$(a)$ Illustrate the use of a stethoscope.
$(b)$ What are infrasonic and ultrasonic sound waves?

(N/A) stethoscope is a medical instrument used for listening to sounds produced within the body,primarily in the heart or lungs. It works on the principle of multiple reflection of sound. The sound of a patient's heartbeat enters the chest piece and travels through the rubber tube via multiple reflections until it reaches the doctor's ears.
$(b)$ Infrasonic sound waves are sound waves with frequencies below $20 \ Hz$. Humans cannot hear these sounds.
Ultrasonic sound waves are sound waves with frequencies above $20,000 \ Hz$ (or $20 \ kHz$). Humans cannot hear these sounds either.