6. Measurement of the Ultrasonic Velocity in Air Introduction Sound wave is a mechanical wave propagating in elastic med

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6. Measurement of the Ultrasonic Velocity in Air Introduction Sound wave is a mechanical wave propagating in elastic media. It can propagate in gases. liquids and solids, but its propagation speed is different in various media. When the vibration frequency of sound wave is between 20 Hz and 20 kHz, it can be heard; when the frequency is lower than 20 Hz, it is called infrasound; and when the frequency is higher than 20 kHz, it is called ultrasound. The measurement of acoustic characteristics (such as frequency, wavelength, sound velocity, phase, etc.) is an important part of acoustic technology. Sound velocity measurement is widely used in acoustic positioning, flaw detection and ranging. In this experiment, standing wave method and phase comparison method were used to measure the propagation velocity of ultrasonic wave in air. Purpose 1. Learn to use standing wave method and phase comparison method to measure the ultrasonic velocity in air. 2. Understand the characteristics of standing wave and the physical meaning of phase. 3. Understand the principle of generating and receiving ultrasound. Apparatus Oscilloscope, Sound Velocity Tester, Signal Generator. theory 1. Measurement of Sound Velocity Sound wave propagates in the air by longitudinal wave. Its propagation speed u is related to the vibration frequency fand wavelength of the sound source as follows: v=fa (6.1) The propagation velocity of sound wave can be calculated by Formula (6.1) by measuring the wavelength of sound wave and the vibration frequency of sound source. The standing wave method and phase comparison method are usually used to measure the wavelength of sound wave. 1.1 Standing Wave Method for Sound Velocity Measurement Standing wave method is to measure standing wave by interference of incident wave and reflected wave under certain conditions. From the wave theory, the acoustic signal generated by the sound source will be reflected back when it is incident vertically on a rigid reflector through the medium, forming a reflected wave. Between the sound source and the reflective interface, the incident wave and the reflected wave interfere to form a standing wave. By changing the distance / between the sound source and the rigid reflector, the vibration amplitude of each particle in the standing wave field also changes, when the distance between the sound source and the rigid reflector satisfies 1-n- (6.2) the vibration amplitude of each particle is the largest, and the particles are in the standing wave resonance state between the sound source and the rigid reflector. The standing wave resonance can be observed by changing the position of rigid reflector x along the direction of wave propagation and keeping the position of sound source unchanged. Formula (6.2) shows that the moving distance of rigid reflector corresponding to two adjacent standing wave resonance states Aris:

(6.3) If the distance between two adjacent standing wave resonance states corresponding to rigid reflectors is measured, the wavelength of sound wave can be obtained, and the sound velocity can be calculated by Formula 6.1. This method of sound velocity measurement is also called standing wave resonance method. In this experiment, an oscilloscope is used to observe the vibration state at the reflecting end to determine whether the particle is in the standing wave resonance state. 1.2 Phase Comparison Method for Sound Velocity Measurement Phase comparison method, also known as traveling wave method, is conducted by comparing the phase difference of two particles on the same traveling wave The distance between two particles with phase difference is half wavelength along the propagation direction of the sound wave emitted by the sound source. Therefore, as long as the distance between two particles with phase difference is measured, the distance between two particles can be obtained. Ad Calculate the wavelength, and the sound velocity can be computed from the wavelength and the vibration frequency of the sound source. In the experiment, keep the position of the sound source unchanged, change the position of the reflective surface, measure the phase difference between the two particles at the source and the reflective surface with an oscilloscope, record the position d of the reflective surface with each change of the phase difference, and calculate the change of the position of the reflective surface Ad with the change of the phase difference. There are two methods to measure the phase difference between two signals by oscilloscope: double tracing oscilloscope method and Lissajous figure method. In this experiment, Lissajous figure is used to measure the phase difference between two positions. The Lissajous figure observed on the oscilloscope is an ellipse. When the position of the reflector is changed, the phase difference between the two signals changes. The Lissajous figure changes periodically from ellipse to line to ellipse to line, as shown in Figure 6.1. When two adjacent straight lines appear, the change of reflector position is the distance Ad between two particles when the phase difference is a.. 92-9=0 e t * 9:19=4 Figure 6.1 Lissajous figures 2. Acoustic transmitting and Receiving-Piezoelectric Transducer 3x

