= Acoustics
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<Acoustics> studies <sound>: its generation, propagation, interaction with matter and detection. In a fluid, a small <acoustic wave> couples a <pressure> disturbance to the motion of the medium. The disturbance transports <energy> without requiring the same parcel of material to travel from the source to the receiver.
A <loudspeaker> and a <microphone> make a simple experiment. Changing their separation measures propagation delay, while changing the <frequency> in a bounded room reveals <resonance> and <standing waves>. A strong microphone response does not by itself imply a large net <sound intensity>: oppositely travelling <waves> can produce a large local pressure while their mean energy transport cancels. <Acoustic impedance> connects pressure and motion for a specified travelling wave.
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