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I know , when video-recording absolutely dark scenes, the mid-priced recorders cant record fully dark video, instead they have some sort of "noise" in image , like this: enter image description here

I see, that something same happens with AUDIO-recording (talking about mid-priced, typical audio-recording appliances). Even in a totally silent room, when we make an audio-recording, when we playback it on PC (without any audio gain) we still hear the some default noise (or whatever it is called).

why the silence is being recorded as noise(even with lavalier mic)? Is that generic behavior?

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    Even the largest anechoic chambers aren't completely silent. What makes you say it's silent where you're recording? – user22688 Oct 24 '17 at 21:43
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    Can I ask why you keep mentioning the lav mic? What special properties do you think it has? – Igid Nov 5 '17 at 22:56
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In a way everything other than the signal you're interested in is noise. Silence is the 'weird' thing, not noise.

There are many types of noise produced in different ways. But the most common (the one you're most probably talking about) is the noise caused by the equipment itself.

As a current flows through a conductor (even more a resistor), some electrons collide with the atoms of the conductor, converting electrical energy into thermal energy, raising its temperature (you can think of temperature as the rate of how fast atoms are moving around their position). This can be expressed as a minuscule variation in resistance, also affecting voltage and current as Ohm's law states.

I'm not 100% sure but I think that's called thermal noise.

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    @T.Todua, that really doesn't make any difference. Lav mics both pick up and make noise themselves. It's no different than using any other mic in this regard, and perhaps even worse. But mostly whatever you are recording INTO is making noise. – user22688 Oct 25 '17 at 14:08
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It is not the silence that is being recorded as noise but noise that is being recorded as noise. Most of the noise is originating from the equipment, some is indeed acoustical but much better masked out by spatial hearing and directed attention during physical presence than when checking a recording.

More expensive equipment tends to add lower levels of noise to recordings.

  • but the same happens with lavalier mic – T.Todua Oct 24 '17 at 21:48
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    Noise can be introduced at several stages in the chain; the mic still sends an electrical signal back to the recorder, which then do some sort of processing and conversion to a digital signal (unless you're recording using analog formats). Also, as several others have mentioned the room might not be quiet. It might sound quiet for your ears, but there can still be ambient noise coming which are filtered out or just not noticeable by your ears. – bjelleklang Oct 26 '17 at 10:58

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