Any vibrating object can be used as the sound source of the surrounding medium, but in order to produce a continuous and single frequency sound wave, the method of electro-acoustic conversion (e.g. electro-acoustic hom) is usually used. In order to avoid the influence of audio area on human hearing and to avoid the interference of surrounding audio on experiment, the laboratory adopts the ultrasonic frequency band. The piezoelectric transducer is the device that transmits and receives ultrasound. Piezoelectric transducers are based on the piezoelectric effect of some crystals (such as quartz, barium titanate, etc.). When these crystals are compressed or stretched, there will be charges and voltages on their surfaces; conversely, when voltage is applied on both sides of these crystals, the crystals will shrink or stretch. Ultrasound transmitter and receiver made of barium titanate piezoelectric material are used in the experiment. The structure of the transmitter and receiver is shown in Figure 6.2. When a single frequency sinusoidal voltage signal is added to its two electrodes, the piezoelectric chip will produce mechanical vibration at the same frequency, thus generating the same frequency of ultrasound. On the contrary, the piezoelectric transducer can also convert the received ultrasonic signal to the voltage signal output from the two electrodes. Vibrating objects have their own natural frequencies, which depend on the properties and geometric dimensions of the material. When the signal frequency added to the piezoelectric chip equals its natural frequency, mechanical resonance will occur. In Figure 6.3. fo achieves the resonant frequency, in this case the acoustic wave emitted is the strongest. Therefore, the frequency of the electric signal should be adjusted to the resonant frequency of the piezoelectric transducer. piezoelectric sheet electrode Contents and Procedures emission surface Figure 6.2 Ultrasonic transducer Figure 6.3 Ultrasonic amplitude with frequency osilloscope ground amplitude 1. Sound velocity measurement by standing wave method 1.1 The sinusoidal signal from the signal generator is applied to the transmitter of the sound velocity tester, and the receiver of the sound velocity tester is connected with the oscilloscope (y) channel), as shown in Figure 6.4. sound velocity tester frequency signal generator to O ground

Figure 6.4 Ultrasonic velocity measurement by standing wave method 1.2 Rotate the distance adjusting handle to make the two ends of the transmitter and receiver of the sound velocity tester be about 4 cm apart, and keep the two ends parallel. Adjust the frequency of the signal generator (the resonant frequency of the transducer is about 37 kHz). Observe the change of the amplitude of the waveform on the oscilloscope. When the amplitude of the received signal is the largest, record the frequency f of the signal generator (f is the resonant frequency), then keep the frequency funchanged in the experiment. 1.3 Slowly rotate the distance adjusting handle to make the receiving end of the sound velocity tester far away from the transmitting end and observe the change of the graph on the oscilloscope. When the amplitude of the waveform on the oscilloscope is maximum, record the position readings of the receiver of the sound velocity tester. Continuously record 10 position readings of the receiver corresponding to the maximum amplitude of the waveform by rotating the handle. The difference between adjacent readings is A/2. 1.4 Compute the wavelength by Ax in successive difference method, calculate the sound velocity by substituting fand à into Formula (6.1). 2. Sound Velocity Measurement by Phase Comparison Method 2.1 On the basis of the connection of standing wave method to measure sound speed. Connect the output terminal of signal generator to Y port (y2 channel) of oscilloscope, and set the oscilloscope to "x-y" mode, that is, the oscilloscope is adjusted to observe the state of Lissajous figure, as shown in Figure 6.5. osilloscope Y ground x S₁ sound velocity tester atto signal generator o O ground Figure 6.5 Phase comparison method 2.2 Slowly rotate the distance adjusting handle to observe the change of the figure on the oscilloscope. When the line in Figure 6.1 appears, record the position readings of the receiver. Rotate the handle continuously to read the position readings of the receiver when 10 straight lines (including one quadrant, three quadrant and two quadrant, four quadrant) appear. The difference between adjacent readings is 1/2. Data recording and processing Record the data required. Calculate the wavelength by Ad successive difference method. Calculate the sound velocity by Formula (6.1) and compute its uncertainty. Attentions 1. The resonance frequency of each sound speed tester is different. In the experiment, we should pay attention to the number indicated by the instrument, find out the resonance frequency of the instrument we use, and fine-tune the optimum value according to the requirements of the experiment.

Figure 6.5 Phase comparison method 2.2 Slowly rotate the distance adjusting handle to observe the change of the figure on the oscilloscope. When the line in Figure 6.1 appears, record the position readings of the receiver. Rotate the handle continuously to read the position readings of the receiver when 10 straight lines (including one quadrant, three quadrant and two quadrant, four quadrant) appear. The difference between adjacent readings is 1/2. Data recording and processing Record the data required. Calculate the wavelength by Ad successive difference method. Calculate the sound velocity by Formula (6.1) and compute its uncertainty. Attentions 1. The resonance frequency of each sound speed tester is different. In the experiment, we should pay attention to the number indicated by the instrument, find out the resonance frequency of the instrument we use, and fine-tune the optimum value according to the requirements of the experiment. 2. In order to reduce the interference, vibration and the area of hand contact with the tester should be reduced. Questions 1. The speed of sound wave propagation is related to temperature and other conditions. When the temperature of air changes, how will the speed of sound change? 2. This experiment chooses the ultrasonic frequency band to reduce the interference. If we need to test the audible frequency band, how should the experimental device be improved? 3. Can we measure the velocity of sound wave in water? How should the experimental device be improved? And can we measure the density of a liquid by using this device?Describe the experimental scheme briefly.