Noise concept?
Answer:
As defined by Cyril M. Harris*, near field (or near sound field) is "the sound field close to a sound source (between source and the far field) where the instantaneous sound pressure and particle velocity are not in phase."
He defines far field as "that portion of a sound field of a sound source in which the sound pressure level (due to this sound source) decreases by 6dB for each doubling of the distance from the source."**
Sound waves behave differently at different distances from a source. This behavior depends on the size of the source, the distance from the source, and the frequency of the sound. Furthermore size is relative, and the only way to compare size in acoustics is relative to the wavelength. I won’t go into the mathematics of it right now, but if you know the size (m) of the source and the frequency of the source (Hz), you can determine the far field distance (when the source begins acting like a point source***) and the near field distance.
The relativity can be explained this way: Imagine you were standing on a dock in a little pond and you threw a pebble into the pond. This would look like a point source, because the wave radiates in all direction spherically from the point where the pebble fell into the water. Now picture what it would look like from the dock if you could throw a giant boulder into that same pond. You wouldn’t see the same spherical waves that the pebble created because the boulder is too big RELATIVE to the pond. It would make a huge splash, that radiates randomly and seemingly without order. But what if you threw that same giant boulder into the OCEAN and could view the water from outer space. This would create the same waves that the little pebble created in the pond and look like a point source. So looking at the boulder splash in the pond from the dock is similar to being in the near field. And looking at the boulder splash in the ocean from outer space is similar to being in the far field. This is how sound waves propagate as well.
Noise is generally defined as any unwanted sound, or sound of a general random nature (i.e. white noise) without any clearly defined frequency components. Therefore, if there are no frequency components the near field/far field concepts explained above might not apply totally for noise. But this should give you a better idea!
Near field noise-Due to high frequency of sound waves
Far field noise-they have very low frequency when they meet you
I have never heard those terms in relation to noise.
My experience with these terms comes from electroacoustics, where generally nearfield and farfield are related to the distance from the source the diameter of the source and the wavelength. The threshold between the two regimes is defined in the link below.
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