The lossy backing is used to absorb the ultrasonic energy of piezocomposite transducers. In order to obtain shorter pulse at the expense of sensitivity, the acoustic impedance of the backing demands to match to the active materials.

To achieve greater acoustic impedance, metallic particles may be selected to be filled in the viscoelastic materials.

The absorption performance can be greatly improved for energy dissipation for viscoelastic materials such as epoxy resin and rubber. The multiple scattering (MS) of metallic particles in viscoelastic materials can increase number of propagation routes and improve energy dissipation at the same time.

Recently, researchers from the Institute of Acoustics (IOA) of the Chinese Academy of Sciences presented the basic MS theory on acoustical performance of the viscoelastic materials containing periodic metallic particles, which made a solid foundation and created good prospect for the application of backing material in broadband ultrasonic transducer.

The MS method is used to analyze absorption performance of viscoelastic materials containing periodic metallic particles, such as tungsten powder for the backing of piezocomposite transducers.

The computational results indicated that the metallic particles scattering and impedance matching are very important for the sound absorption of backing material. The metallic particles in viscoelastic material can improve acoustic impedance and change the absorption performance of the backing.

Preliminary test results showed that these composites behave good acoustical and mechanical performance. Fig. 1 shows that the pictures are the two formulations molding sample.

At the temperature of 24.3℃ and 0.7812MHz frequency test conditions, acoustic impedance of one new formula samples is 6.42MPa•s/m and its attenuation coefficient is 40.6dB/cm. Another formula samples is 4.83 MPa•s/m acoustic impedance and its attenuation coefficient reaches 64.1dB/cm. It has provided excellent backing material for the preparation of medical ultrasonic probe.

Fig. 1: The new molded samples of epoxy resin composite structure (Image by IOA)

If the acoustic impedance needs to be adjusted to match the active materials and the absorption performance is to be improved at the same time, it’s a good way to adjust volume content of metallic particles and mechanical parameters of viscoelastic materials.

Funding for this research came from the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900 and the National Nature Science Foundation of China (863 Program) under Grant No. 11104310.

Reference: 

BAI Guofeng, ZHANG Xiujuan, SUI Fusheng and YANG Jun. Research on Sound Absorbing Mechanism and the Preparation of New Backing Material for Ultrasound Transducers. The 44th International Congress and Exposition on Noise Control Engineering, San Francisco, California, United States of America, 9-12 August, 2015.