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January 18, 2005 to January 26, 2005

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Difference in low Hz response fig 8’s vs. cardioid mics

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Re: Balanced capsule users?

dbock is currently offline dbock

Messages: 94

Registered: April 2004

Location: Hollywood, CA

Member

Hello,

I was wondering if there were any users of Sennheiser's Balanced Capsule, found in the MKH 80 and 800. I never hear about it so I was wondering if it was another potentially great idea lost under the carpet of the fickle pro audio community?

thanks,

David Bock

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Re: Balanced capsule users?

[message #40160 is a reply to message #40145 ]Tue, 18 January 2005 10:51

Schallfeldwebel

Messages: 264

Registered: October 2004

Location: Europe

Active Member

You mean the push-pull capsule principle? As far as I have understood, the MKH 20, 30 ,40, 50 all use this push-pull system.

The MKH 80 and MKH 800 have double capsule variants, in order to create omni, cardioid, hyper cardioid etc. If the principle is alike the Braunmuhl construction of most other switchable large membrane mikes, was never clear to me.

Erik Sikkema

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Re: Balanced capsule users?

[message #40191 is a reply to message #40145 ]Tue, 18 January 2005 15:00

David Satz is currently offline David Satz

Messages: 528

Registered: April 2004

Location: Brooklyn, NY

Gold Member

David, if you have the collection of papers from the AES UK conference in 1998, entitled "The Ins & Outs of Audio"--the first paper in it, by John Willett of Sennheiser UK, describes this type of capsule design and shows some interesting measurement results on twin-tone IM tests. It's called "The Symmetrical Microphone Capsule and the Quest for the Perfect 'Acoustic Window'".

Schoeps claims to have used this type of capsule design first, in 1964, in a microphone which used RF modulation technology. They don't use that technology any more, as you certainly know. I'm not sure exactly which Schoeps model it was, but I suspect that Bernhard Vollmer could tell us.

Also I'm not sure that I would call this design "balanced" but it is, in a mechanical sense, symmetrical.

--best regards

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Re: Balanced capsule users?

[message #40395 is a reply to message #40191 ]Wed, 19 January 2005 16:04

Marik is currently offline Marik

Messages: 143

Registered: May 2004

Active Member

David Satz wrote on Tue, 18 January 2005 21:00

Dear David,

Is there any way I could get a copy of this article?

Particularly, I am fascinated with fig8 push-pull design. A. E. Robertson, in his book "Microphones", mentiones push-pull bi-directional electrostatic microphones, but no details are given--just a scketch of the principle.

What confuses me the most is that in theory, in order to get uniform frequency response of the pressure-gradient biderctional system, the diaphragm should be mass controlled, i.e. tuned to the lowest frequency of the band. It is easily done in ribbon microphones, by using corrugation, but seems impossible to get with a mylar, as it cannot be tuned low enough, or gets unstable.

This problem seems to be unique for fig8 pattern only, as cardioid can be tuned into mid-band and then resistance controlled. On the other hand, would be acoustical resistance of back plates sufficient to get enough resistance for uniform response in bi-directional system?

Am missing something, or is there any trick?

Best regards, Mark Fuksman

P.S. Klaus, I realize my question might be beyond the scope of this forum. However, I have been wondering about it for quite awhile, and would appreciate if somebody could fill with details.

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Re: Balanced capsule users?

[message #40423 is a reply to message #40395 ]

Wed, 19 January 2005 19:48

David Satz

Messages: 528

Registered: April 2004

Location: Brooklyn, NY

Gold Member

Mark, if you don't want to buy the entire collection at $28 for members--it's not one of their larger books--the individual articles can be downloaded from www.aes.org as PDFs. The fee is $5 for members, or I believe they now have a plan whereby you can buy the right to download anything you want for a year's time for a flat fee. (Wish I'd had that last year.) For non-members, an individual preprint like this can be downloaded for $20.

You can start at their preprint search page and enter the author's name (Willett)--currently the only preprint that comes up is titled, "The Symmetrical Microphone Capsule and The Quest for the Perfect 'Acoustic Window'" which looks like it's our baby--it says that it's from the AES UK conference.

