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  1. amanuensis

    A More Accurate Fourier Transform

    It's amazing. There is a super easy criterion to know whether that can be useful. How many points can be plotted in a frequency range (Delta F) where Delta F is -3 dB frequency band around a peak ? Does this number depend on the peak frequency ? Does this number depend on sampling time (Delta...
  2. amanuensis

    A More Accurate Fourier Transform

    It could be very useful in experimental modal analysis. Does the method work with short-duration time signal ?
  3. amanuensis

    Radiation Efficiency

    Do you have Actran for Nastran ?
  4. amanuensis

    modal damping

    I have got the second edition of the book. The chapter 30 is "Theory of vibration isolation" and the equation is in the section "Rigidly connected coulomb damper"
  5. amanuensis

    modal damping

    The formula has been extracted from the "SHOCK AND VIBRATION HANDBOOK" from Harris & Crede, chapter 30.
  6. amanuensis

    modal damping

    Hi, So you need a formula between the two kind of damping... Et voila! ξfriction=(2η/π)2(1-ω2/ω02)2/(ω2/ω02[ω4/ω04-(4η/π)2])
  7. amanuensis

    shaker slip table cross talk excessive?

    Hi IRstuff, It's funny to read your sentense "Sounds like you should be talking to the manufacturer" for a non-native English speakers as me. You use the words "sounds" and "talking" (reminiscent of crosstalk) to say something which has nothing to do with sound and crosstalk. Freud would tell...
  8. amanuensis

    Modal Analysis: Mode Shape and Curve Fitting

    Wow, it's a subtle question. Suppose that Ai are the experimental mode shapes (a set of vector). Their amplitude can be very low. So the modal masses can also be very low. This is not good. When modal mass is normalized, something doesn't change : the Eigen frequency . Because an Eigen...
  9. amanuensis

    acoustic resistor to upshift an electret microphone maximum SPL

    Why do you say that acoustic pressure is proportional to the plate displacement ? Plate Velocity = j * omega * Displacement From Acoustic point of view, Air Velocity is proportional to Delta Pressure / Delta X. Since there is equality between Acoustic Velocity and plate Velocity, we get that ...
  10. amanuensis

    Number of Normal Modes

    What is the main difference between normal and complex modes ? The way the phase is taken into account. With normal modes, the phase exists, but it doesn't matter. Why ? Because the phase is either 0° or 180°. So the amplitude of the mode at each point is given by +something (0°) or -something...
  11. amanuensis

    Failure of attachment points during RTCA DO-160 Vibration Testing

    Other standards : GAM EG13, STANAG 4370
  12. amanuensis

    Number of Normal Modes

    When complex values appear, it means that damping is taken into account. Imaginary part of a complex eigenfrequency is related to damping value (damping loss factor). Imaginary part of a complex eigenvector means that mode is not normal (mode is not a stationary wave).
  13. amanuensis

    modelling of a piston

    If this is an acoustic problem, then a nearing model could be the Helmholtz resonator with no neck...
  14. amanuensis

    Number of Normal Modes

    ...suppose a beam, simply supported, length = L, etc.. Let's consider the 3 first modes (N=3) The modal response is something like this : P(x) = A1*sin(pi*x/L) + A2*sin(pi*2*x/L) + A3*sin(pi*3*x/L). (Sum of the 3 modes) You can imagine P(x) (for all x) as just 1 point of coordinates (A1,A2,A3)...
  15. amanuensis

    Random Vibration Fatigue Cycles

    You're not a gentleman. When you write some lousy messages, I say to myself : " This guy is just an idiot" but I don't write it to the entire community.
  16. amanuensis

    Number of Normal Modes

    When doing modal analysis, you change of space. In modal analysis, the primary space is NOT the space-time. You have to have a little bit imagination. If you understand this, modal analysis will have no secret for you. In modal analysis, the space is NOT the 3d space. A point is NOT given by...
  17. amanuensis

    Random Vibration Fatigue Cycles

    Greg, you annoy me with your "nonsense". Various methods of counting have been proposed in the literature, leading to different results and therefore (for some) to errors in the calculation of the fatigue lives. They are peak count, range-restricted peak count, level-restricted peak count...
  18. amanuensis

    Random Vibration Fatigue Cycles

    You have to get first a time signal of your random vibration input spectrum. Then you need an algorithm to count every time the signal passes through zero. Divide the number, given by your counter, by 2 to get the number of cycles.
  19. amanuensis

    Experimental Modal Analysis

    One word about "tapered peak". As you can see, tapering decreases with peak number. It means that the damping is certainly viscous or hysteretic. You can try to verify for each peak if (Delta f)/f = Constant ( where f is the frequency of the peak and Delta f is the passband located at...
  20. amanuensis

    Narrow band to 1/3 Octave, Sum or Average?

    The question is more complex than one might think. First, you have to know if you work with a random or a deterministic signal. For random signal, things are complicated... Listen to me very well, it's very important : We can say nothing about ONE random signal!! This is perfectly...

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