If frequency decreases, the wavelength:

• A. Decreases
• B. Increases
• C. Remains the same
• D. Is not related to frequency

Answer: (B)

Wavelength is inversely related to frequency when the propagation speed in the medium is fixed. In ultrasound, the speed in soft tissue is essentially constant, so λ = c / f. If the frequency decreases, the wavelength grows. For example, at 1 MHz the wavelength is about 1.54 mm in tissue, and at 0.5 MHz it’s about 3.08 mm. So lowering frequency lengthens the wavelength. The ideas that wavelength stays the same or isn’t related to frequency don’t fit because the product of frequency and wavelength equals the wave speed, which is constant in a given medium.