■ By what kind of structurecan sound be heard? 　■
Sound arises from the object which is vibrating.　That is, the sound is not transmitted in the inside of vacuum although it is transmitted in the inside of the gas in the air etc., and a solid and a liquid. If an object vibrates, nearby air will be pushed and air will become deep only the portion.
The deep portion of air pushes nearby air and the deep portion of air moves.
A thin portion and a deep portion are made in the air. This resembles the situation of the wave spread and transmitted, when a pebble is thrown in the water surface.
The wave of sound is called the dense-sparse wave since the layer of thin air (sparse) and the layer of deep air (dense) are transmitted by turns in it. This wave reaches the ear, and the eardrum vibrates, the signal of that vibration gets across to a brain, and a brain judges this wave as a sound.

Sound is transmitted as a wave. Speaking of a wave, it imagines the thing which sways to the one more nearly perpendicular to a direction of movement as shown in the right figure, but this is called a "traverse wave" and the wave of sound is a longitudinal wave.

進行方向：Advancing direction
When the character of sound is expressed, the figure has a form like a traverse wave, but by machines, such as an oscilloscope, it displays the relation between the width and the time which vibrated, and makes the wave of sound legible.

Longitudinal wave (it sways to a direction of movement and the same direction)→

Traverse wave (it sways to the one more nearly perpendicular to a direction of movement)→

As for sound, the pressure change of air is transmitted as a wave (sound wave). The quantity of this pressure change is called sound pressure.
The unit of sound pressure is international unit Pascal (Pa) of pressure (1 atmosphere = about 1000 hPa = about 100000 Pa). The sound pressure of the minimum sound that human being can catch is abbreviation 20 muPa, and this is a very small value of about 2/10 billion of 1 atmosphere.

The human being's ears are not only surprisingly so highly sensitive, but they correspond to the sound pressure of the quite wide range.

 

Sound gets across also to a liquid.

This is because that gas and a liquid are the same kind of things of the "fluid" which tell roughness and fineness similarly. "Fluid" is gas and a liquid and, unlike the solid, it has fluid character. Moreover, it can say that the density and elasticity of water differ from air in the reason acoustic velocity is faster underwater than the inside of the air.


Telling sound, gas or not only a liquid but a solid often conducts sound.

Solid one can tell well.
When we are standing on the home of a station, although the figure of a train is not in sight, it may be heard that the track serves as Katha Katha. This is to transmit from afar vibration of the train which is still in the distance in a track. It is that the sound transmitted in a track although the sound transmitted in air at this time cannot be heard yet can be heard, because the sound transmitted in a hard solid like a track does not become weaker as easily as the inside of the air.
　That is, it is because the thing transmitted in the inside of a solid also to the sound from a distance is maintaining sufficient intensity which an ear hears. Moreover, in a solid, the speed from which sound is transmitted also becomes early all the time.
　

This is because in the solid the direction of vibration is not only a dense-sparse wave unlike fluid like a liquid or gas.

Reference HPs


http://www.max.hi-ho.ne.jp/lylle/oto1.html
http://www.osaka-kyoiku.ac.jp/~masako/exp/oto/naiyou/seisitu2/