AKG 451 and Ck-8 short shotgun - Question?

[Paul R Johnson]

I asked this on an audio forum and got zero response, so I wondered if the audio folk here had any info.

I was looking for a mic in my store and suddenly remembered it was in an old Zeppelin housing, but when I opened it up, there was a 451 with a CK-8 shotgun capsule, not the mic I was expecting. Anyway - I had a quick search and came across AKG's archive specs and it intrigued me for two things - the capsule was claimed to be good for handheld use on stage as the capsule is physically distant from the actual open end, reducing pops and of course virtually removing proximity effect, and a quick try shows it does indeed work like this. However - my question is on how it actually works? I'm used to interference tubes that generate the hyper/lobar response pattern short shotguns have, but this mic has a solid tube, with a 6mm (ish) slot that runs the entire length. I'd mounted it in the housing with the solid side down, slot up - but a quick experiment doesn't seem to make this that important.

So...... does anyone know how this design actually does it's job? It's clearly different from normal shotguns with slots around the circumference. The blurb doesn't explain. I've never seen another design that has a solid tube with fore/aft slot.

Interference operation is the usual mode for shotguns, but with an open slot, how does interference work? With a solid side, common sense says polar pattern would be distorted too, but the spec and polar patterns look very normal. How on earth does it work?

[John Nantz]

Hey Paul ... where's these audio guys when ya need 'em?

Just tryin' to help ya out here. did a quick search for AKG 451:
https://www.musiker-board.de/search/...51&o=relevance

Lo 'n behold, there's a whole bunch of hits! at least 5 pages worth. Just need to go to the right board ....

[Greg Miller]

Microphone design is basically magic. I can't imagine the amount of testing & adjusting involved before a final design evolves.

Here's some info on the CK-8 (down lower on the page, below the CK-1):
AKG CK 1 and CK 8

Compare this with the side slot design of the earlier EV644:
Electro-Voice Model 644

I suspect that Harry Olson may have researched similar directional mic types during his time at RCA, but alas my Olson books are at a different location so I can't give you any specific references right now.

[Pete Cofrancesco]

I could never understand why they were called shogun mics. Maybe they should have consulted a gun expert first.

[Greg Miller]

Originally Posted by Pete Cofrancesco

I could never understand why they were called shogun mics. Maybe they should have consulted a gun expert first.

Gee, surely it can't be because they have a long tube in front, that you aim at the target, as with a shotgun.

[Greg Miller]

Originally Posted by Paul R Johnson

the capsule was claimed to be good for handheld use on stage as the capsule is physically distant from the actual open end, reducing pops and of course virtually removing proximity effect, and a quick try shows it does indeed work like this. However - my question is on how it actually works? I'm used to interference tubes that generate the hyper/lobar response pattern short shotguns have, but this mic has a solid tube, with a 6mm (ish) slot that runs the entire length.

Any true interference tube design, with a long tube in front, has the capsule at the back end and some sort of slots or openings along the length of the tube. Obviously a single long slot works *if* the rest of the design was based on that. I'd guess that the inside of the slot is not entirely open into the tube, but might have some sort of acoustical resistance to "tune" everything to work just right. As I said elsewhere ... "magic." Since the capsule itself is kept at a distance from the sound source (i.e. the speaker's mouth) there's less proximity effect. The capsule isn't exposed at the head of the mic, but rather has a significant volume of air in front of it (like a mic in a zeppelin), so there's less susceptibility to plosives and wind noise.

The side opening(s) admit(s) sound waves that are somewhat out of phase with the sound coming down the tube from the front ... different phasing depending on which part of the slot the sound goes through, so it's not just one single number of degrees, but a range. The phase difference causes partial cancellation of the amplitude, thus reducing the pickup at off-axis angles. You can find numerous articles online about interference tube operation.

[Pete Cofrancesco]

Originally Posted by Greg Miller

Gee, surely it can't be because they have a long tube in front, that you aim at the target, as with a shotgun.

