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Soundscape, stereo effect etc. in live recordings

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Ferenc

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Recently I recorded a live rock concert and observed an interesting phenomenon. It was an open-air act, in a small court of a building, bounded by walls. I stood right behind the mixing desk, with cardioids taped to my eyeglass frames. The distance from the stage was about 25-30 m. Sometimes I had to move because people were walking around, conversating, drinking etc. When listening back to the recording, I did observe (not too disturbing, but noticeable) changes in the tone color and sound field.

Next day there was another concert in the very same location. I was not interested in recording, so the situation offered the possibility of some exploration. I heard a profound difference between positions of exactly at the center and moving sideways by as small as about 50 cm! At the center, the whole sound somehow "jumped" to the front and the stereo effect became less natural.

What's the explanation of this effect? (I suspect some - untypical - interference effects...) Have anyone observed similar? What are the implications for the practice of live recordings?

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It's called interference patterns. You don't normally hear them so obviously because the brain filters out some minor changes and can't detect others. It's a bit like how your brain tries to adjust perception of your eyesight to maintain colour balance regardless of what lighting you're in.

Basically, if you're wearing the mics, any movement at all is going to change the sound of the recording. Very small, very slow movements are less likely to be noticed.

What you're actually hearing are the acoustics of the room you were in [and their interaction with the speaker placement, crowd movement, etc.]. This is normal and if you pay close enough attention you can actually hear it yourself just by walking around.

I'd suggest making a simple mount for the mics and using it next time rather than wearing them. If you're running FOH [if you're at the sound board] then you'll be limited as to placement [having to keep it with you] but at the very least, mounting them and not moving them makes for a -consistent- recording that you can EQ and edit later.

I have seen friends make simple mounts for twin-condensor "binaurals" by gaff-taping a pencil to the top of an extended mic stand, then taping the mics to the end of the pencil. Not elegant, but it does at least work.

Since you're using cardioids I'd suggest using a Y-pattern, as you don't have to deal with the leakage you get with omnis.

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I may be way off here but I have had this occur once outdoors while recording in a static location. I attributed it to the changing thermal currents coming from the ground, crowd, sun light (as it was a very hot day) . This in effect would be similar to the doppler effect changing pitch as sound moves either to or from the subject. In my case the tonal changes particularly in the upper frequencies were most noticeable.

Any physics guru's out there care to comment on this?

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  • 4 years later...

Physics

A=S*sqrt((T+459.4)/459.4)

A= The speed of sound at Temperature

T = Temperature

S=Speed of Sound at 0 degrees F. = 12,600.353 inches

459.4 = absolute zero or , zero degrees Kelvin

12,600.535*sqrt((73+459.4)/459.4) as an example , will yield the speed of sound at 73 degrees . In an open air mass , there are pockets of density differences , and differing temperature areas due to ambient air and body mass of the crowd .

Denser air will affect the equation as well because the sound will slow down in some places , more than others . What happens then is Phase variances in the overall sound field , that are at first not noticeable , and then on closer examination in a different environment where you are not distracted by the crowd , it becomes apparent

In some case it is Very pronounced due to thermal activity in the Air .

Experiment ,.........: large Glass bowl , fill it with cool Tap water , next Slowly pour into it , Warm water right in the middle of the Bowl , look thru the side of the bowl , and you will actually see the thermals , this is happening in the Air around you , you just cant see it , until the effects of it show , Leaves floating Up , Feathers , dust etc .

In a Semi contained area , like a partially walled outdoor theater , the sound gets compressed as well by the partial walls . reflections from each wall travel back into the area , converging with sound from the other wall ( center is now NOT such a great place to be ) in that situation closer to the front of the stage , or all the way at the back would be best for a stereo image .

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