Why you should never use multi pattern mics

[GeorgeToledo]

I'm interested in this too. Pearl microphones developed TL-4 in the early 70's. It's a very early transformerless (hence the name TL) low noise fet microphone with one amplifier for each side. It had 4mm between the membranes and according to the founder and designer Rune Rosander it didn't have any noticeable phase issues below 8K.
I don't know if it was the first of its kind but Rune was very progressive. In the 50's he developed their rectangular capsules which they still use. Schoeps CMT-20 released in 1965 is often claimed as the first solid state microphone. I have an article about Pearl TC-4 from the same year. KM84 was released in1966 and U87 in 1967.

Both TC-4 and TL-4 are great microphones. Today Pearl also have the DS70, a quad microphone with two capsules mounted on top of each other at an angle of 90 degrees.

Fairchild F22 was also released 1965, often cited as the first solid state mic as well.

 

[Marik]

Something I was thinking about reading this thread: "Ribbon" is just a shape, not necessarily an operating principle. You can make an electrostatic ("condenser") ribbon capsule, and you can also make a planar magnetic capsule. They're just both not tremendously useful for recording audio. Planar magnetic drivers, which have the operating mechanism of ribbon drivers but the plate construction of electrostatic drivers, are used in headphones sometimes, but I've never seen one used as a capsule. I've never seen an electrostatic ribbon used for anything, even though it's technically possible.

Classic corrugated ribbon is the only medium, which works as a mass controlled system (IOW, it is tuned into the lowest frequency), so it IS an operating principle. Any planars, electrostatics, etc. work as a totally different system, where the diaphragm is tuned into a midband, so to make them work correctly there needs applied acoustical resistance. That was done before--for example, Fostex printed ribbon is a planar.

The aforementioned Bruno Velotron was the very early example and was not implemented correctly. First, its ribbons were not corrugated--just flat strips. Since they were not exactly tensioned they were tuned in the lower mids. Technically, that was impossible to make them corrugated, as the ribbon would have too much excursion and would just get out of balance and get stuck to the charged plates. Even as it is had problems of popping, hitting the plates, and breaking insulation. Also, the response was like a mountain--large peak around 200Hz region with a predictable 6dB8 slope.

Best, M

 

[Marik]

350VDC at the ribbons WOW

Because the ribbons are not tensioned the spacing between stators and diaphragm is quite a bit larger than conventional condenser mics, which required much higher bias voltage. Even then the mic was notorious for the diaphragm hitting the plates and destroying insulation. If the spacing was tighter then it would've added a good amount of air load (acoustical resistance) to the diaphragm, flattening response.

Best, M

 

[retsoor3]

For M/S recording, duplicate omni to two tracks, spread to taste L/R and use a cardioid for middle This works well in the right space.

 

[Matt Nolan]

For M/S recording, duplicate omni to two tracks, spread to taste L/R

How does that even work?

 

[kingkorg]

Skip to 7:40. I promise i wasn't aware of this video before starting the thread

 

[k brown]

So funny he came out with this. In response to my e-mailing him about adapting his now-famous submini 5840 CF mic circuit for use with 6AK5 mini pentode, I built a C-37A tribute identical to the one shown below, but with an Oktava 219 capsule, polarised by grid leakage, just like the C-37A (and the Altec 'Lipstick', too).

(Incredibly generous guy - he also sent me schems of a couple of simple mic pres using the 6AK5. This was over 20 years ago, before he was quite as famous as now.)

I bought two of those Calrad DM15 crystal mics and re-jiggered them to look C-37A-like; the one in the photo has an Audio Technica 4041 capsule in it (no tube, just direct out from FET). You can't see it in the photo, but the lettering on the body reads "F O N Y". GEEK!

I've always thought the C-37A was the most quietly beautiful mic ever made. Also have a soft spot for it since it was born the same year I was!

Though the original Calrad head grille looks very similar to the Sony, it's holes are too small and too far apart, so sounds like crap! So I built a new one of mesh.

For the one with the 219 capsule, I used the 6AK5 instead of the 6AU6 because the Calrad body is a bit smaller than the Sony, and 6AK5 is only about 2/3 the length of the 6AU6.

I did use exactly the C-37A 6AU6 circuit in an Octava 319 mod (in it's original, but heavily modified body).

