Which fraction of a wavelength is recommended for the matching layer thickness?

• A. One-half
• B. One-eighth
• C. One-third
• D. One-fourth

Answer: (D)

In ultrasound transducers, the matching layer is designed to be a quarter-wavelength thick in that material. This specific thickness helps energy move efficiently from the transducer into the tissue by both canceling unwanted reflections and transforming impedances between the two media. When the layer is one-quarter of a wavelength, the reflections from the two interfaces end up out of phase and partially cancel, reducing overall reflection and improving transmission. At the same time, a quarter-wavelength layer acts as an impedance transformer, so the effective impedance seen by the transducer is closer to that of the tissue, maximizing power transfer. The actual thickness is t = λ_m/4, where λ_m is the wavelength in the matching material (λ_m = v_m/f, with v_m the material’s sound speed and f the operating frequency).