Topic: Neumann Omni Question

[stevetoney]

OK, as a relative newcomer to the really nitty gritty details of this hobby, I don't understand what the difference, in laymans terms, between the AK30 and AK31 omni capsules are.  Can someone explain this to me in common terms.  For example, I don't understand the following terms; diffuse field equalized vs. free field equalized and free sound field vs. diffuse sound field. 

I guess the real bottom line is, which is more appropriate for live music recording?

***

AK30 Description
It is a diffuse field equalized pressure transducer with a flat frequency response up to 10khz (in the diffuse field). In the free sound field this microphone has a boost of approximately 7db at 10khz.


AK31 Description
AK 31 is a free-field equalized pressure transducer. The sensitivity in the free sound field is flat up to 20 kHz. In the diffuse sound field there is a roll-off above 5 kHz.


Thanks oh great and knowledgeable Icculus.

Steve

[stirinthesauce]

The 130 would be better suited for PA taping (IMO) as generally you are taping at a distance.  The slight HF bump will produce some directional charecteristics of the mic.  The 131 is better suited for being close to the sound source (really close), which most of the times tapers are not.  Of course if using in onstage applications, the 131's may be better.  I've owned free field omnis and used to great results.  I currently own diffuse field omnis and use those to good results as well.  Differring applications require different mics.

I'm not great at explaining mic characterstics as some so here is something I snagged from Michael Hartkopf's old page.

[Diffuse-Field (-Equalization)
The expression diffuse-field means a field of sound that consists of reflexions (reverberations, echo). It is the opposite extreme of the free-field. In fact any room and most locations outside are mixes of free-field and diffuse-field. The nearer to the source, the more the free-field conditions rule over the diffuse-field conditions and vice versa. An important set of points in the room is the radius of reverb. The radius of reverb is the set of all points where the intensity of direct sound is equal to the intensity of all reverbs and echoes. In opposition to free-field equalized microphones, diffuse-field equalized ones respect the fact that when they are addressed from the rear, their diaphragm is in their own shadow of sound and by this have a reduced sensitivity at high frequencies. So diffuse-field equalized microphones have a light treble-boost (don't confuse it with a presence-boost, which has a fall towards the high frequencies!) while free-field equalized ones are ruler flat. Diffuse-field equalized microphones are used in the distance from the source, where there is more reverb and echo than direct sound. The typical application is as ambience mic.

Free-Field (-Equalization)
The expression free-field means a field of sound without any reflexion (reverberation, echo). The sound spreads from the source into all directions and either travels to infinity or is killed at the borders of the field (anechoic chamber etc.). In the free field, there is direct sound only. A free-field equalized microphone is constructed in a way that the sound that arrives the diaphragm from the front causes a ruler flat frequency response curve. Free-field equalization ignores the effect that the dimensions of the microphone itself must be taken into account when sound addresses the mic from the rear, because in this case the diaphragm is in the sound-shadow of the microphone. Free-field equalized microphones shall be used anywhere direct sound is stronger than reverberation. i.e. inside the radius of reverb. Because main mics, such as AB, MS or OSS/Jecklin disk, must be set up inside the the radius of reverb, free-field equalized mics are the right selection for these applications. The nearer the mics at the radius of reverb the more they may have a little (!) treble boost.

more from Hartkopf on subject

Omnidirectional
Often it is a highly underestimated Polar Pattern. It has no preferred direction, which means it picks up sound from any direction with almost the same intensity. Because it is a Pressure Transducer that works behind the omni pattern, we get an extended bass response, no close-up effect, a clear and linear reproduction through the whole frequency range. The frequency response is constant in a very large angle around the microphone. Unfortunately, the larger the capsule the larger the difference between a free-field equalized element and a diffuse-field equalized one. The dimensions of the capsule itself must be observed when viewing at the sound field. In case sound comes from the rear to the mic, the diaphragm is in the sound-shadow of the whole mic (including capsule). This results in a small loss of trebles. So you have to decide whether you want to use the mic in a distance from the sound source (where is more reverb than direct sound) - e.g. as ambience mic - or inside the radius of reverb, where there is more direct sound than reverb. Diffuse field equalization results in a bit of treble boost. For distant miking, you have to select a diffuse-field equalized omni microphone, and for close miking (including AB setups, OSS/Jecklin discs, MS setups) the correct selection is a free-field equalized omni microphone. However, in fact, the more distant to the sound source, the more treble boost the mic may have.
Single-diaphragm omni capsules are constructed a as closed cylinder of which the front end is the diaphragm. Only a very tiny hole allows the atmospheric pressure to go into the capsule (otherwise it would be a barometer). Sound cannot pass the hole, so it is acoustically tight.

edited for late night brain fart.  Confused on model numbers.

