Why does sound wave over 20000Hz or lower 20 Hz

[yu_wing_sin]

Why does sound wave over 20000Hz or lower 20 Hz cannot produce the horizontal wave?

Although this is the reason why people can't hear those sound waves, is there any reasons or theory that can explain this phenomenon?

 

[mathman]

People can't hear those sounds because the human ear is unable to, not because of the physics of the sound waves. Dogs can hear much higher frequencies than humans.

 

[Nam_Sapper]

But nonetheless there are limits to the the efficient propagation of waves.

For instance, the amount of energy to move walls of air at frequencies below 20 Hz is so large that the only way to efficiently do so is with 'speakers' and special cabinets which are huge, and by using special 'rooms' tuned to the frequencies you want to produce. Basically, the lower the frequency, the more energy it costs, for a given effectiveness or perceived volume, and this is partly a limiting factor in the production of sound, as well as in the reception of it. (Your ears are too small to efficiently receive low frequencies, however, whales and elephants are able to easily hear these sounds.)

At higher frequencies, limits are imposed by the speed at which air molecules can be forced to vibrate and impact upon one another, and also, the frequency is affected by the density (space between the molecules) of the air.