Like light, sound too undergoes reflection from any hard surface and obeys the same laws of reflection, which light obeys, that is angle of incidence is equal to the angle of reflection, the reflected wave, the incident wave and the normal at the point of incidence lie in the same plane. The reflected sound is referred to as an echo. When we produce sound before a reflector, whether we hear the echo or not depends on the speed of the sound in the medium and the distance between the source/observer (which is the same in this case) and the reflector. To perceive two sounds they should have a time gap of at least 0.1 second. In a closed enclosure, one can hear multiple sounds (echoes) even after the source of sound stops producing the sound due to multiple reflections at different points on the surfaces of the enclosure. This phenomenon is called reverberation. Reflection of sound
is used in various applications like stethoscope, echocardiography, sonar etc, although ultrasonics is used for things with a frequency greater than that of the audible range. The sounds whose frequency is less than 20 Hz are infrasonics, and greater than 20 kHz are ultrasonics. The human ear, that helps us hear sound, consists of three parts called the outer ear, middle ear and the inner ear. The outer ear consists of a pinna that helps in capturing the sound; the middle ear comprises of an eardrum and an auditory canal, hammer and anvil that amplify the incident sound; the inner ear consists of an auditory nerve that converts the amplified sound into electrical signals and sends it to the brain for perception.
In case of hearing difficulties, one uses hearing aids that consist of a microphone and an amplifier.