The propagation speed of an ultrasound wave is equal to the product of which two quantities?

• A. Pulse duration
• B. Period
• C. Number of cycles in the pulse
• D. Frequency

Answer: (D)

Propagation speed is set by multiplying frequency by wavelength: v = f × λ. Frequency is how many cycles pass each second, while wavelength is the distance the wave travels in one cycle. Their product gives the distance traveled per second, i.e., the speed. In a non-dispersive medium, this speed is constant because changes in frequency are accompanied by inverse changes in wavelength, keeping the product the same. Among the given options, the quantity that participates in the speed equation is frequency, since it is one of the two factors in v = f × λ; the other listed quantities (pulse duration, period, number of cycles) don’t form that speed relationship by themselves.