´t be sure of any of the small peaks, and You can´t be sure the compliance is correct.
     

     

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  12. FileFixer

    FileFixer Well-Known Member

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    Put that recorded Pink Nose in ARTA software and see frequency response in octave scale with corected -3dB compensation from Pink Noise.
    Or you can upload recorded sound somewhere and I can see and post right plots in this topic. ;)
     
  13. FileFixer

    FileFixer Well-Known Member

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    @tnsilver It have sense calculated from formula...

    [​IMG]
     
  14. trinhsman

    trinhsman AK Subscriber Subscriber

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    All of this makes me feel dumb. I can't follow any of it. For me it's simple. Does it sound good?
     
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  15. Nashou66

    Nashou66 AK Subscriber Subscriber

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    Me too !!!

    Another thing is with replacement styli the compliance is not really known and most are not even close to
    the original manufactures spec for that cartridge.

    So here is another question. Is there a way to find the compliance of the stylus's suspension your working on?

    I mean some replacements do give their compliance numbers but i take it they are not exact .

    Nashou
     
  16. guiller

    guiller Toscaninichus Australis

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  17. trinhsman

    trinhsman AK Subscriber Subscriber

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    Now I'm really confused. Just glad there are people out there that understand this stuff so I can just listen.:)
     
  18. ConradH

    ConradH Addicted Member

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    IMO, it's almost certainly that broad peak between 10 and 20 Hz, which is about where you'd expect to find it. Having recently done a spreadsheet that took into account every individual piece of a tone arm to calculate the effective mass, I give credit to anybody that can get it down under 8 grams or so. Not at all easy, but with most cartridges you don't want it that low.
     
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  19. tnsilver

    tnsilver AK Subscriber Subscriber

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    Don't worry about it. I had no idea about it either in the first round of vinyl superimacy back in the 70's and it's still mostly academic. It's very similar to figuring out how well a VW beetle can do if we mount it on an 18 wheeler suspension, or visa versa. The resonant frequency is just an uncontrollable cascading "poing" that happens when we subject any spring to vibrations. This frequency changes with the varying of the spring's compression. This always happens, even in a cart, b/c the cantilever is essentially a spring. All we can do is just make sure that frequency is something we can't hear and can't trigger by walking on the floor or by shutting the garage door.

    The trick is matching the tonearm effective mass (which is just another name for "inertia") to the cantilever springiness (compliance). There's a formula for this that dates back a couple centuries. It tells you what's the "right" effective mass of a tonearm, taking into account the weight at the end of the tip, and the springiness of the cantilever. All of this aims at keeping the resonant frequency low within the range of a goldilocks zone that's accepted by convention. That range is between 8Hz (8 cycles per second), which is just above the 6Hz of footfall, and 12Hz, which is just arbitrarily bellow the 20Hz that we can hear.

    It's worth mentioning that effective mass is the amount of resistance a tonearm exerts to the changes in direction the stylus wants to move to, as a result of reading the groove modulations. A tonearm has it's own inertia, so when a springy stylus wants to suddenly go right while the tonearm is still going left - it's going to encounter resistance. This resistance is going to generate vibrations in the cantilever and if the two don't match, we, in theory, can degrade the sound quality. That's all there's to it.
     
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  20. trinhsman

    trinhsman AK Subscriber Subscriber

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    OK, so how do I adjust my suspension on my Marc Morin modified AR XA so I can use it in my main living area without getting foot fall skips? That's really all I need to know how to do. For now, the XA is in the basement system on a concrete and tile floor and SOUNDS FANTASTIC. Bring it upstairs and put it in the mix with my Audible Illusions and Pass Labs gear and it sounds OK, and I can't walk by it without it skipping.
     

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