Sound

Loudness, Pitch, Quality

A musical sound is distinguished from other sounds by three characteristics: Loudness (depends on amplitude), Pitch or shrillness (depends on frequency), and Quality or Timbre (depends on waveform, allowing us to tell apart two sounds of the same loudness and pitch from different sources). Color and Texture are not physical properties used to describe sound in acoustics, so they are not correct choices.

Vibrations of a simple pendulum (neglecting air resistance), Vibrations of a tuning fork struck against rubber pad, Vibrations of a loaded spiral spring

Free vibrations occur without external force. Examples include simple pendulum (neglecting resistance), struck tuning fork, and loaded spring. Table vibrations and diaphragm vibrations are forced vibrations.

Reflection of sound

SONAR (Sound Navigation and Ranging) works on the principle of reflection of sound waves. Ultrasonic waves are sent and their echoes are received to determine distance and location of objects.

Applied frequency equals natural frequency

Resonance occurs when the frequency of the externally applied periodic force matches exactly with the natural frequency of vibration of the body, resulting in maximum amplitude.