The paper number is MAL-02. Its précis is: "The microphone, as the very first link in the chain, is the most important part of the recording and reproduction process. As such, it ought to be as neutral as possible, an 'acoustic window' neither adding to, nor subtracting from, the original. However, microphones can introduce noise and other non-linearities that distort the original signal. We will look (and listen) at these distortions and show how a symmetrical transducer can minimise these non-linearities and more closely approach that ideal 'acoustic window'."

I've downloaded many papers from the AES Web site and it has always worked well for me; I recommend the service.

--best regards

Re: Balanced capsule users?

[message #40461 is a reply to message #40145 ]

Thu, 20 January 2005 01:10

John Monforte

Messages: 64

Registered: January 2005

Location: Miami FL USA

Member

Hello Klauslabbers,

I have only recently found this forum and have been lurking around here a while. Now I feel there are a couple topics where I can add my $0.02.

FWIW, I am an electrical engineer who taught transducers at a university for many years.

David,

As you know, mics are part fashion and part patent (along with other things). Both have kept these mics somewhat obscure.

This forum mostly concerns itself with large diaphragm condensers (or more accurately, Branmuhl-Weber style). Perhaps 90% or more. In the world of mics, this type of capsule is less than 1% of the population. Admittedly, this is about high performance mics here. Still, in the universe of music, far outside of Pop, Rock, Country and Jazz (where LDCs are used extensively) lies the classical realm where these things find a home. There are many high performance mics that are virtually unheard of by engineers who record non-classical (and that sounds like it could be a thread).

I used the MKH20 when it came out for a magazine review. I liked it- and the concept - a great deal, but I was not thrilled with the sonics of the electronics behind the capsule. I was using (and continued to use) B&K4003s. If I were to expand my classical toolkit, I would get the fig8 version. It is the best in class.

Other engineers use them too. Notably John Eargle, a multiple Grammy winner, who extolled the virtues of the Sennheisers to me. I won't go any farther than that or I will deserve berating by Klaus for hearsay.

The idea of having the diaphragm flanked by a stator on each side (can we call them BOTH backplates?) doubles the capacitance and sensitivity. This can be used to lower "self noise" (a mild misnomer) or reduce capsule area to reduce wavelength loss improving HF directivity. Kudos.

Another benefit is the reduction of distortion due to the push-pull nature of the design. This is a typical arrangement for electrostatic loudspeakers. Unlike loudspeakers, the excursion as compared to the area is very small, so the improvement is negligible. This is a fancy way of saying the diaphragm doesn't curve much in its excursions.

I am not sure this method is applicable to B-W capsules, but I would have to consider that much more carefully.

Mark,

You make many correct statements about mass controlled operation, etc. but the problem with condensers is that they are negatively stable, unlike ribbons which are positively stable. This will need some explaining...

EXPLANATION: Imagine a small sphere (think, a marble) that lies inside a large one (say, a basketball). The marble lies at the bottom at rest. Add a force and it displaces. Remove that force and it returns to the original state. Positive stability. Now place that marble on the exact top of the basketball. When it is displaced, it displaces itself even further. Negative stability.

A charged diaphragm at the exact center of a pair of plates has equal forces acting in each direction. Displace it from the center and the nearer one will attract the diaphragm more strongly while on the other side and, well you can take it from here...

To counteract this force you apply what physicists call a mechanical restoring force. Tension. Resonance is entirely determined by mass and restoring force. As you add tension you raise resonance. Fine for the omnis and cardioids, but as you note this is not good for fig8s.

My apologies if the science is too thick here, but its not like I tossed out any math!

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Re: Balanced capsule users?

[message #40730 is a reply to message #40461 ]

Fri, 21 January 2005 15:59 Go to previous messageGo to next message

Marik

Messages: 143

Registered: May 2004

Active Member

David Satz wrote on Thu, 20 January 2005 01:48

Thank you David,

Hopefully their plan materializes--too many papers to download there.

John,

Very cool analogy!

Once, I used to service Quad electrostatic speakers and back then, learnt this problem of instability of push-pull designes. The cure was reducing polarizing voltage, or increasing the membrain tension.

John Monforte wrote on Thu, 20 January 2005 07:10

As you add tension you raise resonance. Fine for the omnis and cardioids, but as you note this is not good for fig8s.

