The main characteristics of ultrasonic transducers include sensitivity, directionality, and blind spots, which are explained as follows:
Sensitivity
Before understanding sensitivity, it is necessary to first introduce Sound Pressure Level (SPL).
SPL is the effective sound pressure measured on a logarithmic scale relative to a reference value, defined as:
SPL = 20*log(P/Pref); The unit of measurement is decibels
Where P is the sound pressure and Pref is the standard reference sound pressure. Typically, Pref is defined as 0.0002 ubar (derived from the human auditory threshold). But for the convenience of expressing the performance of the transducer, sensitivity is used to represent the received intensity.
The unit of sensitivity is usually V/Pa or mV/Pa (V: receiving voltage, Pa： Pascal). The emission intensity is defined as the sound pressure obtained at a certain distance after a standard condenser microphone (SCM) inputs a specific frequency and power signal to an ultrasonic transducer, as shown in Figure 6. Reception sensitivity is defined as a fixed sound pressure emitted by an ultrasonic transducer, received by a standard condenser microphone, and tested at a certain distance in parallel with the ultrasonic transducer, as shown in Figure 7. Next to a standard condenser microphone, the transducer receives this sound pressure and converts it into a voltage signal. By comparing the two, the sensitivity of the transducer can be determined. So, the output voltage of the receiver can be used to determine its sensitivity level. The higher the output voltage of the receiver, the higher the sensitivity, and vice versa.
Directionality
The directionality of an ultrasonic transducer is defined as the angle at which the ultrasonic intensity decays by -3 dB at a specific distance from the transducer relative to the maximum intensity defined as zero dB in the main emission direction, as shown in Figure 8. Ultrasonic transducers are affected by the wavelength of ultrasonic waves and the size of the emitting surface. The higher the frequency, the smaller the wavelength, and the narrower the pointing angle; The larger the size of the launch surface, the smaller the pointing angle.
Blind Area
Blind zone refers to the minimum detection distance of the ultrasonic transducer, and the main factor affecting the size of the blind zone is the ringing time. When the transducer receives an electronic signal of a specific frequency, the piezoelectric ceramic generates vibration and emits ultrasonic waves accordingly. However, this vibration, as a circuit, does not immediately stop. After the main vibration, it gradually tends to calm down to rest with the help of the circuit board. Damping layer. The period from the end of the main oscillator to the static state is called the ringing time. The ringing time will affect the blind zone range. Taking the transceiver as an example, we use the time difference between the input signal and the reflected wave to measure the distance of the object to be measured.
