# pressure wave

• August 9th, 2010, 02:16 PM
DesertRose
pressure wave
hi, it is said that when a sound wave travels through the air, the molecules vibrate about it's equilibrium position along the direction of the wave motion. my question is this; why are regions of high and low pressure created and why is it called a pressure wave?
any assistance would be greatly appreciated!!
thanks!
• August 9th, 2010, 03:06 PM
Cold Fusion
Things like speakers and our vocal cords vibrate forwards and backwards; the forward motion compresses air since it is limiting the space that the air can exist in, therefore making it denser. The backwards motion creates a partial vacuum, which is due to new area being allotted for the air to exist in. Because of atmospheric pressure, the air is pushed into this empty space simply because there is nothing to stop that pressure from moving into there.

This motion causes areas of high and low pressure in the air; I do not think that you absolutely need a binary system of + and - to create sound fundamentally....it just so happens that things like speakers and vocal cords have an excursion limit, and must return to their original position before 'bouncing out' again; otherwise we would spit out our vocal cords! :shock:

Even when considering a shock wave create by something like an explosive, due to the elasticity of air, I believe the air essentially 'over bounces' itself and in relieving the high pressure, creates a bit of low pressure in its wake, though I do not think it would be equal and opposite to the high pressure. You also get low pressure from explosives due to the fact that they tend to suck up air in the atmosphere in order to catalyze the reaction.
• August 9th, 2010, 04:59 PM
Harold14370
Re: pressure wave
Quote:

Originally Posted by DesertRose
why is it called a pressure wave?

If one were to graph the pressure as a function of distance from the source, it would be a moving sinusoid. The graph would look like a wave in the water where the peaks and troughs in sound pressure would be like the peaks and troughs in the height of the water wave.
• August 10th, 2010, 12:52 PM
DesertRose
Ok thanks a lot!!!!!