When comparing penetrating power in coarse-grained materials at the same frequency, which vibration mode generally penetrates deepest?

When comparing penetrating power in coarse-grained materials at the same frequency, which vibration mode generally penetrates deepest?

• A. Transverse
• B. Longitudinal
• C. All would have the same penetrating power
• D. Shear

Answer: (B)

In coarse-grained materials, how deep ultrasound can go is mainly determined by attenuation from scattering at grain boundaries. At the same frequency, longitudinal waves travel faster, so their wavelength is longer. A longer wavelength tends to interact with the grain structure less strongly, reducing scattering and energy loss, which lets the wave penetrate farther. In contrast, shear (transverse) waves move more slowly and have a shorter wavelength at the same frequency, making them more susceptible to scattering and attenuation by the grains. That extra loss means they don’t travel as deeply. So, the longitudinal mode generally penetrates deepest.

In coarse-grained materials, how deep ultrasound can go is mainly determined by attenuation from scattering at grain boundaries. At the same frequency, longitudinal waves travel faster, so their wavelength is longer. A longer wavelength tends to interact with the grain structure less strongly, reducing scattering and energy loss, which lets the wave penetrate farther. In contrast, shear (transverse) waves move more slowly and have a shorter wavelength at the same frequency, making them more susceptible to scattering and attenuation by the grains. That extra loss means they don’t travel as deeply. So, the longitudinal mode generally penetrates deepest.