Acoustic Pressure Equalizers & ORFT/NOS Micing with Omni

Posted: Thu Jul 19, 2007 2:21 pm

Reading Wieslaw Woszczyk's interesting article on Acoustic Pressure Equalizers on the DPA Microphones site http://www.dpamicrophones.com/Images/DM01554.pdf, I was struck by a provocative statement on page 21:

"Stereo pair miking both for solo instruments and for entire ensembles can be more successfully accomplished with omnidirectional microphones fitted with diffractive spheres. Near-coincident arrangements can be used as well as spaced apart arrangements. In a near-coincident one, the image width and the positions and spread of sources can be controlled as with directional microphones; there is no limit to the width of image due to physical microphone separation because widening of the microphone angle produces further widening of image."

I don't own a set of APEs, but am intrigued with the idea of fitting them on my pair of DPA 4006TLs and deploying them as a main stereo pair using a near-coincident technique such as ORTF or NOS. Has anyone here tried this? I frequently work in acoustically inferior halls where the AB spaced pair technique would be inappropriate because the microphones would pick up too much of the room. Do the APEs offer a way to enjoy some of the benefits of omni micing without the drawbacks? I would love to find a way to use the DPA 4006TLs as a main pair in these types of situations.

-- James

Posted: Fri Jul 20, 2007 11:09 am

I'd be careful of getting any omnis too close together for stereo micing... It is just way too easy to get a monoish mush. This is especially true for larger ensembles.

APE spheres are awesome to have. They can do amazing things for focusing the sound of the omnis in a recording. I use 50m sphere here all the time- I purchased them for decca tree work, but I use them for lots of other stuff as well.

The only time I've ever been able to get away with a pair of omnis closer than about 2.5 feet apart is for classical guitar recording. For that, I'll arrange omnis (usually schoeps) on a stereo bar and it sounds quite good. The reason why it works is that the sound source is small and the width of the mics proportionally works like it would in a larger ensemble. Image still isn't perfect, but for a classical guitar, I find that to be less of an issue than with other instruments.

--Ben

Posted: Fri Jul 20, 2007 12:04 pm

I think it's important to delineate whether the main pair is to be used solely or if there will be augmentation mics (specifically outriggers.)

I would agree that even with the 50mm balls (in reference to the Schoeps since I don't have or use DPAs), a near-coincident pair would hardly be considered a good practice. Although, that being said, there's only one sure way to figure that out and that is to simply try it!

I've had some very wierd luck when I accidentally had omni caps in place of cardioid caps before (it's an old story - barely enough time to set up, threw up a pair of Schoeps, accidentally put up a pair of omnis instead of cardioids on a single pole, no time to monitor prior to the downbeat.... If it weren't for the outriggers, the result would have been unusable. However, in the end, it worked out just fine. I even repeated the "experience" a couple times just for giggles and it worked.

I guess what I'm trying to say is - give it a try! The cones (or balls for the schoeps) aren't that expensive and at the very least they will come in seriously handy in the future. So you can at least not have to think of it as money lost if you buy them.

J.

Posted: Fri Jul 20, 2007 3:21 pm

I know this isn't really what you were asking about, but wouldn't a Jecklin disk accomplish what you're after? I use one with my Schoeps omnis, and I really like it.

Posted: Sat Jul 21, 2007 7:02 am

It is quite interesting to look at the following graphs at the Schoeps webpage:

http://www.schoeps.de/D-2004/specs-ka.html

There is not the dramatic change in directivity you might expect, only a few dB in the 2kHz region and practically no change over 5kHz. Of course you will have a slightly better stereo image than without the balls attached if you apply ORTF or NOS setups, but only in the overtone regions. The bass and fundamental tones will be more or less monophonic (depending on the distance between the capsules and frequency). Schoeps write about KA40/50:

Quote:

Thus it can improve overall balance in terms of both level and clarity. This is not due so much to the relatively modest change in directionality, but rather to the elevated response plateau which is achieved without a lessening of bass response.

regards
Hermann

Posted: Sat Jul 21, 2007 12:15 pm

Thanks for the replies. I plan to order the 50mm APEs and will do some experimentation. The article suggests adding some absorbing material to the spere (an old foam windscreen?) to provide further attenuation at lower frequencies and even better rejection of off-axis sounds.

I'll try and compare the omnis/APEs with a cardoid pair - both in ORTF - the next time I have the chance.

As to using omni outrigers, I do this all the time with larger ensembles, but when recording a string trio, for example, I find it's overkill.

I have been using the jecklin disk for the past few months. I like the accuracy of the stereo image when using the disk, but it still doesn't provide enough rejection of the room when working in an unfavorable acoustic.

Of course, in a better world I will just go out and buy a great pair of cardioids capable of holding a candle to the 4006s!

Cheers!

-- James

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