Method for measuring vibration of mechanical wave transducers using laser doppler method
An acoustic output of therapeutic devices used in shock wave therapy can be obtained by measuring the pressure waves radiated from the pressure wave transducer in water. Since the pressure waves used in shock wave therapy generate severe bubbles in water, measuring the surface vibrations of the pressure wave transducer using the laser Doppler method includes significant noise caused by the bubbles. This invention utilizes a filter generated to eliminate the noise in the vibration signals measured by the laser Doppler method, which is caused by the bubbles. The filter is generated by comparing the vibration signals measured in degassed distilled water with the reference signals measured in air at the minimum output setting of the shock wave generation device. Using the filter, it is possible to estimate the vibration signals of the pressure wave transducer generated underwater under the set conditions of the specific shock wave generating device.
1. A method for measuring a surface vibration of a mechanical wave transducer of a mechanical wave generating device underwater using a laser Doppler method, comprising:
generating a filter using a vibration signal obtained from measuring a vibration of the mechanical wave transducer in conditions where bubble generation due to a mechanical wave is suppressed underwater using a laser Doppler method, and a vibration signal of the mechanical wave transducer measured in air using the laser Doppler method at the same output setting of the mechanical wave generating device; and
estimating a surface vibration signal of the mechanical wave transducer underwater using the filter.
2. The method of claim 1 , wherein degassed distilled water at a minimum output setting of the mechanical wave generating device is used as a condition where bubble generation due to the mechanical wave is suppressed.
3. The method of claim 1 , wherein the filter ϕ(f) is defined by a following equation:
Φ
(
f
)
=
F
{
β
(
t
)
)
/
F
{
α
(
t
)
)
[
Equation
]
Here, β(t) denotes a surface vibration signal of the mechanical wave transducer measured underwater using the laser Doppler method under the condition where bubble generation is minimized due to the mechanical wave, and α(t) denotes the surface vibration signal of the mechanical wave transducer measured in the air using the same method under the same output setting, F denotes Fourier Transform, t denoted time, and f denotes frequency.
4. The method of claim 3 , wherein the surface vibration signal w(t) generated underwater while varying the output setting of the mechanical wave generating device is calculated using a following equation:
w
(
t
)
=
a
(
t
)
*
F
-
1
{
Φ
(
f
)
}
[
Equation
]
Here, * denotes convolution, F −1 denotes an inverse Fourier transform, and a(t) denotes the surface vibration signal of the mechanical wave transducer measured in the air while varying the output of the mechanical wave generation device.
5. The method of claim 3 , wherein the surface vibration signal w(t) generated underwater while varying the output setting of the mechanical wave generating device is calculated using a following equation:
w ( t )= F −1 [ϕ( f )· F{a ( t )}] [Equation]
Here, F −1 denotes an inverse Fourier transform, · denotes scalar product, F denotes Fourier Transform, and a(t) denotes the surface vibration signal of the mechanical wave transducer measured in the air while varying the output of the mechanical wave generation device.