I eventually gave up building CF tube mics because they sound nearly identical to a properly-biased FET. Since they are doing no gain, they have no tube 'color'.

 

[Gus]

Look for the Shure article
"Unique Directional Properties of Dual-Diaphragm Microphones"

 

[ccaudle]

Look for the Shure article
"Unique Directional Properties of Dual-Diaphragm Microphones"

Also available as an AES preprint by Guy Torio, preprint 5179, and a paper on a similar topic, preprint 4800.

The version as a whitepaper from Shure is here:
Dual diaphram directional properties paper

Preprint 4800 is a similar discussion around single diaphragm mics, but not available directly from Shure.

 

[azi]

New to this thread some 6 years after the fact. Trying to grasp how the 180 degree response graph (green line) in the original post was created/measured?

 

[soliloqueen]

New to this thread some 6 years after the fact. Trying to grasp how the 180 degree response graph (green line) in the original post was created/measured?

...by turning the capsule around...?

This thread is a month old................................

 

[Marik]

Look for the Shure article
"Unique Directional Properties of Dual-Diaphragm Microphones"

Thorough studying of this paper is a must for anybody interested in microphones!

 

[azi]

...by turning the capsule around...?

This thread is a month old................................

Oh and oh, mixed up my threads. Okay, but with the capsule reversed and given the same test signal from front polar axis, wouldn't there be mechanical filtering and interference from the non energized membrane and additional phase cancelation issues? It seems you could not do an apples to apples comparison unless you could reverse the membrane, not the capsule?

 

[ccaudle]

wouldn't there be mechanical filtering and interference from the non energized membrane and additional phase cancelation issues?

That is kind of the point, you want to see what effect those items have on the frequency response. For an ideal cardioid device (not realizable in practice) there would be a large level difference, and flat with respect to frequency to sounds from the rear side. How much the real microphone varies with respect to that theoretical ideal gives you an idea of how it will sound, and what will be problematic in terms of rejection from the rear.
Although to really be sure you are seeing the frequency response of the back side you would need an anechoic environment, otherwise the front side will be picking up a lot of reflections from walls, floor, etc. I do not know what environment the original measurement in.

 

[soliloqueen]

Oh and oh, mixed up my threads. Okay, but with the capsule reversed and given the same test signal from front polar axis, wouldn't there be mechanical filtering and interference from the non energized membrane and additional phase cancelation issues? It seems you could not do an apples to apples comparison unless you could reverse the membrane, not the capsule?

the cardioid pattern is created by that phase cancellation and mechanical filtering. if there were no through holes (and rear membrane, if the capsule is designed to account for it) the capsule would, unintuitively, be omni. air pressure is a hell of a drug. the only thing flipping the membrane would do is allow it to short out against the backplate at high excursions. i can tell just from the phrasing of your question that you've made a lot of completely understandable, totally logical and also wrong assumptions about what's being measured and why. welcome to microphone design! you're going to have a blast here. no sarcasm intended.

a capsule with no vents is omni because it's a simple barometer. all it can do is detect changes in air pressure. there's no mechanism for it to know from what direction those changes are coming because everything arrives at the diaphragm at the same time. add vents to the rear, and suddenly sound from the rear arrives at the membrane at two different times! calibrate this difference carefully, and you can cause the two waves to interfere destructively when they meet at the membrane if they're coming from a certain direction, and now you have a polar pattern! in a capsule with 2 diaphragms, the filtering that comes from the diaphragm is accounted for in the design of the capsule. the 180 degree measurement is performed precisely to measure that mechanical filtering and interference because it tells us a lot about the performance of the capsule.

 

[tomas.borgstrom]

I made a post earlier in this thread that I generally prefer solid state single pattern microphones when I make true stereo recordings, except grand piano where I tend to use M269 or M49. Well I disproved myself when I recorded a Chopin record this week and I really preferred the sound of DPA 4006. So sharp and suited for the elegant compositions. Next time I won't bring the fragile and heavy Neumanns. Perhaps I'll sell them after reading this thead . Sorry a bit of topic.

 

[k brown]

For classical piano, SDCs or ribbons are the only way to fly! ; - )

 

[Icantthinkofaname]

I've been thinking about sharing this for a while now as i've never read anything about this before. Please understand that there are exceptions, and if you like using your favorite LDC in figure of eight (or omni) there is nothing wrong with that, but you might want to understand what is really going on.

