Application Cases

Experiment Name: Experimental Study on Acoustic Source Radiation Characteristics of Cylindrical Shell Structures in a Free Field

Experimental Equipment: Anechoic water tank, signal generation module, ATA-ML100 underwater acoustic power amplifier, hydrophone array, charge amplifier, Donghua data acquisition card, computer, accelerometers

Experimental Procedure:

Figure 1.1: Experimental Setup Diagram

This experiment was conducted in an anechoic water tank. The experimental setup for the cylindrical shell structure acoustic source is shown in Figure 1.1, and the experimental measurement systems are shown in Figures 1.2 and 1.3. The cylindrical shell acoustic source used in the experiment has a height of 0.5 m (excluding the end caps), an outer diameter of 0.5 m, an inner diameter of 0.497 m, and end cap heights of 0.014 m. The material is iron. The interior is filled with air. A radial mechanical excitation was applied to the inner surface at an axial position of 0.35 m. The cylindrical shell structure acoustic source was placed in an anechoic water tank with a background noise level of 66 dB. During the experiment, a signal source generated band-limited white noise signals (500 Hz – 3 kHz, amplitude 1 Vrms). These signals were amplified by an underwater acoustic power amplifier module to 800 VA. A hydrophone array located 6 m from the source measured the far-field sound pressure. Accelerometers were used to measure the surface vibration of the cylindrical shell acoustic source. Due to the length limitation of the accelerometers, the vibration velocity was measured 1 m below the water surface, while the sound pressure was measured 4 m below the water surface. Signals were acquired using a Donghua data acquisition system and processed with MATLAB to obtain sound pressure and cylindrical shell surface vibration velocity. The distribution of watertight accelerometers on the surface of the cylindrical shell structure acoustic source is shown in Figure 1.4. Positions 1, 3, and 9 are spaced 45° apart. Position 10 is exactly between positions 1 and 9, and position 2 is between positions 1 and 3, resulting in 22.5° spacing. Watertight accelerometers were installed axially every 7.1 cm. A total of 60 watertight accelerometers were used, allowing the measurement of vibration velocities at 60 different positions. The experiment was repeated multiple times, and the average results were obtained. A known spherical source was also placed at the same location as the cylindrical shell acoustic source to serve as a standard sound source. The signal source similarly generated band-limited white noise signals (0–3 kHz). A Donghua data acquisition system collected sound pressure data, which were stored as frequency spectra. During the experiment, a rotating device rotated the cylindrical shell a full 360°, with a hydrophone array measuring the sound pressure data on the envelope surface of the cylindrical shell. The rotation step was 18°, resulting in a total of 20 measurement sets. The experiment was repeated multiple times, and the average results were obtained. The spherical source did not require rotation; only one set of data was collected.

Figure 1.2: Experimental Measurement System for the Cylindrical Shell Structure Acoustic Source

Figure 1.3: Experimental Measurement System for the Spherical Source

Figure 1.4: Accelerometer Distribution Diagram

Experimental Results:
In the anechoic water tank experiment, the influence of reverberant sound on the sound field was neglected. The radiated sound power level of the acoustic source can be expressed as:

Where:

• $N$ is the number of small surface elements dividing the measurement envelope surface of the hydrophone array.

• $S_j$ is the area of the $j$-th small surface element.

• $p_j^2$ is the mean square sound pressure corresponding to the $j$-th small surface element on the measurement envelope surface of the hydrophone array.

• $p_{\text{ref}}=1\mu \text{Pa}$ is the reference sound pressure.

The measured radiated sound power level and surface average velocity of the cylindrical shell acoustic source, calculated in MATLAB, are shown in Figure 1.5. The measured radiated sound power level of the spherical source, calculated in MATLAB, is shown in Figure 1.6.

Figure 1.5: Radiated Sound Power Level and Surface Average Velocity of the Cylindrical Shell Structure Acoustic Source in the Anechoic Water Tank

Figure 1.6: Radiated Sound Power Level of the Spherical Source in the Anechoic Water Tank