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In a sound wave, troughs are regions of low (with respect to an average value) pressure, and, accordingly, low density. So, the answer to the title question, troughs are rarefactions. The answer to the body of your question: the amplitude is always positive. But acoustic pressure level, a logarithmic measure of the effective pressure of a sound relative to a ...


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Sound is a pressure wave in a fluid medium. That medium has a "static"or average pressure and sound is simply a small change of that pressure (as a function of time). Negative sound pressure just means that the current pressure is a little lower than the average pressure. For example, static air pressure is around 100,000 Pascals (Pa) . Normal ...


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