Topic: Quick omni question

[voltronic]

The sphere causes a slight increase in upper-midrange response in the forward direction--basically a "presence" boost--while smoothly rolling off the high frequencies at the sides and back. The graphs on http://www.schoeps.de/en/products/ka40/graphics show the on-axis part of this effect. It's a subtle effect which allows the microphones to focus well when placed at greater than usual distances.

I'm a bit confused by the Schoeps link you provided: The Schoeps KA 40 shows the presence boost you described, but then the graphs seem to show the opposite effect of what you said regarding high frequency attenuation off-axis.  I'm looking at the bottom graph showing increased HF attenuation as you move increasingly off-axis, which is labeled "without KA".  The top graph "with KA 40" does not show this off-axis attenuation.  To me, these graphs are saying that the KA 40 is compensating for the off-axis HF rolloff that the capsule exhibits on its own.  The M150, on the other hand, shows the high-frequency roll-off off-axis starting at 2kHz.

Maybe so. But for stereo recording, why would you want reflected sound to be picked up with full-strength high frequencies? That only smears and confuses the imaging, unless the venue is very dry. If anything, in spaces with typical reverberation, I think it's desirable for omni microphones to de-emphasize the upper-mid and high frequencies even more than a small-diaphragm capsule naturally does, when those frequencies aren't arriving on axis.

I have not found this to be the case with these particular microphones.  If you want to hear for yourself, here is a very minimalist independent record label in my area where they only use a single pair of Earthworks omnis for their recordings.  http://www.dtrmusic.com/

[DSatz]

The upper graph on Schoeps' Web site shows only the on-axis response with the sphere attached, while the lower graph shows the response at various angles of incidence without the sphere. So you're right: these graphs don't show the effect of the sphere off-axis. It would have been better IMO if both graphs showed the response at the same set of angles--one with and one without the sphere.

--As for Earthworks mikes and other omnis based on very small pressure transducers, for any given type of microphone with almost any given set of characteristics, a person can eventually learn how to use those characteristics to best advantage, and to choose venues and music so that good-sounding recordings result. I don't mean to disparage that one bit; I have respect for a person who can get a good sound with less-than-ideal equipment. And probably 90% of the LPs ever made, excepting the top classical labels, were recorded on equipment that engineers today wouldn't choose for its sound quality.

Anyway, for most recording with omnis, I think most engineers have developed their way to estimate the best initial microphone placement, and their expectations regarding the sound quality to aim for, based on the way traditional small-diaphragm omnis work, with definite directivity at high frequencies. It's part of the traditional idiom for listeners as well as engineers.

--best regards

[voltronic]

The upper graph on Schoeps' Web site shows only the on-axis response with the sphere attached, while the lower graph shows the response at various angles of incidence without the sphere. So you're right: these graphs don't show the effect of the sphere off-axis. It would have been better IMO if both graphs showed the response at the same set of angles--one with and one without the sphere.

Now that makes more sense.  I had thought they were showing that that the response at all angles with the sphere was more or less equal on that graph; I hadn't considered that they simply chose a graph that doesn't show them compared against one that does.

[yates7592]

Here's a link to quite a detailed paper on how the DPA APE's modify the off-axis frequency response on the 4006 and 4003 mics:
http://www.bgavinsound.com/Reference/Music/DPA%20Microphones/article%20-%20acoustic%20pressure%20equalizers.pdf

Interestingly, section 11 refers to 'increasing stereo separation in stereo-pair miking' - using near-coincident pairs of directional omni's (NOS/DIN??) with APE's to further widen the stereo image. Does anybody have any experience with this? Would you end up with a hole in the middle? I would like to try it but sounds too good to be true!

[Gutbucket]

It would be somewhat like Jecklin-disk baffled omnis with a smaller disk, with seperation at low frequencies limited by the narrow spacing and directionality comming into play a bit higher in frequency than with a standard Jecklin setup.

When I was using the DIY spherical attachments I mostly used a Decca-tree like arrangement with the left and right microphones spaced about 1 meter apart with a 90 degree angle between them (+/- 45 degrees) and a third in the center facing forward.  With the center microphone in play, I could have pointed the left and right mics 180 degrees apart, fully to the sides, without a 'hole'.  That would have ahieved an even better ambient seperation, however more important to me was orientating the left/right mics to take advantage of their on-axis presence range frequency response boost for the primary direct sound arriving from in front.

[Gutbucket]

Interestingly, section 11 refers to 'increasing stereo separation in stereo-pair miking' - using near-coincident pairs of directional omni's (NOS/DIN??) with APE's to further widen the stereo image. Does anybody have any experience with this? Would you end up with a hole in the middle? I would like to try it but sounds too good to be true!

Besides their on-axis presence boost, my reasoning for using the spheres was partly as a way of optimizing around the limitation of a more narrow spacing than I would have prefered to use- in my case a total spread of 3 feet instead of 6 feet apart using three microphones.  Using two microphones those numbers would be smaller, but I wouldn't consider 12" enough without a baffle between the microphones. It would work of course, but I don't think a NOS/DIN arrangement would provide optimal seperation for a main stereo pair using omnis with APE spheres and distance mic'ing like most are doing here, although it might be an appropriate choice for mic'ing a piano or something.

[Gutbucket]

Here's a photo I just came across while looking for something else, showing a spaced pair of the DIY APE baffled omnis oriented 180 degrees apart.  This is a center/rear-facing pair (note the drum kit in the background, which was positioned center stage).  The left/right pair was spaced 3', facing +/- 45 degrees, and is out of frame.