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How much difference (in dB) is approximately between a broadband impulse noise which is mistakenly calculated back to a near field distance and a measurement, which considers the near field? For example: A noise is measured in 2m distance with 80dB and then it is calculated back to 118dB at 0.025m. Or: The same noise, but the near field characteristics are included in the measurement. What is the approximately decibel difference between those measurements?

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This question has no definite answer. The beauty of the far field lies within the fact, that you don't need a very rich knowledge about the geometry of the object you're surveying.

In the picture you provided the sound source seems to be some kind of engine. Parts of the engine are especially loud, some parts do not contribute to the overall noise level. Different parts may also have different resonance/vibration patterns, and therefore sound differently.

Your near field measurement has a very high dependency, of where exactly the microphone is placed. Along the exhaust pipes I expect higher values, but the away-facing side of the engines flywheel may even be a bit quieter than expected.

So there is no "approximate difference" between far field extrapolation and near field measurements, as it's not even clear, whether this would result in a positive or negative difference.

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    $\begingroup$ It's a good estimate for the average of the near field. But if you have a very heterogenious machinery, your location based variation around this average will be very large. $\endgroup$ Mar 3, 2018 at 15:30
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    $\begingroup$ Ah ok thank you. Just one more thing. You said that the difference can be positive (more decibels than measured) and negative (less decibels than measured). Why can it be also negative? $\endgroup$
    – J. Scott
    Mar 3, 2018 at 15:46
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    $\begingroup$ As stated in the answer: If you put your microphone next to one of the major emission points (like an exhaust, or where parts are rattling against each other) you expect to measure a higher value than what your far field extrapolation predicts. But if you place the mic somewhere down near the massive support structure and the rubber damping elements where the machine rests upon, you have a good chance, that it will be a bit quieter compared to the far field calculation. $\endgroup$ Mar 3, 2018 at 15:58
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    $\begingroup$ My estimation - well I won't (as I can't) give you any numbers. But same rationale applies here: Putting the mic along the skin, right next to the slap, will yield high values. Mic at the opposite side of the head will yield low values. But if you hit the dummy's ear, and you are interested, whether his eardrum is in any danger, let me tell you: It is. Additional to any sound wave, your palm forces the air inside his ear, suddenly increasing the static pressure a lot. Additionally, your opponent's muscles are tense, awaiting the blow, hindering pressure equilibration. $\endgroup$ Mar 3, 2018 at 18:07
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    $\begingroup$ It's not about the eardrum (it is of course in danger). It's about the sound pressure which can harm the inner ear. $\endgroup$
    – J. Scott
    Mar 3, 2018 at 18:09

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