Why mic positions cause inaccuracy in frequency readings in your ETD's

  • 1.  Why mic positions cause inaccuracy in frequency readings in your ETD's

    Posted 01-07-2024 18:29

    People ask Professor Steven (me) why when we change the mic or change its position, we get frequency errors and variations. They say, "I understand why I get amplitude variances, but not frequency variance." Here is why. Every signal processing engineer (my peers) know that when we drop down in amplitude closer to the noise floor, we loose signal-to-noise ratio (SNR) but also get frequency error estimates. What is happening is that the partials we need to detect are too low and 'smear' in frequency and get pertubated in frequency by the noise as well. What we want are very well defined sharp spectral lines of the partials. The underlying FFT (Fast Fourier Transform) of the ETD has a window function that widens (by necessity) that spectral line, and we take the peak of that lobe through interpolation to estimate that frequency more precisely. Some ETD's do that better than others, by the way. When we use a 'rectangular window' FFT, we only have about 13 dB SNR to play with. Below that, we cannot detect the peak of the lobe very well and hence we are off the frequency in error. Hence, we get a noisy reading in frequency in our ETD. The PianoSens device eliminates this because it it picking up only string vibrations, not mic placement errors coming from nodes (nulls) from interference patterns.

    Kind and eager to teach signal processing to the PTG community.

    Steven Norsworthy

    Professional Signal Processing Engineer, Lecturer, Inventor

    Professional Musician, Pianist

    Steven Norsworthy
    Cardiff By The Sea CA
    (619) 964-0101