Yes, but I am still wondering how they did it, for example, in MKH30, which AFAIK, is a single diaphragm PP fig8 capsule. Is there a special acoustical arrangements, or a knotch filter EQ in electronic circuit?

Best, Mark Fuksman

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Re: Balanced capsule users? [message #40738 is a reply to message #40730

]Fri, 21 January 2005 16:46

David Satz is currently offline David Satz

Messages: 528

Registered: April 2004

Location: Brooklyn, NY

Gold Member

Mark, you're right that the Sennheiser MKH 30 is a single-diaphragm figure-8 microphone with push-pull capsule construction. The mechanical damping is relatively low in this capsule design, so its "natural" frequency response would be rather mountain-shaped if left alone.

But the Sennheiser MKH series isn't one amplifier with a set of interchangeable capsules like the Schoeps CMC series or the Neumann KM 100; each amplifier circuit can be tailored specifically for the one and only type of capsule that ever goes onto it. So the amplifier for the MKH 30 boosts its capsule's low-frequency response (it's just a simple 6 dB/octave slope) and I honestly don't know whether they play with the top end or not.

The fact that a flat low end on a figure 8 doesn't give the same sound quality as an equally flat pressure transducer has been discussed here before--see the thread, "flat low response in fig. 8 mics?" for example.

--best regards

Re: Balanced capsule users? [message #40792 is a reply to message #40145 ]Sat, 22 January 2005 00:57

John Monforte

Messages: 64

Registered: January 2005

Location: Miami FL USA

Member

For those that have been interested enough to read down this far, it behooves us to point out the nature (physics) of a fig8 pickup - portions of which need to be included in a few other threads running around here. Stick with me through the concept discussion and I will reach some powerful conclusions at the end.

Think of an acoustic space which has pressure variations all around like the height of the ground around your house. Keep in mind that, for sound anyway, this terrain changes continually over time.

A pressure sensor merely records the sound pressure at a point in space - like the height of a given point in your backyard. It is not a stretch to realize that the pressure at a given point is affected by any sound waves passing by - thus without the benefit of math we can say that it is omnidirectional.

A velocity mic measures the pressures at two closely spaced points and registers the difference (which, over time, is a called a variation) in pressure between them. This is the "slope" of the acoustic terrain around the mic. Just like measuring the slope of your backyard, it is meaningless to measure that at a single point. We get a better idea of the slope by measuring at two points that are as far apart as we can make them. This measures the "grade" (or in the acoustic world "pressure gradient" of that point in space.

Now, if those points are far enough apart there might be wavelengths small enough (which is also to say frequencies high enough) that go undetected or poorly detected. This manifests itself as a high frequency limitation. If we bring the two points closer together we have little difference to detect at all, thus our sensitivity is poor. We want the spacing as small as possible while simultaneously being as large as possible! Engineers have to solve questions like these all the time.

Now, we can go back in the house and think about sound sources. (This discussion is avoiding math through the use of fancy hand waving and lots of analogies!)

When a sound radiates it expands too. As it expands is loses strength over a series (or a pair!) of points. Now our gradient measurement is picking up pressure differences that are a result of propagation and not just the wave's velocity (pressure slope). A point source of sound has level that changes as the inverse square of the distance, or - to avoid any sense of math even in my adjectives - the level change between a pair of points is more noticeable when they are close to the point source. Try this: Listen to your speaker from one foot away and compare that to one inch away. Repeat for 10ft and 9ft 1in. We call this PROXIMITY EFFECT.

First Big Thing:

** Proximity effect occurs in all mics that sense sound at more than one point, which is to say any mic that is not omni will have proximity effect and no manufacturer can design it in or out, or even choose more or less. **

Did you ever notice how sound sources get more trebley as you get farther away from them? And how much more pronounced that is for headphones rather than loudspeakers? This proximity stuff is frequency dependent! Chew on that for a moment, I'll wait here.

Second Big Thing:

** Any frequency response measurement for a directional mic that doesn't specify distance to a source is a waste of perfectly useful paper.**

At a large enough radius away from the spherically radiating point source the wave front appears not be be curved at all. That was Christopher Columbus' point exactly. What we now see as "plane waves" might actually be from a distant point source. Either way, plane waves do not attenuate over distance so our velocity measurement is more true and has no frequency dependent weirdness.

