UPSC NDA II 2018 General Ability Solved Paper

Concept:Ultrasound refers to sound waves with frequencies above the upper limit of human hearing.Explanation:Sound waves are classified by their frequency ranges.Audible sound for humans lies between $20\ \mathrm{Hz}$ and $20,000\ \mathrm{Hz}$ ($20\ \mathrm{kHz}$).Frequencies below $20\ \mathrm{Hz}$ are called infrasonic waves.Frequencies above $20\ \mathrm{kHz}$ are called ultrasonic waves or ultrasound.Humans cannot hear ultrasound, but some animals (e.g., dogs, bats) can.Therefore, the frequency of ultrasound waves is greater than $20\ \mathrm{kHz}$.Answer:Option D: Greater than $20\ \mathrm{kHz}$ is correct.