SPL is inversely proportional to which variable?

• A. Wavelength
• B. Frequency
• C. Amplitude
• D. Time

Answer: (B)

SPL is about the pressure amplitude of the ultrasound wave as it propagates, and in tissue that propagation is strongly affected by attenuation. Attenuation in soft tissue increases with frequency, so higher-frequency waves lose more energy over the same distance. That extra loss reduces the remaining pressure at a given observation point, which lowers the measured SPL. In other words, as frequency goes up, the observed SPL tends to drop because more of the wave’s energy is absorbed along the way. This is also why higher-frequency ultrasound gives better resolution but shorter penetration: you pay with greater attenuation, so SPL at depth decreases with frequency.

Wavelength is related to frequency (λ = c/f), so higher frequency corresponds to shorter wavelength, but the key point for SPL is the frequency-dependent attenuation, not the wavelength itself. Amplitude is the immediate driver of SPL, but for a fixed drive the frequency- dependent loss in tissue governs why SPL decreases with increasing frequency. Time doesn’t set SPL in this context.