How does diaphragm size/polar pattern relate to mic applications?

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do you like the sound of the TLM-103 for vocals, which is what i assume you use it for? how much different does it sound than the Ui 87? keep preaching about the response of the mic, cuz i'm interested in how an unflat response is considered good. since i hear crap about how you should always go for the flat response.

~the turtle
 
seriousturtle said:
do you like the sound of the TLM-103 for vocals, which is what i assume you use it for? how much different does it sound than the Ui 87? keep preaching about the response of the mic, cuz i'm interested in how an unflat response is considered good. since i hear crap about how you should always go for the flat response.
~the turtle
Wow, difficult to answer, but I'll try.

I've used the 103 for vocals, acoustic guitar, mandolin, fiddle, and a host of other instruments. It's a very clear sounding mic with a nice mid range warmth, and no shrillness. I'm not a big fan of the U87 but a lot of people love them.

Finally, "flat" is hard to define. The TLM103 is flatter than a U87, but that's not the point. You want to go for the most flattering mic for a particular instrument, but it must be a mic that doesn't add unpleasant coloration.

The flattest mics are/were made by B&K for test measurements and they're ruler flat (literally) usually from about 10 Hz to around 30 or 40 kHz. They are also pretty boring as mics for recording most music.
 
Part 3 (or 4 or whatever)

Well, I haven't heard from David Satz as far as permission to reprint his post here, but I don't think he'll mind (since he's a nice guy), so here it is:

RockyRoad wrote:

Could some kind person explain to me how the physics of these things work, and how sound from behind an omni mic such as the KM183 can get around the metal side casing and into the mic.

Sorry for the dumb questions, but I'd like to know why things arent as they seem on the surface.


"Why are things not the way they seem?" is a question that I _so_ wish people would ask more often than they do. Most folks seem to stop noticing that things aren't the way they seem, and start behaving as if that appearances are all that matter. To me that's the essence of that form of spiritual death which we in this society call "adulthood." It's why I believe that only children should be allowed to vote or own property--but failing that, there should be a law (or better yet, a general agreement) that grown-ups ought to answer all honest questions honestly. Then maybe we would not be such a culture of deception and self-deception, and people would retain their ability to notice things that don't make sense.

The replies from Sean and Scott are spot on, but I'd like to try to help you visualize what these two types of microphone are doing. Again, the relevant categories are "pressure transducer" (basically omnidirectional) and "pressure gradient transducer" (basically figure-8, but by using dual diaphragms and other tricks, any other first-order directional pattern can be synthesized including cardioid and super- or hypercardioid).

The model of a pressure transducer is a barometer. It measures air pressure in the space around it. The simplest, grade-school science barometer is a sealed tin can with air in it. The lid of the can will flex in proportion to air pressure changes in the room around it; you can attach a stick to the lid, and calibrate the stick's motions in terms of whatever units of air pressure you want to use (inches of mercury or the standard metric unit, which is "bars").

The thing is, the can will get squeezed by increasing air pressure or it will expand in times of low air pressure, regardless of which way you "aim" it. In fact the concept of "aiming" a barometer doesn't really exist because it's integrating and responding to a phenomenon that is all around it. You just set it up in whatever physical orientation is convenient for you, and it works.

You could think of the barometric pressure in a daily weather report as being the response of the barometer at 0.000011574 Hz if you want (one cycle per day). Essentially a barometer is a microphone with response down to DC. And that is a real-world characteristic of pressure transducers: their low-frequency response can be extended as far down as you like. Most pressure microphones have some small vent built in to prevent them from bursting when transported by air, but they can very well be dead flat to below 1 Hz or 5 Hz, certainly to any audible frequency.

OK. So the pressure transducer works precisely _because_ only one side of the diaphragm (the lid of the can) is exposed to the air pressure that is to be recorded; the air on the other side of the diaphragm is a constant mass, and the diaphragm flexes in order to equalize the pressure on both its sides.

The other major category of transducer is pressure-gradient, which is a fancy way of saying that its diaphragm is exposed to the sound field both on the front and the back, so it responds to the difference between the pressure that exists on the front and the pressure on the back. If the pressure presented on both sides at a given moment is identical, there is no net motion and no output. If the pressure on the front is greater than the pressure on the back, the diaphragm will move toward its backplate (assuming a condenser microphone). If the opposite is true, the diaphragm will move outwards, away from the backplate.

