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I seem to remember reading about this phenomenon with omni mics before somewhere a long time ago,

but I just took the opportunity to record a noisy vent in one of my rooms and I recorded it with an MK5 mic set in cardioid, and then I recorded it in omni, with no change of position of the mic.

The cardioid mic had an average level of -52.9 dB and a peak of -38.8 dB.

The omni mic had an average of -35.5 dB and a peak of -21.5 dB.

Nothing changed on the preamp level or position of the mic, only the polar pattern.

But it sounds like there is a whole lot more sub-bass and rumble on the omni, and the level is boosted quite a bit (which, I can understand that the whole room as opposed to the front of the mic is being recorded, but the sub-bass baffles me).

Do I have a defective mic or does an omni pattern pick up a lot more sub-bass and rumble, and how come I didn't know about this before? Is this known about or written anywhere?

Could I use this to an advantage for recording guns, for instance?

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3 Answers 3

As with answer 1, You are hearing proximity effect, except what you are hearing is due to the Cardioid patterns lack of bass response. Proximity effect is easy to hear on every polar pattern (except Omni of course) By comparison to Omni, any sound source at a distance from the mic will sound significantly bass light. This is a huge problem for inexperienced live vocalists or musicians in a studio not rooted to the spot.

The Omni pattern of most condenser mics will give you the most accurate sound spectrum and a far superior low frequency extension, often lower than 20Hz.

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afaik, as opposed to pressure-gradient transducers, omnis work much like barometers: they simply convert any air pressure change to electricity. meaning, they even have to be high-pass-filtered or otherwise adapted so as not to transmit the ambient air pressure!

pressure-gradient microphones, on the other hand, as we all know, use constructive and destructive interference to produce a certain polar pattern.

in fact, a short google research on this was quite revealing: http://www.tonmeister.ca/main/textbook/node472.html

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The proximity effect shouldn't occur with an omnidirectional microphone. The Schoeps CCM 5/MK5 frequency response chart shows that the omni configuration has a very flat frequency response down to 20 Hz, whereas the cardiod configuration begins to attenuate low frequencies at 100 Hz. In addition, you're probably picking up more LF in the room that the cardiod is rejecting.

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