Ultrasonic waves are sound waves with a frequency higher than the upper limit of human hearing, which is typically around 20 kilohertz. Option 1 states that ultrasonic waves have a frequency between 20 hertz and 1000 hertz, which is incorrect as this range falls within the audible range of human hearing. Option 2 states that ultrasonic waves have a frequency between 1000 hertz and 20000 hertz, which is partially correct as the lower limit is correct but the upper limit is incorrect. Option 4 states that ultrasonic waves have a frequency less than 20 hertz, which is also incorrect as this range falls within the infrasonic range, not ultrasonic.

Therefore, option 3, which states that ultrasonic waves have a frequency more than 20 kilohertz, is the correct answer.