Waves and Sound

Ultrasound waves have frequencies above the audible range for humans, which is typically above 20 kHz. Therefore, option A correctly states that ultrasound frequency is above 20 kHz.

Sound waves are longitudinal waves because the particle displacement is parallel to the direction of wave propagation, unlike transverse waves where displacement is perpendicular.

Sound requires a medium to travel and cannot propagate through a vacuum because there are no particles to transmit the vibrations. It can travel through solids, liquids, and gases.

TV transmission coverage is limited primarily because the Earth is spherical, causing radio waves to travel mostly in straight lines and thus limiting the range due to the horizon. This curvature prevents signals from reaching very far distances without relay stations or satellites.

At ordinary temperatures, sound travels approximately 1120 feet per second. For an echo to be distinctly heard, the reflected sound must return after at least 0.1 seconds, meaning the reflecting surface must be at least 56 feet away (since sound travels to the surface and back, the total distance is 112 feet, so one way is 56 feet).

Ultrasonic waves are sound waves with frequencies higher than the upper audible limit of human hearing, which is about 20 kilohertz (20,000 hertz). Therefore, ultrasonic waves have frequencies more than 20 kilohertz.