3.4.4 Experimental procedures
1)Using the resonance interference method to measure the speed of sound
(1)Connect the experiment instrument based on the circuit shown in Figure 3.4.4.Turn on the oscilloscope and the signal generator.Adjust the frequency of the generator to～35 000 Hz and input the signal to transmitter transducer.Adjust the oscilloscope to make the sine signal from the receiver is clear and in the center of the oscilloscope screen.Attention:before using the instrument,it should be preheated for at least 15 mins.
Figure 3.4.4 Connection diagram of resonance interference method
(2)Determine the best working point(resonance state)of piezoelectric ceramic transducer.Carefully adjust the frequency of signal generator in the range of 34 000 kHz and 36 000 kHz to maximize the wave amplitude displaying on the screen of the oscilloscope.The working frequency of the signal generator is the natural frequency of the transducer.Record the frequency and keep it through the whole experiment.It should be highlighted that the end surfaces of two transducers are parallel to each other to receive the maximal signal in this method.
(3)Determine the maximal starting position.Observe the change of the wave amplitude on the oscilloscope by slowly moving the receiver transducer.When the amplitude of the wave on the screen reaches the maximum,one can record the first position as starting point.Then,continue to slowly move the receiver transducer to change the distance between the receiver and the transmitter and record 10 positions li in Table 3.4.1 where maximal amplitude appeared in the screen in turn.
(4)Record the frequency f from signal generator and the temperature in the laboratory.Calculate the wavelength of ultrasonic sound λ by successive difference method.Calculate the speed of the ultrasonic sound wave v propagating in air by function v=f λ,then compare the calculation result with the theory result based on Equation(3.4.7)and calculate the relative error.(https://www.daowen.com)
2)Using the phase comparison method to measure the wavelength
(1)Connect the circuit shown in Figure 3.4.5.Repeat step(1)and step(2)in the content above to achieve resonance state of the piezoelectric ceramic transducer.
(2)Press the X-Y button of the oscilloscope to observe Lissajous pattern on the screen.Slowly move the receiver transducer and observe the Lissajous pattern.At the same time,record the position li in Table 3.4.2 when the same pattern appeared one after another(prefer to select a direction of linear state).
(3)Record the frequency f from the signal generator and the temperature in the laboratory.Calculate the wavelength of ultrasonic sound λ by successive difference method.Calculate the speed of the ultrasonic sound wave v propagating in air by function v=f λ,then compare the calculation result with the theory result based on Equation(3.4.7)and calculate the relative error.
Figure 3.4.5 Connection diagram of phase comparison method
