Using an attenuation coefficient of about 0.5 dB/cm/MHz, what is the total attenuation in decibels for 3 cm depth at 2 MHz?

• A. 1.5 dB
• B. 9 dB
• C. 3 dB
• D. 6 dB

Answer: (C)

Attenuation adds up with both depth and frequency. The coefficient 0.5 dB per cm per MHz tells us how much energy is lost per centimeter for each MHz of frequency. To get the total loss, multiply by the path length and by the frequency: 0.5 × 3 × 2 = 3 dB. So the beam loses 3 dB over 3 cm at 2 MHz. This also illustrates why higher frequency or longer travel distance increases attenuation. The other numbers would result from omitting either the depth or the frequency factor.