According to the wave theory, the wave velocity has the following relationship with wavelength and frequency: v = f λ, and the wave velocity can be obtained as long as the frequency and wavelength are known. In this experiment, the transducer is controlled by a low-frequency signal generator, and the output frequency of the signal generator is acoustic frequency. The wavelength of sound wave is measured by standing wave method (vibration interference method) and traveling wave method (phase comparison method). The picture below shows the experimental device for ultrasonic sound velocity measurement.

Measuring wavelength by standing wave method

After the plane wave emitted by the sound source is reflected by the front plane, the incident wave and the emitted wave are superimposed, and their wave equations are as follows:

The composite wave after superposition is:

Y =(2 acso2px/l)cos2p feet

Cos2px/L = 1, and each point has the largest amplitude, which is called antinode, and the corresponding position is:

X = nl/2 ( n =0， 1，2，3……)

Each point with cos2pX/l = 0 has the smallest amplitude, which is called a node, and the corresponding position is:

X = (2n+ 1)l/4 ( n =0， 1，2，3……)

Therefore, the wavelength can be obtained by measuring the positions Xn and Xn- 1 of two adjacent antinodes (or nodes).

Measuring wavelength by n-phase comparison method

The ultrasonic wave emitted from the transducer S 1 reaches the receiver S2, so there is a phase difference between S 1 and S2: j = 2px/l where L is the wavelength and X is the distance between S 1 and S2? 8? 8。 Because when x changes a wavelength, the phase difference changes by 2p. Using Lissajous figure, the wavelength of ultrasonic wave can be measured.

Experimental focus

N Know how to transmit and receive ultrasonic waves.

N deepen the understanding of theoretical knowledge such as vibration synthesis and wave interference.

Master standing wave method and phase method to measure sound velocity.

Matters needing attention

N Ensure that the end faces of sensor S 1 and S2 are parallel.

The output signal frequency of the N signal generator is consistent with the resonant frequency f 0 of the piezoelectric transducer.