Application Cases

Experiment Name: Experimental Study on Ultrasonic Wave Propagation in Subwavelength Waveguides

Experimental Principle:
Preliminary theoretical studies predict that acoustic waves propagate as regular plane waves within subwavelength waveguides with relatively low transmission losses. To investigate the propagation characteristics of acoustic waves in waveguides, this experiment employs the time-of-flight (TOF) measurement method. The experimental setup and testing principle are illustrated below.

Figure a: Apparatus for Measuring Acoustic Velocity in Subwavelength Waveguides

In the subwavelength waveguide experimental system:

1. A signal generator generates a tone-burst driving signal in channel 1, with fewer than two pulses and a low repetition frequency to prevent interference between pulses.

2. Channel 2 is set to the same signal and connected to an oscilloscope as a trigger signal.

3. An RF power amplifier (ATA-8035) amplifies the driving signal and adjusts the amplitude to the set value.

4. An impedance matcher ensures efficient application of the driving signal to the ultrasonic transducer, reducing reflected power and minimizing system power loss, heating, and damage caused by reflected power.

5. The ultrasonic transducer (11) converts electrical signals into acoustic signals.

6. An acoustic coupler (7) transmits the acoustic waves to the input end of the waveguide.

7. Adjacent to the input end, a branch of the waveguide (6) is present.

8. A hydrophone (8) measures the signal at a distance of 2 mm from the tube opening.

9. An identical hydrophone (10) measures the signal at another tube opening.

10. The acoustic signals are compared on an oscilloscope to determine the time delay $t$, with microsecond-level precision.

11. The distance $L$ between the two tube openings is measured, yielding the acoustic velocity $v=L/t$.

12. A fixing plate (5) secures the entire measurement system for acoustic wave generation, transmission, and reception.

13. Water tanks (12, 9, 13) store the medium inside the tubes and isolate the acoustic wave generation and reception sections to prevent mutual interference.

Testing Equipment:
Signal generator, ATA-8035 RF power amplifier, oscilloscope, hydrophone, ultrasonic transducer

Experimental Procedure:
The ultrasonic wave generation section consists of a signal generator and a piezoelectric transducer. The signal generator produces an excitation signal, and the piezoelectric transducer, driven by the inverse piezoelectric effect, generates pulsed acoustic waves. The pulse cycle count is set to 2 to avoid interference. The piezoelectric transducer is fixed at the bottom of a water tank filled with pure water. The ultrasonic wave generation and transmission sections are not in direct contact but are connected via the liquid medium inside the tube. The front end of the subwavelength waveguide features a funnel-shaped acoustic coupler designed to guide acoustic waves from the piezoelectric transducer into the narrow tube. Since the experimental method relies on time difference, the subwavelength waveguide has two branches. Acoustic signals are detected at both branches, and the time delay between them is used to calculate the acoustic velocity. Finally, hydrophones are used to receive the signals.

Figure b: Acoustic Velocity Measurement Results for Different Materials

Figure c: Point-Line Plot of Acoustic Velocity vs. Frequency for Different Materials

Experimental Results:
The acoustic velocity measurement results for different materials are plotted as a point-line graph for better visualization, as shown in Figure c. Analysis of the graph reveals the following:

• For the subwavelength waveguide made of steel, the measured acoustic velocity in the tube wall is close to the longitudinal vibration velocity of the material itself (approximately 5000 m/s). The acoustic velocity of the medium inside the tube is also close to that of pure water (approximately 1480 m/s). This indicates that for steel subwavelength waveguides, at subwavelength scales, no solid-liquid coupling occurs between the tube wall and the internal medium.

• For waveguides made of brass, the measured acoustic velocity in the tube wall is lower than the material's intrinsic acoustic velocity.

• For waveguides made of acrylic, the measured acoustic velocity in the tube wall is higher than the material's intrinsic acoustic velocity. Since the acoustic velocity of the medium inside these waveguides could not be measured, further assessment of coupling effects is not possible.

• For waveguides made of quartz glass, the measured acoustic velocity in the tube wall is higher than the material's intrinsic longitudinal vibration velocity, while the measured acoustic velocity of the internal medium is lower than that of pure water. This indicates that solid-liquid coupling occurs in quartz glass subwavelength waveguides at subwavelength scales.

  
  

Figure: Specifications of the ATA-8000 Series RF Power Amplifier