[DSatz]

Free field = a fairly extreme situation in which nearly all sound reaches a microphone directly, "free" of obstacles and sound-reflecting surfaces, i.e. in an anechoic environment, or at some elevation outdoors--or else so close to the sound source(s) that the microphone picks up little if any reflected sound.

Diffuse field = the extreme opposite in which there is little if any direct sound. The angles from which sound reaches the microphone are essentially random; the path lengths (and thus the arrival times) from the sound sources to the microphone are extremely diverse. In a pure diffuse sound field there is no possibility of localizing any sound sources.

These two terms, and the scenarios they represent, are familiar to acousticians; measurement microphones are always one or the other. Neither extreme corresponds to the typical usage of omnidirectional microphones for modern stereo recording of music, however. For a variety of reasons having to do with the size of a microphone and the effect that this has on its pickup pattern at high frequencies (causing even an "omnidirectional" microphone to become distinctly directional), some kind of choice is unfortunately necessary.

Neumann offers only those two strongly defined types; most other manufacturers' omnidirectional microphones fall in between those two extremes, however, because an in-between characteristic will generally fit better with contemporary recording practice. When these two types are the only alternatives, however (as it is currently with Neumann and as it used to be with Schoeps, for example), people's preferences vary: Classical engineers tend to go for the free-field type more, while pop and rock engineers go for the diffuse-field type. This is because the free-field type would simply sound lacking in high frequencies when placed at typical stereo recording distances in a normally reverberant space. The diffuse-field type suffers from an excess of high frequencies in such placement--but where pop and rock are concerned, "too much" seems to be preferred over "not enough," while for classical (especially classical musicians, as opposed to engineers and producers) it is often the other way around.

With the small Neumanns the polar patterns of the two types are absolutely identical, so you could choose either one, make your recordings, then use an equalizer to make the result sound just as it would have sounded if you'd used the other type of mike instead. Or you could record with either type of microphone and equalize it to some point between the two extremes, which would usually sound better anyway.

--best regards

[stevetoney]

Both responses...awesome!  Thanks, that really helps and FWIW, I searched the archive and this seems to be the first such response on this subject so it's really appreciated. 

By the way...both of you truly are Icculus!!!

[silentmark]

+t's 

[DSatz]

tonedeaf, thank you (I think), but is an Icculus like a cross between Icarus and Daedalus, or is it more like a succubus doing calculus while on cannabis?

--best regards

[illconditioned]

Thanks!

Could you explain *why* a diffuse field will behave as it does?  In other words, there is some physical process that gets less high frequencies to the mic from reflected sources?  Is it just that high frequency components decay as they reflect (so the diffuse mic compensates for this with a high frequency boost)?

OK, if the high frequencies dissipate, what is the *ideal* diffuse source?  We know the ideal free field source is an anechoic chamber, what is the ideal diffuse source?

By the way, this was explained to me in another way.  Someone told me that a diffuse source is like putting the mic on an (infinite?) surface and putting that mic + surface in an anechoic chamber.

Oh yeah, suppose I've got a "standard" omni mic like a DPA4060.  Is that diffuse or near field, or some compromise?  Is this listed in the spec sheet?

Ah, shit, now I've confused myself.

  Richard

[stevetoney]

tonedeaf, thank you (I think), but is an Icculus like a cross between Icarus and Daedalus, or is it more like a succubus doing calculus while on cannabis?

--best regards

Icculus was the mythical prophet in Phish's imaginary Gamehenge saga.  This was Trey Anastasio's senior project, which he created and set to musical form when he was in college many many years ago.  It was actually quite an innovative and insightful story that is not unlike the story of the Hobbit in Lord of the Rings.  Obviously, it's a looooong story, but to make it short, there was a book...the Helping Friendly Book...that was written by the great and knowledgeable Icculus.  That means that the characters of the story were intended to 'read Icculus'.  Get it?  Read-Icculus...ridiculous!!!  Trey has that kind of sense of humor!

Anyhow, I liken the knowledge that you have to that contained in the Helping Friendly Book!!!  Thank you very much!

[F.O.Bean]

Also, what he is saying is, if you pointed the free field, say directly vertical(upwards), then it would have a slight HF bump at 5kHz regarding the sound source, right?

But the diffuse field, it doesnt matter where its pointed regarding the sound source, it will still have a HF bump, correct? which is DEF necessary from far-field recording IMO as omnis can pack a PUNCH, especially at jamband shows.....

