A Newbie Asks About Omni Microphones

[Aled Hughes]

The Acoustic Pressure Equaliser -- APE -- or the sphere/ball attachment for some omni mics modifies both the frequency and polar responses, with the aim of enhancing on-axis clarity and subduing the off-axis ambience.

It does add brightness to on-axis sources through a pressure build-up effect but, more importantly, it also alters the direct/reverberation ratio, reducing the level of high frequency sound arriving off-axis. This is used, essentially, to give better stereo imaging in reverberant spaces.

Increasing the size of the sphere results in the effect working down to a lower frequency (and vice versa). Most sphere's are about 40mm, but they can range from 30-50mm.

It does a completely different thing to diffuse-field equalisation which is intended to ensure equal sensitivity and frequency response for both on and off-axis sounds.

EQ will obviously affect both on-axis and off-axis sources equally.

In contrast the near-field/diffuse-field equalisation is intended to compensate for a physical effect that occurs only with close on-axis sources.

In practice, you can boost the HF of a direct-field omni (flat for close on-axis sources) when used in the diffuse-field to restore a reasonably natural spectral balance.

Going the other way, cutting the HF of a diffuse-field omni to restore the tonal balance of a close on-axis source will also result in any off-axis spill sounding duller. Whether that's an issue depends on the circumstances, of course. A better option in this situation is simply to orient the mic so that sound from the wanted close source arrives side-on to the capsule (at 90 degrees) since this doesn't result in an on-axis pressure build-up and thus no HF peak!

Thanks Hugh.

I’ve always found my OM1s to respond well to a hefty EQ boost when placed in the diffuse field.

[Mike Stranks]

So maybe that's the answer for me... bearing in mind my financial situation, the fact I already have OM1s and the relative (now) infrequency with which I'm recording in the diffuse field... some judicious enhancement in post...

Thanks all for comments and suggestions.

[Aled Hughes]

Thinking some more about this, and struggling to get my head around a couple of things.

Wouldn’t the net effect of

The Acoustic Pressure Equaliser… does add brightness to on-axis sources through a pressure build-up effect but, more importantly, it also alters the direct/reverberation ratio, reducing the level of high frequency sound arriving off-axis.

be very similar to the HF boost built into mics such as the KM183, which must be a directional boost seeing that the mics do not exhibit it at 90 degrees?
IE, wouldn’t the KM183 and a Line Audio with APEs both show increased sensitivity to on-axis HF, and by extension therefore have attenuated off-axis HF response?

On the same point, this:

…diffuse-field equalisation which is intended to ensure equal sensitivity and frequency response for both on and off-axis sounds.

seems to me to contradict this: (regarding sources arriving 90 degree off axis)

doesn’t result in an on-axis pressure build-up and thus no HF peak!

I think I’m missing or ignoring something fundamental here…

[Hugh Robjohns]

...be very similar to the HF boost built into mics such as the KM183, which must be a directional boost seeing that the mics do not exhibit it at 90 degrees?

If APEs did the same thing there'd be no need for diffuse field omnis.

The diffuse field omni is designed to capture sounds from all directions, in a diffuse field, with a equal level and a flat overall frequency response.

An APE is designed to narrow the polar pattern -- so not equal sensitivity to all directions -- as well as introduce a slight HF boost for on-axis sources -- so not a flat overall response in the diffuse field either!

think I’m missing or ignoring something fundamental here…

I suspect it's that omni mics behave differently when close to the source, and when distant in a reverberant space. And so they are engineered to work correctly on one or other situation.

APEs do a different thing again...

[Aled Hughes]

Thanks Hugh, that’ll be it!

So the 90 degree placement ‘trick’ only works for close sources? It would make no difference whether the mics were pointing forward or upward in the diffuse field?

[Hugh Robjohns]

So the 90 degree placement ‘trick’ only works for close sources?

Correct.

It would make no difference whether the mics were pointing forward or upward in the diffuse field?

Correct again.

[Aled Hughes]

There, that’s firmly lodged!

Thanks again

[jimjazzdad]

A short tale of diffuse and near field omnis with EQ: I made a cock-up on a location recording of a violin and piano. I accidentally mixed a Schoeps MK2 capsule with a MK2H capsule in the main stereo pair (I put it down to eyesight, but maybe I am doddering too). I didn't notice anything wrong monitoring on location with cans but when I got back to the studio... Anyway, I compared the FR plots for both capsules and created an EQ curve in my DAW to boost the HF on the MK2 to make it sound like a MK2H. No one - not the producer nor the musicians - ever noticed. (I should mention that there were spot mics to help the main pair, but even soloed, the mismatched mains sounded fine after EQing)

[Drew Stephenson]

This is where having microphones with good off-axis response helps I think.

[Ariosto]

So the 90 degree placement ‘trick’ only works for close sources?

Correct.

I record a grand piano with two DPA 2006C omni mics (which are diffuse field) in the upright position very close to the strings with open lid, and this works very well. So I can recommend this technique - which I originally heard of from Hugh - some time ago.

[Sam Inglis]

MBHO / Haun in Germany make good modular small-diaphragm mics that are more affordable than Schoeps etc, and have a diffuse-field omni capsule in the range.

[Mike Stranks]

Thanks Sam!