Big Thing Finale:

** All of this discussion has nothing to do whatsoever with transducer type. None of this can be altered through electronic manipulation because it all happened in the acoustic world before any electronics could be applied. Resistance is futile.**

Topics For Those Who Want To Prod Me Into More Didactic Soliloquies:

Not discussed here:

- The gradient measurement is not "flat" which is to say constant over frequency.

- The transduction laws of dynamic transducers are also not constant over frequency.

- Resonances caused by the unavoidable act of hanging a mass under tension warp responses too.

- And on all three counts, the frequency variations are not those dinky "+3dB at 10KHz" variety. I mean 20-40dB across the audio band!

Gotta go - brain's hurtin'...

Re: Balanced capsule users?

[message #40800 is a reply to message #40792 ]

Sat, 22 January 2005 02:54

Marik

Messages: 143

Registered: May 2004

Active Member

John Monforte wrote on Sat, 22 January 2005 06:57

Big Thing Finale:

....except of one (if did not miss a point here)--simple electornic combination of fig8 and omni will give us a cardioid pattern.

And yes, I would love to hear about three other topics you've mentioned at the end.

David Satz wrote on Fri, 21 January 2005 22:46

--best regards

Thank you David,

That's all I wanted to know. On a second thought, there is no need for HF EQ. Nature of this type of the transducer is such, that below tuning frequency it works as a resistance controlled

system (that's where it needs the EQ, you've mentioned), and above--as a mass controlled one. What happens here is that against of our intuition, the force on diaphragm in a "true" bi-directional system actually increases with rate 6db per each octave with frequency raise, in "natural way" compensating for a fall of mechanical system, and giving overall flat response.

To explain this phenomenon of doubling force per octave, I'll try to follow John's analogy. Let's say we have two points--one with elevation 500' and another 550', with distance between them 100'. If we connect these two points, we get a slope with certain angle in relation to a flat surface. Now, let's pretend the distance between these two points only 50', but their elevation is still 500' and 550' respectively. If we connect these two points now, the angle will be twice as much, and if we were to clime it, it would take us more force than in the first case. Hope it makes sense.

Best regards, Mark Fuksman

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Re: Balanced capsule users? [message #40810 is a reply to message #40800 ]

Sat, 22 January 2005 07:59

David Satz

Messages: 528

Registered: April 2004

Location: Brooklyn, NY

Gold Member

John and Mark, those were some very nice, accurate explanations in common-sense terms. My one quibble (John) is that a plane wave does decrease in strength somewhat as it progresses, simply because air motion could only be frictionless in a vacuum--which of course is a paradox. And the attenuation with distance ("adiabatic loss") isn't exactly flat with regard to frequency, plus it depends on the temperature and humidity of the air. But that's a mere footnote.

--I'd like to post something here which, of course, I will instantly take down if Klaus says it's against policy. It's an ad that Gotham Audio ran in "db" magazine in the late 1970s or perhaps the early 80s. It explains the practical effects of pressure vs. pressure gradient design more succinctly than I've seen anywhere else. I've rearranged the text in this image file; originally it was all a single column:

index.php/fa/808/0/

There's just one overgeneralization: What Temmer says about the omnidirectional setting of multi-pattern microphones is correct for dual-diaphragm capsules such as Neumann and AKG multi-pattern microphones use, but not for single-diaphragm capsules as made by Schoeps. In single-diaphragm multi-pattern microphones the omnidirectional setting is a pure pressure transducer--a "classic" omni--which does not behave like a directional transducer with regard to proximity effect or sensitivity to wind, breath, and handling noise.

I work part-time as a free-lance translator for Schoeps, so to try and balance my bias (if that's not a tad ridiculous when what I'm posting is a Neumann ad) let me mention that a couple of years ago Shure introduced the KSM 141, a studio condenser microphone with a two-pattern capsule in which the omnidirectional setting is a pure pressure transducer. The patent which used to protect the pattern control of Schoeps' multi-pattern capsules expired some years ago, so Shure was perfectly free to do this.