[DSatz]

illconditioned, a diffuse sound field is built up from the reflections in a room. The sound is bouncing off the walls (literally) as well as the floor, ceiling, people, furnishings, etc., and with each bounce there is some attenuation because some of the sound energy turns into miniscule amounts of heat. The thing is, this happens very unevenly at different frequencies, with high frequencies being absorbed more than midrange frequencies in general. Even just traveling through the air, the losses are a little greater at high frequencies, but the main high-frequency losses come from absorption when sound is reflected.

Add to this the fact that an omnidirectional microphone isn't omnidirectional at these same high frequencies--if you want flat response on axis, you pay for it with rolled-off response off-axis. The standard frequency response curves which the manufacturers all print are free-field response curves, meaning that they show the microphone's frequency response for sound arriving as "plane waves" on the 0-degree axis of the microphone. Move 30 degrees to the right or left, however, and the picture changes considerably at high frequencies. That information is contained in the polar response graphs which, unfortunately, not very many people know how to read (but it's not actually hard to figure out, and HUGELY worth learning).

There is no typical environment that produces an ideally diffuse sound field in the sense that an anechoic chamber offers an ideally direct (free) sound field. You can approach it in some large Gothic cathedrals where the sound hangs around in the air for six or seven seconds after a singer has already let go of a note, but it is a theoretical construct as is the perfect anechoic chamber (real ones all have limits and problems).

I'm not very familiar with DPA's product line so give me a moment to go look. Or better yet, let me tell you what I would be looking for, and then I don't have to go look. If this is a microphone of the more or less usual diameter for a small-diaphragm condenser, then it will show an on-axis elevation of several dB (5 - 6 - 7 - 8) at around 8 - 9 - 10 - 11 kHz if it is a diffuse-field type. (For smaller microphone diameters the frequency range would be higher.) A free-field type will always have flat high frequency response on axis; in a sense no one cares about its off-axis response since essentially no sound is reaching it from off-axis anyway.

If the microphone is made to be extremely small then these distinctions will no longer matter within the audible range. That may seem like the ideal, but it's not--because such small microphones have rather high equivalent noise levels and anyway, most engineers seem to find that their recordings sound better with some directionality in the microphones at high frequencies, though I suppose that could vary with the circumstances and with people's personal tastes (he said, trying to avoid any arguments with Earthworks users).

I hope this answers your questions to some extent.

--best regards

[easy jim]

Once again many thanks DSatz for the excellent and informative posts.  +t

[stevetoney]

Pretty obvious Neumann put alot of thought into the design of the AK30 and AK31.  What's somewhat interesting is that they haven't ever modified the design of the AK50 to offer an additional option to match what the majority of music recordists desire in that capsule...namely the warmer boost on the low end of the hyper capsule.  I think most music people prefer a warmer version of the AK50.  Oh well, guess that Neumann can't make every variation to meet every desire and need of every sub-group that uses their products.

Thanks once again for such insightful input on this interesting subject.

Steve

[anhisr]

I just "fix" the AK50 in post.  I have even saved the eq so I don't have to set it every time.  Just made it look more like the AK40 cap. 

[F.O.Bean]

I just "fix" the AK50 in post.  I have even saved the eq so I don't have to set it every time.  Just made it look more like the AK40 cap. 

Now thats a GREAT idea. Care to share what settings you have it preset for? Hmmm, I need to hear more of your recordings, Jerry. I guess I could go thru that Phish you sent me. I transferred all but like 6 shows so far

Might have to get some ak50's and ak40's then. Always LOVED the ak40 sound!

[DSatz]

tonedeaf, the Neumann AK 50 is evidently a compromise between the requirements of close speech and general applications (including music recording), with the speech applications coming out somewhat ahead in the bargain (-10 dB at 50 Hz). One could assume that film and video dialog recording and high-end P.A. systems were the primary market that Neumann envisioned for this capsule. But its directionality is very consistent across the audio frequency range, and as a result, what anhisr describes is a perfectly reasonable strategy for music recording.

One of the "secrets" of making a microphone respond well to equalization is to keep its polar pattern as consistent as possible across the audio frequency range; the other is to start from basically flat frequency response, i.e. a generally neutral/natural/transparent sound quality, without much if any special "personality" or "character." Most of the capsules in Neumann's KM 100/KM 180 series have a certain "sparkle" to them as the result of their slight high-frequency enhancement, but it's not glaring--just a slight push in a glamorous-sounding direction.

It's not an emphasis that I usually prefer because I mainly record trained professional musicians who already have the good stuff in their own sound; adding a layer of simulation onto the real thing doesn't generally give a very convincing result. But in some respects that little bit of built-in EQ can make untrained voices sound more like trained ones, which certainly is a neat trick.

--best regards