I take it you've never fired an actual shotgun as opposed to a rifle.

[John Nantz]

Greg - that is a microphone documentation gold mine. Never seen anything as well done as those two links. The product photography is absolutely great, the write-up about each mike is super helpful, and there are the polar patterns, wiring diagrams, and even … audio samples.

Some of the E-V mics and mic stands, with their brushed and polished metal finishes, look so good they should be on the wall unit for display.
The Electro-Voice 652 looks like it was something straight out of “War of the Worlds” movie.

Really, really nicely done!

They should keep Phil occupied and out of mischief for a little while!

[Greg Miller]

Originally Posted by Pete Cofrancesco

I take it you've never fired an actual shotgun as opposed to a rifle.

Sure I have. In both cases you aim the tube at the target.

The half-power point, where the pattern is down 3dB from on-axis, is around 90º to 100º for typical shotgun mics. They do not pick up 100% on axis and no pickup at all anywhere off axis.

An interference tube mic pattern is most directional (like a rifle) at high frequencies, certainly more like a shotgun pattern at middle frequencies, and like a firecracker at low frequencies. Of course you're not going to find any mic that admits only one air molecule directly on axis.

For that matter, the *sound* from a firearm has some amount of directional pattern related to frequency. But God help us if we revive that thread! ;-)

[Greg Miller]

Originally Posted by John Nantz

Greg - that is a microphone documentation gold mine.

Yep, one of my favorites.

Originally Posted by John Nantz

The Electro-Voice 652 looks like it was something straight out of “War of the Worlds” movie.

Beautiful styling. Too bad it rolled off at 7kHz, I'm sure that's why it was never popular.

And look at how those little plastic rings change the response. That's what I mean about mic design being "magic." (Of course Harry Olson probably found the formula to calculate stuff like that in the 1930s.) To my simplistic way of thinking, the small plane of plastic turns it into a boundary mic at higher frequencies, and boundary mics have higher acoustic gain.

[Paul R Johnson]

Thanks guys, I didn’t explain very well. I’ve seen the akg material and what I’m trying to find is an explanation of how a slot works as a function. A capsule with open access to sound fronts approaching from a wide range of angles is not an interference tube. The normal physics of how interference tubes doesn’t work for this mic. Especially confusing when you consider that without the small side slot, we would just have a microphone at the end of a tube, with a very limited ability to collect sound from anywhere other than forwards, so very, very directional. We don’t use this because the tube would be resonant and skew frequency response badly. Maybe the long slot simple breaks the resonance, but still makes the tube behave like a sealed tube? Maybe being open in that way also doesn’t unbalance the polar pattern? What I hoped for was some detail of which bit of the science is doing the work, and why it doesn’t work strangely? Is there an element of pressure gradient coupled to pressure operation, and no interference cancellation or what? Clearly it works, I’m interested in how, and why is this design no longer used? One AKG document talks about gradient and interference combination but that seems a huge simplification of what must be going on here. The fact it can hear from directions no entry point aims at hints at pressure operation, but the opposite side seems to capture very differently from the other side making it extremely odd having a deaf side to one side and a wide angle of acceptance on the other? That’s a very strange microphone. I’ll shoot some video to explain better if I get a chance today.

[Paul R Johnson]

akg-ck8 on Vimeo

Speaking into the solid side and the slotted (open side) sounds the same. A really interesting design that does appear to do everything claimed, and is a pretty forgiving microphone.

[Greg Miller]

Originally Posted by Paul R Johnson

A capsule with open access to sound fronts approaching from a wide range of angles is not an interference tube. The normal physics of how interference tubes doesn’t work for this mic.

But it seems that it *does* work, doesn't it? The only thing that would provide "open access" would be complete lack of a tube or any other enclosure in front of the transducer. For one thing, there is still a tube, albeit with a small slot. A slotted tube does *not* provide "open access," any more than a tube with a pattern of perforations would. The width of that slot (you said ~6mm) is quite small compared to the wavelength of audio frequencies, so it will have a significant effect on them. Additionally, I suspect there is some sort of acoustic resistance or diffusion behind the slot: maybe a mesh, maybe a fabric, maybe some combination or some special material(s).

Consider a point sound source far away, at roughly 90º off axis. The sound from that source is not a single beam like a laser beam; it radiates spherically once it leaves the source. The energy will hit the entire length of the slot, and a small amount of energy will enter at every point along the slot. And it then probably goes through some unknown acoustical resistance. After that, some amount of the sound from each theoretical point will go toward the transducer at the tail end of the tube. The energy from the part of the slot nearest the transducer will arrive first. The energy from the part of the slot farthest from the transducer will arrive last, after it travels along the length of the tube. The energy from intermediate parts of the slot (intermediate distances from the transducer) will arrive at the transducer at varying times. The result is that the coherent wavefront from the source will arrive at the transducer spread over some finite amount of time. Different times equals different phase relationships. So the sum of the energy at the transducer, at any given instant, is less than would be expected. That causes directional attenuation: the mic is less sensitive off axis than on axis.

Originally Posted by Paul R Johnson

Is there an element of pressure gradient coupled to pressure operation, and no interference cancellation or what?

From what I recall, an interference tube design starts with a pressure gradient capsule (at the back end of the tube). So both principles are at work.

Originally Posted by Paul R Johnson

One AKG document talks about gradient and interference combination but that seems a huge simplification of what must be going on here.

I imagine most mic descriptions intended for us laymen are huge simplifications. If you read anything by Harry Olson, there's a ton of calculus involved behind the theory. Then someone in the lab has to perform a string of experiments to refine the design and get physical reality to agree with the theory. AKG (or any other manufacturer) is not going to put all that detail into their product description, because only a small number of end users would understand it. (And other companies might be inclined to steal it.)

Originally Posted by Paul R Johnson

The fact it can hear from directions no entry point aims at hints at pressure operation, but the opposite side seems to capture very differently from the other side making it extremely odd having a deaf side to one side and a wide angle of acceptance on the other?

Sound waves (except for highest frequencies) are going to diffract around the tube and reach the slot (as your test demonstrates).

When you test the mic by talking close to the slot, the sound from your voice does not reach all parts of the slot at the same intensity or at the same time, but a distant sound source would. You're changing the distance / time / phase relationships compared to the relatively distant sound source for which it's designed. So the result is bound to be different.

Remember, too, that sound waves are subject to a lot of diffraction, moreso than light waves, so I think that affects how closely a mic design follows the design objectives.

You might want to look for the book, Elements of Acoustical Engineering by Harry Olson. It contains some basic discussion, with a lot of supporting math. So it is confusing but at the same time enlightening.

[Paul R Johnson]

Greg - thanks for this some ideas there I will have a look at. I think the thing that is counter intuitive is that sound coming in from the side appears to be equal in volume on the slots side and the solid side. Perhaps the slots on a conventional shotgun are not 'active' parts at all (which I had assumed) but are simply ways of creating access without the thing falling apart?

If it is pressure operation, that would explain the similarity between open and closed side performance if it's pressure only?

Got me intrigued this one!

[Greg Miller]

Paul,

I imagine there's a good mathematical explanation, but I am not a mathematician.

I think there are two issues, and I'm not sure which one has you stumped, or maybe both.

1.) The slot is definitely there for a reason! I believe this is a type of interference tube design. Do you agree with that? Do you understand how it works?

2.) The fact that you seem to hear the same thing whether you talk toward the side of the tube with the slot, or the side 180º away from that. I think this is mostly a matter of diffraction. I am trying to think of a good experiment to prove (or disprove) that, one that you can try without an anechoic chamber. Do you understand and agree with the diffraction concept?

Is there something else puzzling that I am missing?