Acoustic transmission

Acoustic transmission is the transmission of sounds through and between materials, including air, wall, and musical instruments.
The degree to which sound is transferred between two materials depends on how well their acoustical impedances match.
In musical instrument design.
Musical instruments are generally designed to radiate sound effectively. A high-impedance part of the instrument, such as a string, transmits vibrations through a bridge (intermediate impedance) to a sound board (lower impedance). The soundboard then moves the still lower-impedance air. Without bridge and soundboard, the instrument does not transmit enough sound to the air, and is too quiet to be performed with. An electric guitar has no soundboard; it uses a microphone pick-up and artificial amplification. Without amplification, electric guitars are very quiet.
Stethoscope.
Stethoscopes roughly match the acoustical impedance of the human body, so they transmit sounds from a patient's chest to the doctor's ear much more effectively than the air does. Putting an ear to someone's chest would have a similar effect.
Building acoustics.
Acoustic transmission in building design refers to a number of processes by which sound can be transferred from one part of a building to another. Typically these are: