Sound production and propagation

Sound is the vibration in the "elastic" medium that the human ear feels, and it is the rapid and small fluctuations of pressure. Sound is produced by the vibration of objects, such as the paper cone of the speaker, the plucked strings, etc. These vibrating objects are called sound sources. After the sound source utters a sound, it must pass through a certain medium to broadcast it to the outside. This medium can be gas, liquid and solid. After being disturbed by the vibration of the sound source, the molecules of the medium also vibrate, so that the energy spreads outward. However, it must be pointed out that the molecules of the medium only vibrate back and forth near their equilibrium position before being disturbed, and do not move outward with the sound waves. When the vibration of the medium molecule is transmitted to the human ear, it will cause the vibration of the eardrum of the human ear, and finally produce the sensation of sound through the auditory nerve. For example, the paper cone of a speaker will vibrate when the voice coil passes through an alternating current. This vibration causes a change in the density of adjacent air particles, and then spreads along the medium to distant particles, and finally reaches the receiver, as shown in the figure. It can be seen that during the propagation of sound waves, the vibration direction of the air particles is parallel to the propagation direction of the waves, so the sound waves are longitudinal waves.