I almost never use multi pattern mics. There are several things happening at the same time, so i'll try to break them down.

The first issue is something I call ''electrical polar pattern fighting the mechanical one''. The other issue is the fact that sound is picked up by two membranes spaced apart by the thickness of the backplate. So you get phasing issues.

Electrical polar pattern fighting the mechanical one.

Let's take double sided/dual membrane K67(or k47, c12) capsule as example. Now imagine only one side is active, the other is not and for all intents and purposes doesn't exist in cardioid mode. You get frequency response like this:

We are interested in the green line which is 180 degree response. Most people think of cardioids as if they reject everything coming from the rear. This couldn't be further from the truth. 180 degree response is just 5 db bellow the 0 degree response at 5K where our ears are quite sensitive. This 180 degree response is created by the sound hitting the FRONT diaphragm coming through the mechanical delay network of the capsule's backplate, and not the REAR diaphragm which is in this mode disabled.

Now imagine we activate the rear diaphragm in such manner that we form figure of eight pattern. And we do get indeed, with an ideal capsule, F8 pattern with identical responses. The sound coming from each side of the capsule is now dominated by the sound coming from both physical diaphragms. We have achieved this by applying polarization voltage, and now both diaphragms are engaged. But what happened to that green 180 degree response we had in the previous image which was created by the sound coming through the rear diaphragm and the backplate and eventually hit the front diaphragm in opposite phase? Did it disappear? Well that's the thing people don't usually think about. It is still here, masked by the electrical signal created by the rear diaphragm. It is still just -5db at 5K, and it is mixed with the electrical signal. However it is PHASE SHIFTED by the distance between the diaphragms (approximately backplate thickness)!!! Of course this is true for the sound coming from both sides, there is no front or rear diaphragm per se, the capsule is symmetrical.

The issue of two diaphragms
This is basically explained above, but to simplify it even further, by using double sided capsule you actually have two microphones facing in opposite directions, however due to the capsule nature they are always somewhat phase shifted due to diaphragm separation. If you line up two mics as close as possible for any source in front of them, why would you do it any different in this case? Ideally you wouldn't, the same principle applies. Which brings me to the point. See how simple and elegant construction of a ribbon mic is. One ribbon, symmetrical construction, the sound coming from all around is hitting the diaphragm (the ribbon) at the same time, no delay of any sorts, no backplate in between, and no ''electrical'' signal messing with the ''mechanical''.

The same really goes for omni mode, or any pattern in between.

There is also one more issue with a lot of multi pattern mics. The electrical paths from the front and back diaphragm of the capsule to the impedance converter are rarely identical. Which can make all sorts of issues, mediocre both polar and frequency responses for example. In some, quite common topologies, one side of the capsule can stay charged even though the pattern is changed, which could mean you are using some random pattern between F8 and omni without realizing, yet thinking you are in cardioid.

If I want F8 response I simply use a ribbon mic. If I want omni, I just go for omni mic which utilizes one diaphragm. There are some different, yet exotic solutions, special capsule designs that try to avoid these issues. I personally use, if I have to, dual out mics such as Townsend Labs Sphere which allow me to virtually move diaphragms closer to each other by time aligning signals. The electronic path from both sides is also identical. But again, it will never give me phase coherent signal as ribbon or omni mics.

So how would you use a ribbon mic on something like drums? Is there a specific distance that's recommended? I haven't ever used a ribbon mic, though there are a couple affordable ones I've considered picking up (like the one from Superlux).

 

[kingkorg]

I use ribbons for F8, so mostly overheads/room/ms. They can reject cymbals way better than cardioids if placed right on toms for example.

 

[soapfoot]

I've only done it a couple of times, but there's something special about a 77DX on snare. I tried it after seeing a photo of Al Jackson... some distance (6-8"/15-20cm), not as close as you'd put an SM57 or a padded KM84. It's not a regular thing for me, but it held up just fine and is "that sound" if you want that period thing.

I frequently use an M160 under snare, and it's one of my two preferred choices of hat mic (on the occasions when I use a hat mic)

Coles 4038s are of course great overheads for a certain type of thing