We know that a sound source produces a sound wave and a high pressure area is followed by a low pressure area, while they travel with 300m/s. My question is, if the sound source travels with 50m/s in the same direction, is there a chance that the sound source will be in a low pressure area? I know that the high/low pressure areas travel towards the direction of the sound wave, but isn't there a high/low pressure area oscillation at the point where the sound wave left? Consider the sound wave to constantly emit sound.
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$\begingroup$ in case you cannot comprehend my question, what I am basically asking is: after a sound is generated, a high pressure area moves away at the speed of sound, right? Is there a low pressure area that follows that high pressure area? or normal pressure area follows the high pressure area? $\endgroup$– sloupiocCommented Jul 18, 2015 at 22:54
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2 Answers
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Yes, based on your comment. There is a low pressure area which follows the high pressure but it also travels at the speed of sound, so you wouldn't catch up to it at 50 m/s.
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Consider the typical powered speaker; a paper cone on a solenoid. When current is applied to the solenoid, the cone moves (forward or backward, depending on direction of current).
Let's say you apply a steady DC current to the solenoid and the cone pushes forward. It will push a high-pressure wave ahead of it, and then pressure against the cone will equalize back to ambient normal. When you let off the current, the cone springs back to its initial position, which creates a low-pressure area. Again, the pressure quickly equalizes to ambient.
(If the direction of current is reversed, the cone first draws back then springs forward. It's the same process in reverse.)
Now consider the speaker traveling forward through the air. Because it is traveling, the pressure against the front is already greater than ambient (and the pressure behind, less than ambient). When the speaker cone pushes forward, it further increases the pressure ahead of it (which change travels outward as a sound wave) and when it pulls back the pressure ahead decreases - whether the decrease in pressure drops below the "ambient" air pressure (e.g. the consistent pressure five meters to the left of the speaker's path) depends on whether the speaker cone travels backwards faster than the speaker body moves forward, with respect to the air.
At any rate, the average pressure of the sound wave will be the same as the ambient air pressure through which it is traveling, for a continuously emitting source.
