Characteristics of the Ultrasound Transducer
There are many characteristic parameters of ultrasound transducers, and only the following three are briefly introduced.
1. Frequency characteristics of the ultrasound transducer
The crystal of the portable ultrasound transducer itself is an elastic body, so it has its natural resonant frequency. When the frequency of the applied force is equal to its natural frequency, it will produce mechanical resonance, and the maximum electrical signal will be generated due to the positive piezoelectric effect. On the other hand, when the frequency of the applied electricity is the same as the natural frequency of the piezoelectric crystal, mechanical resonance should occur due to the inverse piezoelectric effect. The amplitude of resonance is the largest and the elastic energy is also the largest. At this time, the piezoelectric body obtains the maximum deformation vibration and produces ultrasound output through the medium. Experiments have proved that when the frequency of the applied force or electricity is not consistent with the natural frequency of the crystal, the amplitude of the electrical signal and the amplitude of the deformation vibration generated by the ultrasound transducer crystal will become smaller. It can be seen that they are both a function of frequency.
Frequency characteristics of the ultrasound transducer
If a certain value of voltage is applied to the piezoelectric crystal and the frequency of the applied voltage is changed, the loop current or impedance will change with it. When the voltage frequency is a certain frequency FM, the current has a maximum value Imax, and when the voltage frequency is another frequency fn, the current has a minimum value of Imin. The phenomenon that the current of the piezoelectric crystal changes with frequency (see the figure above) shows that the equivalent impedance of the ultrasound transducer crystal is a quantity that changes with frequency. If you continue to increase the frequency of the voltage, you can also find that a series of current fluctuations appear regularly, and the maximum value of the fluctuation (corresponding to fm1, fm2...) is sequentially reduced, while the minimum value of the fluctuation (corresponding to fn1, fn2... which is increasing in turn, FM is called the minimum impedance frequency of the piezoelectric vibrator (also called the maximum transmission frequency); fn is called the maximum impedance frequency (also called the minimum transmission frequency).
2. Transduction characteristics of the ultrasound transducer
The energy conversion characteristics of ultrasound transducers include two aspects: electrical energy-mechanical energy-ultrasound energy, and ultrasound energy-mechanical energy-electrical energy. The former belongs to the transmitting process, and the latter belongs to the receiving process. The conversion between energies will inevitably produce losses (producing unhelpful energy consumption), and the performance of the high frequency linear probe is characterized by the conversion efficiency:
Motor conversion efficiency = output mechanical power/input electrical power
Machine-to-sound conversion efficiency = radiated ultrasound power/input mechanical power
Therefore: Electroacoustic conversion efficiency=radiated ultrasound power/input electric power
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3. Transient characteristics of the ultrasound transducer
Most of the ultrasound transducers work in the pulse state, and the response rate of the transducer to the pulse is called the transient characteristic, which is also an important indicator. The transient characteristics of a transducer are related to its frequency characteristics. In short, the wider the frequency spectrum of the transducer, the better its transient characteristics, and the narrower the allowable ultrasound pulse width. Here, the pulse width described refers to the length of time that the ultrasound is emitted intermittently, in seconds, which is different from the frequency (the number of ultrasound vibrations per second).
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