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I am confused here, though I have been reading about it for years. What is the difference between a real value and an absolute value? For example, when do we take the absolute value of pressure or the real value of pressure in acoustics?

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closed as unclear what you're asking by ACuriousMind, John Rennie, knzhou, CuriousOne, user36790 Jun 21 '16 at 2:54

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  • $\begingroup$ en.wikipedia.org/wiki/Absolute_value $\endgroup$ – user108787 Jun 20 '16 at 9:07
  • $\begingroup$ What about the definition of the absolute value is unclear to you? I'm not sure what you're asking. $\endgroup$ – ACuriousMind Jun 20 '16 at 13:55
  • $\begingroup$ I am just trying to understand the difference in real life situation between absolute value and real value. $\endgroup$ – ezE Jun 20 '16 at 14:24
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Difference between real and absolute value in general: Look at count_to_10 's answer.

For acoustics and preasure measurement: Absolute pressure - pressure against perfect vacuum. Real pressure: Usually defined as the pressure against a reference-environment. Also called differential pressure. For example the pressure of the air inside a football against the ambient pressure.

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  • $\begingroup$ So the pressure microphone usually measures Absolute pressure? $\endgroup$ – ezE Jun 20 '16 at 9:35
  • $\begingroup$ Depends on the microphone. Some measure the difference against the environment by connecting the environment (or any other reference-volume) and the volume where the pressure has to be measured by a tube (only separated by the sensor). This is called differential pressure. Absolut pressure sensors measure against a perfect vacuum. Relative pressure sensors measure the pressure against ambient conditions. $\endgroup$ – Scotty1- Jun 20 '16 at 9:48
  • $\begingroup$ This is interesting, So we have three types of pressure measurement 1. Relative (ambient condition), 2. Diffrential (reference environment) and Absolut (vaccum). The first two we use real value of pressure and for the third we use absolut value of pressure? $\endgroup$ – ezE Jun 20 '16 at 11:22
  • $\begingroup$ Well... Pressure measurement is always in relation to a reference-value. The three kinds of measurement just define different reference-values, but the way of measuring the pressure is the same. And absolute pressure is "real" pressure + reference pressure. So for example in acoustics the ambient pressure (reference pressure) is a static pressure of $p_{\infty}=101325\,Pa$. If you have a soundwave with a dynamic or "real" pressure of $p_{Sound}=1000\,Pa$ you get a total/absolute pressure of $p = p_{\infty}+p_{Sound} = 102325\,Pa$. $\endgroup$ – Scotty1- Jun 20 '16 at 11:56
  • $\begingroup$ But take care: If calculating the Sound Pressure Level (SPL) the reference-pressure is NOT the ambient pressure. The SPL calculates the logarithmic loudness of a sound relative to hearing threshold. Thus the reference-pressure is the sound pressure of the threshold. $\endgroup$ – Scotty1- Jun 20 '16 at 11:59
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In a complex number $z=a+i b$,the real part is $a$ and abs is $\sqrt {a^2+b^2}$,.

I am not an expert of acoustics but in general, when pressure is given in complex quantity and you want to measure its magnitude then take absolute. If you are comparing two complex numbers then equate real with real and imaginary with imaginary.

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