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[klaus]

David, thanks for untangling the complex issue of published mic specifications, where hearing, testing and marketing intersect.

Here is the basic problem: we have a largely uneducated user community, most of whose members believe they can judge the quality of a microphone by the flatness of its response (or its low noise floor, or high overload point.) Sadly, they can't, but the market teaches that they respond to those claims....

I don't believe the user community should be held responsible for that outcome. That would be like saying: Volkswagen Diesel car owners are responsible for falsified emission tests because they want low-emission, high MPG cars.

[Kai]

Indeed, ruler-flat response for an omni is useful, and easily achieved. You may decide that flat response on-axis is what you want, or flat response to a uniform reverberant sound field, as in some sphere-mounted mics, is what you want. Both can be called "flat" but they are different. If flat response were a predictor of usefulness, we could use inexpensive electret capsules that are indeed flat, some of them to above 30 kHz, and in all directions.

There is not "ruler flat" mic out there, not even the mentioned omnis.
"Flat" in the sense a manufacturer mean's it is just his own definition.

Here's an example: The most used mic's for measurement are the Bruel&Kjaer 4133/4134 omnidirectional capsules. They are used as reference for measuring other mics.
Now look at their directivity chart as published by B&K:
You see the frequency response variations relative to the angle of sound, a variation of 14dB @ 20kHz !
(BTW: this is not the actual frequency response, but needs to be combined with the "actuator measurement" of the mic for the final result, which is again not ruler flat).

This alone show that for the normal user frequency response measurements are simply misleading, as he's not able to interpret what he sees.

Best
Kai

[David Satz]

Kai, I'm quite sure that you know this, but for the sake of the discussion: If you'd chosen a pressure capsule one-half the diameter of the one whose curves you showed, the disparities in its response for different angles of sound arrival would have begun an octave higher. The microphone would have had "truly" omnidirectional response for one further octave of its range, and the net disparity within the audible range would be distinctly less.

If one believes that an omnidirectional microphone should be "as omnidirectional as possible," that change would definitely represent an improvement. And at least in theory, this halving of size could be repeated until the capsule was 100% omnidirectional up to the limit of human hearing or even beyond. By then, of course, it would be too noisy for many professional recording applications--but let's set that aside for the moment.

More to the point, most engineers who record music with omnidirectional microphones don't really want microphones that are completely non-directional at the highest frequencies. Such microphones might be seen as "theoretically perfect" in a certain way, but the ones that exist aren't generally considered any better-sounding on that account. Manufacturers never see much demand for them for music recording purposes.

On the contrary: Rolling off the high-frequency content for reflected sound is integral to the way in which omni mikes are nearly always used when recording music. There's even a fair amount of professional music recording practice--with many wonderful-sounding recordings over many decades to show for it--that's based on increasing the high-frequency directivity of pressure microphones even further, and provoking that directivity increase to begin at frequencies lower than where would it begin otherwise. This is most often done by embedding the transducer in the surface of a sphere; the Neumann M 50 is the best-known example of this technique. As a side benefit, the microphone then gets something of a "presence boost" in the front hemisphere, prompting the use of even greater miking distances--which (in a good acoustical environment) improves the "blend" without giving up "focus."

Those benefits can be gained only by setting aside any misplaced, Platonic idealism that one may have about omnidirectional microphones. When people have an impulse toward that kind of on-paper perfectionism, hooray--but it's important to realize when that impulse is beneficial, vs. when it would get in the way of making good recordings.

--best regards

[klaus]

Of the things that impress me most about astrophysicist Neil Degrasse Tyson is his ability to explain complex scientific phenomena in terms I can understand.

Before this thread drifts too far off into opposite direction, could someone please channel Tyson and condense the last two posts so that the average reader of this forum can understand what is argued here?

Thanks,
KH

[David Satz]

OK, I'll try:

(1) Kai said that no pressure transducer of typical size (e.g. with a 1/2" diameter capsule or larger) can be truly omnidirectional at high frequencies. Off-axis, it will always have some high-frequency rolloff relative to its on-axis response.

