418
5 Laser Specklometer, Speckle-Optical Diagnostics and Laser …
suggested if the muscle is presented as a simple model of a string system. In this
case, if the skin oscillation amplitude does not exceed the limits of the longitudinal
radius of the speckle with load increment, then gradual tension of the string muscle
should lead to smooth increase of frequency of muscle “humming” and as a result to
smooth dependence of the average frequency. But histograms of all the three integral
characteristics examined above point to surges of muscle tone under some loads. In
accordance with the accepted model, it can be assumed that load increment must
cause increase of the number of involved muscle fibers that leads to enhancement of
the oscillation amplitude of the resonant skin above the longitudinal average speckle
radius. But if the amplitude of skin contractions is equal, for example, to a doubled
average longitudinal speckle radius, then pulsations of the light field (and thus of
the electrical signal from the photoreceiver) will occur with a double frequency of
skin oscillations; with the amplitude, which is equal to three longitudinal radii; with
a tripled one, etc. These frequencies are named “nodal” for definiteness.
It should be mentioned that in our case consideration of harmonics above the third
one does not have any sense as the recorded signal passes through a LF filter with the
cutoff frequency of 50 Hz first. Pulsation amplitudes of the electrical signal, which is
concentrated on the mentioned nodal frequencies, are maximal and proportional to
the contrast of the speckle-structure, and they amount to maximal contributions into
the average frequency during loads corresponding to the occurrence of these nodal
frequencies. During all the intermediate loads, the electrical signal has intermodal
frequency values, but along with this it has a strongly pronounced phase modulation. The latter causes spectrum smearing of its energy with the width of signal
frequency order, which will appear on the graph of the average frequency in the form
of a dip between two surges. If the considered presentation is true, then the presence of maxima on dependences of the average frequency on the load is evidence of
firmly fixed amplitudes of skin oscillations, which are easily calculated basing on the
longitudinal size of a speckle. But this aspect requires additional study. It is worth
mentioning that analysis of the informative content of spectrograms and search for
informative parameters is very complicated [151, 152]. It is known that traditional
methods of estimation of spectral characteristics of processes are based on the application of Fourier transformation either directly to their realizations (Cooley–Tukey’s
method) or to statistic estimates of autocorrelation functions of time dependencies
(Blackman–Tukey’s method). Meanwhile, resolution and accuracy of the obtained
estimations are usually limited with the finite duration of the used realizations of
the process or with a finite interval of values of autocorrelation functions, respectively. Special smoothing functions [153] are usually used for decreasing the negative
impact of process truncation. Namely, the rational choice of the latter ones is of the
greatest interest as algorithms of the fast Fourier transformation are well known and
have become a traditional tool of researchers long ago.
The choice of informative parameters of intensity fluctuation spectra of the scattered radiation is a serious problem because of the multiparameter character of the
current task specified by the change of the registration conditions as well as by
the object of the study itself in the process of measuring. The first group of factors
should include difficulties connected with reproducibility of geometry of experiment
5 Laser Specklometer, Speckle-Optical Diagnostics and Laser …
suggested if the muscle is presented as a simple model of a string system. In this
case, if the skin oscillation amplitude does not exceed the limits of the longitudinal
radius of the speckle with load increment, then gradual tension of the string muscle
should lead to smooth increase of frequency of muscle “humming” and as a result to
smooth dependence of the average frequency. But histograms of all the three integral
characteristics examined above point to surges of muscle tone under some loads. In
accordance with the accepted model, it can be assumed that load increment must
cause increase of the number of involved muscle fibers that leads to enhancement of
the oscillation amplitude of the resonant skin above the longitudinal average speckle
radius. But if the amplitude of skin contractions is equal, for example, to a doubled
average longitudinal speckle radius, then pulsations of the light field (and thus of
the electrical signal from the photoreceiver) will occur with a double frequency of
skin oscillations; with the amplitude, which is equal to three longitudinal radii; with
a tripled one, etc. These frequencies are named “nodal” for definiteness.
It should be mentioned that in our case consideration of harmonics above the third
one does not have any sense as the recorded signal passes through a LF filter with the
cutoff frequency of 50 Hz first. Pulsation amplitudes of the electrical signal, which is
concentrated on the mentioned nodal frequencies, are maximal and proportional to
the contrast of the speckle-structure, and they amount to maximal contributions into
the average frequency during loads corresponding to the occurrence of these nodal
frequencies. During all the intermediate loads, the electrical signal has intermodal
frequency values, but along with this it has a strongly pronounced phase modulation. The latter causes spectrum smearing of its energy with the width of signal
frequency order, which will appear on the graph of the average frequency in the form
of a dip between two surges. If the considered presentation is true, then the presence of maxima on dependences of the average frequency on the load is evidence of
firmly fixed amplitudes of skin oscillations, which are easily calculated basing on the
longitudinal size of a speckle. But this aspect requires additional study. It is worth
mentioning that analysis of the informative content of spectrograms and search for
informative parameters is very complicated [151, 152]. It is known that traditional
methods of estimation of spectral characteristics of processes are based on the application of Fourier transformation either directly to their realizations (Cooley–Tukey’s
method) or to statistic estimates of autocorrelation functions of time dependencies
(Blackman–Tukey’s method). Meanwhile, resolution and accuracy of the obtained
estimations are usually limited with the finite duration of the used realizations of
the process or with a finite interval of values of autocorrelation functions, respectively. Special smoothing functions [153] are usually used for decreasing the negative
impact of process truncation. Namely, the rational choice of the latter ones is of the
greatest interest as algorithms of the fast Fourier transformation are well known and
have become a traditional tool of researchers long ago.
The choice of informative parameters of intensity fluctuation spectra of the scattered radiation is a serious problem because of the multiparameter character of the
current task specified by the change of the registration conditions as well as by
the object of the study itself in the process of measuring. The first group of factors
should include difficulties connected with reproducibility of geometry of experiment