Can I ask a general question, without hijacking/diverting the thread ? What would be the typical range of spacings you might use for a spaced pair of omni AB mics on a single bar ? From smallest to largest spacing..to encompass the variety of sources we might encounter: full width orchestra, chamber group, string quartet, solo piano, wind ensemble, organ, solo flute etc etc. I'm trying to fashion a lightweight but extendable/retractable AB arm which might encompass all these possibilities. I should have a working prototype to be able to submit a photo for your viewing pleasure in a few days, once I have the appropriate dimensions locked in.......

For me, about 25 to 70cm for a main pair of mics. I use manfrotto 154b for the wider stuff.

Generally one can say the spacing is head width 6-8" to 3 feet. So a one meter bar is pretty standard.

Tests have shown which runtime difference between the signal for left and right you need for it to be localized completely at the position of the left or the right speaker. And we know how many dB difference between coincident signals you need to achieve the same effect. The rest is mainly geometry and mathematics to calculate which angle and/or distance between the mics you need to project the subject to the full basis between the loudspeakers.

You can use this calculator by Prof. Sengpiel.

"Aufnahmebereich" ("recording angle"?) is the total angle from left to right (e.g. of an orchestra, seen from the tripod of your mics) that will be projected into the basis between the loudspeakers by the current mic configuration, anything further left/right in reality will be cut off and fall on the loudspeaker.

As you can see there, anything up to 0.514 m spacing between omni mics (or any other with no angle) will lead to a recording angle of 180°, so a microphone system would have to be aligned with the frontmost intruments of an orchestra. If you are further away, you would need more distance between the mics. In reality, a little less distance then what the page calculates will be more realistic for a better virtual spacing between loudspeakers with regard to spacing of sound sources in reality. (You can see that in the graphics, too.)

It's nice to play around a bit with that web page. It shows the pure theory and can be very helpful. It is valid for the main microphone setup for stereo over loudspeakers. Not for close AB and not for room mics.

Lutz

Visualization of ORTF Stereo Microphone System mic angle 110° spacing 17 cm equivalence system - Array with two microphones Time of arrical difference mic - Stereo recording angle SRA time difference level difference mic orchestra angle degrees visua

Same in english

Returning to an older thread, what would your recommendation be for a string quartet concert recording, AB spacing for a pair of Omni mics. No spot mics, pair located within the ensemble, rather than in front of it, around 2.8 to 3m high

My experience has shown that smaller spacing (38-50cm) gives a good representation without a 'hole in the middle' and this seems to be borne out in the Sengpiel SRA calculator linked by fred2bern above. Richard King recommends 70cm, which seems a little wide. Any thoughts...and adjustment for seated vs standing ?

This following thread elicited good debate and experience to bear on the spacing issue, with much evidence supporting the wider (70-100cm) spacing: chamber ensemble recording...spaced pairs?

Ok, so for anyone who's interested, I've generated a string quartet recording at both the pm rehearsal and evening concert, identical 70cm AB spacing, identical mic-pre gains.

The only differences are: variable location of mic stand and height.

Rehearsal recording has mic stand at 8 feet height, and located on the midpoint of a line joining cellist to 1st violin.

Concert recording has the stand raised to 9 feet, and relocated to about 10cms on the audience side of a line inside the gap/space between 1st and 2nd violins and cello/viola (in other words, very close to the actual centre of the quartet)

I know...I should have only changed 1 variable, not 2 ....!

I'll post a couple of samples for comparison.....

I found the spacing of the omni pair has to do with the size of the hall and the distance between the mics and players. Smaller the hall, closer the distance, smaller the spacing you need to position the omnis, and vise versa. Another word, the spacing should not be dogmatically set to a particular distance. It is a good idea to point the microphones outward slightly, directing them to the players, a bit like ORTF, perhaps not at 120 degrees. In a string quartet setting, once you point the mics outward slightly, you can reduce the spacing between the two mics. This can improve the phase coherence.