(2) I agreed, but said that this disparity between on-axis and off-axis high-frequency response can be lessened, and the frequency at which it begins can be moved upward, by making the transducer smaller. If you take that far enough, the on-vs.-off-axis response disparity can even be moved up out of the audible range entirely--though doing so also brings a noise problem that can be significant for some types of recording.

(3) While it might be conceptually attractive to use microphones that are perfectly omnidirectional all the way up to 20 kHz, and such microphones are available, most engineers don't seem to prefer them for most music recording (even in situations where their higher noise levels are acceptable). Rather, a high-frequency rolloff at the sides and rear of the microphone seems to be generally perceived as an actual advantage for most music recording. And there has long been a special demand for pressure microphones in which the narrowing of the pickup pattern begins even lower than the size of the diaphragm alone would dictate--e.g. with noticeable directivity even in the upper midrange--which is even farther from the ideal of a "perfectly omnidirectional omni."

(Does that help?)

[klaus]

Yes.  Very much. Keep at it!

[soapfoot]

Mr. Satz also pointed out that many of the technologies that have become extremely desirable for certain applications-- like the Perspex sphere in the M50 (and similar spheres that are add-on accessories to other omni mics)-- are specifically designed to increase directional response at high frequencies, technically moving the mic even further away from textbook "omni" response than it otherwise would be. This can be subjectively desirable in some applications, such as ensemble recording in the diffuse field, where it can be perceived to increase "reach," or emphasize the source itself above the reverberant sound

[Kai]

...add-on accessories to other omni mics)-- are specifically designed to increase directional response at high frequencies...

There are certain advantages omni's have, e.g better low bass response in the mid and far field, that no cardioid or figure of eight mic can achieve and the lack of proximity effect in the nearfield.
So it might be desirable to use an omni, but still maintain a certain amount of directivity in the high mid and treble range. This is what those (mostly globe-form) accessories are made for.
They usually boost the treble too.

There is another fundamental difference between omni and cardioid/eights mics:
omnis are recording air pressure, cardioid/eights air movement.
If you look at room resonances as standing waves, where the cardioid/eight sees a maximum, the  omni sees a zero and vice versa.
So if one has problems with rome modes it might help to switch the mic type.

BTW: there is a 1/2" omni with almost perfect omnidirectivity up to 20 kHz:
The AKG CK5 CK22 capsule for the C451 system.
I doesn't sound very good for recording IMO, I think AKG build it for measurement purposes mainly.

Regards
Kai

[soapfoot]

There is another fundamental difference between omni and cardioid/eights mics:
omnis are recording air pressure, cardioid/eights air movement.
If you look at room resonances as standing waves, where the cardioid/eight sees a maximum, the  omni sees a zero and vice versa.
So if one has problems with rome modes it might help to switch the mic type.

VERY important point here; one I had never considered. Thanks for that, Kai.

[Jim Williams]

BTW: there is a 1/2" omni with almost perfect omnidirectivity up to 20 kHz:
The AKG CK5 capsule for the C451 system.
I doesn't sound very good for recording IMO, I think AKG build it for measurement purposes mainly.
Regards
Kai

AKG also made the CK-22, an omni with non-directional characteristics. It is a side slot omni with a metal coverered capsule front, rather unusual looking. The signal enters the slots and hits the rear of the capsule, not the front. It is exceptional for pipe organ recordings.

[Kai]

AKG also made the CK-22, an omni with non-directional characteristics.

This is the one I meant, my rememberance had let me down.

Funny, I have some of those and recording the ambience sound of a pipe organ in an all marble church in Carrara, Italy is the only thing I ever found this ones useful.

Even as a measurement mic it's not very impressive, as it's not as linear in the upper range as I'm used to from my Bruel & Kjaer capsules.

BTW: another omni I'd like to mention is the B&K / DPA 4004 and 4007. They are smaller then usual, so directivity is appearing higher then with 1/2" capsules and is well controlled.
They are noisier then bigger ones, but very useful for high level sounds.

Regards
Kai