What are the units of the attenuation coefficient?

• A. Np/m
• B. dB/cm
• C. Hz
• D. m/s

Answer: (B)

Attenuation describes how the signal weakens as it travels, so this coefficient is a rate of loss per unit length. We express loss on a logarithmic scale because it makes additive handling of multiple segments intuitive, and decibels provide that scale. Therefore, expressing the attenuation coefficient in decibels per unit length—such as dB per centimeter—fits the common engineering convention and lets you multiply by distance to get the total loss. While Nepers per meter (Np/m) would describe the same decay using natural logs, dB/cm is the standard in many contexts, including clinical ultrasound. The other options don’t measure loss: frequency (Hz) is how often the wave cycles, and speed (m/s) is how fast it travels.