--best regards

* Attachment: The Truth about Patterns.JPG

(Size: 127.63KB, Downloaded 96 time(s))

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Re: velocity transducers [message #41400 is a reply to message #40792 ] Tue, 25 January 2005 16:38

recordista

Messages: 46

Registered: May 2004

Location: Reno, NV

Member

John Monforte wrote on Sat, 22 January 2005 06:57

Proximity effect occurs in all mics that sense sound at more than one point, which is to say any mic that is not omni will have proximity effect and no manufacturer can design it in or out, or even choose more or less.

How would you classify the Electro-Voice "Variable D" designs?

Quote:

Topics For Those Who Want To Prod Me Into More Didactic Soliloquies

I've enjoyed your lucid explanations thus far. More, please (maybe time for a new thread?)

Kurt Albershardt

Reno, NV

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Re: Balanced capsule users? [message #41460 is a reply to message #40145 ]Tue, 25 January 2005 22:13

John Monforte

Messages: 64

Registered: January 2005

Location: Miami FL USA

Member

Recordista,

Thanks for your kind words.

Cardioid mics (but not the Branmuhl-Weber style ones) have ports in the rear to permit a portion of the sound wave to appear at the rear of the diaphragm. If the rear was wide open you would get fig8. This is most effective at a narrow range of frequencies and deteriorates as you move away from the center of their action. Take a peek at some off-axis curves to see what I mean.

The Variable D has a long damped series of openings that act as a short distance to cancel higher frequencies and progressively longer ones for lower ones. I haven't looked deeply into the science, but it does appear that the off axis rejection is improved over a slightly wider range of frequencies.

David S.,

A microphone picking up a point source at a tremendous distance will be receiving a signal that is degraded because of the huge distance as you say. But the signals picked up by the front and back points of entry will interact without proximity in a manner identical to plane waves since there are no level differences that occur as it traverses from the front to back of the mic.

Plane waves are a theoretical construct anyway, since planes have infinite size. This is a point of departure between engineers and physicists. I once worked next door to a team of researchers who measured g factor (a physical constant) to ever increasing decimal accuracies. At stake was the very nature of the universe. Engineers need only a few decimal places and life is great, + or - 3dB. For the purposes of an engineering discussion, a plane radiator is considered one whose diameter is about 10 times its wavelength. A point source has a diameter about 1/10 the radiated wavelength.

Marik,

I owe you a clarification too.

The Big Thing Finale says that, no matter ribbon or condenser, the pattern was created acoustically and its natural limitations (proximity, sensitivity, wavelength loss, etc.) cannot be undone electronically on the resultant signal.

Microphone arrays can be used to create different pickup patterns. If we keep these signals separate (like the Soundfield B format), we can recombine them at any time and create new patterns. However, once mixed they can't be unmixed. For the fig8 pickup I was talking about, they were mixed acoustically before transduction.

Thanks for listening!

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Re: Balanced capsule users? [message #41491 is a reply to message #41460 ]Wed, 26 January 2005 01:30

recordista

Messages: 46

Registered: May 2004

Location: Reno, NV

Member

John Monforte wrote on Wed, 26 January 2005 04:13

Quote:

How would you classify the Electro-Voice "Variable D" designs?

The design is nearly immune to proximity effect.

Quote:

no matter ribbon or condenser, the pattern was created acoustically and its natural limitations (proximity, sensitivity, wavelength loss, etc.) cannot be undone electronically on the resultant signal.

Translate a three dimensional world into a simple electrical signal and something just gets lost...

Kurt Albershardt

Reno, NV

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Re: Balanced capsule users? [message #41494 is a reply to message #41460 ]Wed, 26 January 2005 01:40

Marik

Messages: 143

Registered: May 2004

Active Member

John Monforte wrote on Wed, 26 January 2005 04:13

Recordista,

Thanks for your kind words.

Variable D (D stands for distance) was invented by Wiggins. The idea behind it is ingeniously simple--instead of usual mass controlled diaphragm, to use a resistance controlled one. If it were to be used with single acoustical back-path, LF response would fall with rate 6db per each octave (see above discussion with David Satz). By using three carefully tuned paths, the "right" pressure gradient for LF, MF and HF could be maintained for a flat response.

Quote:

Marik,

I owe you a clarification too.

Thank you John,

I felt (even wrote) I was missing something.

Best regards, Mark Fuksman

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