In soft tissue, the attenuation coefficient in dB/cm is approximately equal to one-half of which quantity, expressed in MHz?

• A. Distance, in cm
• B. Frequency, in MHz
• C. Wavelength, in cm
• D. Propagation speed, in m/s

Answer: (B)

In soft tissue, attenuation grows roughly in proportion to frequency. The practical rule of thumb is that the attenuation coefficient α (in dB per centimeter) is about 0.5 times the frequency (in MHz): α ≈ 0.5 f (MHz) dB/cm. This means the attenuation per centimeter is about one-half the operating frequency expressed in MHz. For example, at 3 MHz, the attenuation is about 1.5 dB/cm; at 5 MHz, about 2.5 dB/cm. This happens because higher-frequency waves experience more absorption and scattering per unit distance.

The other options don’t fit because attenuation per centimeter isn’t determined by distance alone, nor is it tied to wavelength, which is derived from speed and frequency, or to propagation speed itself. The key factor here is frequency, hence the one-half relationship in dB/cm.