The thing is, if you just hang a microphone diaphragm out in space, it will be pushed around by wind or by air currents of any kind (including if you just blow on it) but it won't pick up much in the audio frequency band because it's a thin element and the pressure from sound waves will tend to be identical on both sides of the diaphragm, at least until you get up to the high frequencies (which we'll talk about some other day), and when the pressure is the same on both sides of the membrane there is no net movement and no output. But before I explain why this type of arrangement picks up sound at all, let's observe that we've actually encountered something that is true of pressure gradient microphones generally, which is that they are much more sensitive to wind, breath noise and "popping" of consonants in vocal pickup than their omnidirectional counterparts are (when the omnis are pressure transducers).

The trick which makes a pressure-gradient arrangement work for recording sound is that the sound reaching the back of the membrane is delayed momentarily, by setting up a delay chamber in between the back vents of the microphone and the back of the diaphragm. If you can make the pathway for sound even just a tiny fraction of an inch longer before the sound reaches the rear of the diaphragm, then you will cause a phase shift between the sound reaching the front and the sound reaching the back. That phase shift will be different at different frequencies, of course, so there will really be only one frequency (plus its exact integer multiples) at which a maximum of difference in pressure will result between the front and back of the diaphragm. At that frequency the resulting microphone will have its highest sensitivity to sound. But if you arrange things so that this frequency occurs somewhere other than at the very top or the very bottom of the audio range, you can do other tricks with damping and filtering so as to flatten the overall response.

The thing is, this more complicated type of microphone is also sensitive to the direction from which sound is arriving, because if sound is arriving from in front, it will strike the front of the diaphragm immediately, then when it reaches the rear input ports it will pass through the acoustic delay chamber and eventually reach the back of the diaphragm--so there will be a continually varying difference in the air pressure on the two sides of the diaphragm, and that's what moves it and produces a signal. But if the sound is coming from behind the microphone, it will reach the back inlets first, and pass through the delay chamber at the same rate of speed as the original wave is traveling outside the microphone; by the time both waves reach the two sides of the diaphragm, they will be in phase with one another and the result is no net motion of the diaphragm. (That's if the microphone is a single-diaphragm cardioid.)

That should be enough to establish a basic viewpoint, I hope. (End of David Satz' post)

And we'll now head into "when and why to use what, and how" when we do the next posting on this.
 
Whew! That's some pretty heavy stuff....it'll take a while to digest. This was my second pass through, and I have a question: if cardiod condensers only receive the sound wave from the front of the diaphragm, why are they all have "vents" (don't know if that's the correct term or not) behind the diaphragm? Wouldn't that be superfluous? Maybe I missed something. Or, maybe it's too damn late for me to be trying to comprehend this stuff...I'll try again in the morning.

Great post though. I hope I'm not too dense to get it.
 
Duh...

Harvey Gerst said:
Read the very last paragraph again. Cardioids work because they have vents that let sound into the back side of the diaphragm.

That's what I get for reading something complicated at 2:14 A.M. Okay, in a minuteI'm gonna start to be afraid to ask any more questions since I'm 0 for 2 so far, but until then I'll assume that we're working under the "there are no dumb questions" mindset and plunge ahead. Let me try again, and please enlighten me (again) if I'm wrong. So far, I understand that:

Pressure transducer=omnidirectional

Pressure gradient = condenser patterns such as figure 8, cardioid, hypercardioid etc.....

If I'm understanding this correctly, condenser mics need to be powered with "phantom power" because the pressure that they are receiving on the front of the diaphragm is balanced by pressure on the back of the diaphragm. The difference in pressure - caused by a "phase delay" that can be measured in microseconds - between the front and the back of the diaphragm is very small, and for this reason, the signal needs to be "amplified" by an electric charge.

Further, the directional "pickup pattern" is determined by the design of the diaphragm capsule - more to the point, the pattern is determined by the amount of "delay" engineered in to the back of the capsule. Because if an equal amount of pressure reaches both the front and the back of the diaphragm at the same time, it won't move at all and there will be no sound picked up from the direction that caused this to happen. But sounds coming from any direction that causes the diaphragm to have more pressure on one side or the other will be picked up because they're slightly "out of phase".

Am I getting any closer? I have some other questions but I'll hold them until I feel like I undstand this issue better

Sorry if I seem dense, I'm just trying to fully understand some of the basic concepts before you move on to the REALLY DEEP stuff. Could you give a couple of examples of some common industry standard mics which relate to the pressure transducer vs. gradient schism?

Thanks for your patience. I'll get it eventually (I hope!). Believe it or not, I am trying. :cool:

Chris
 
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By George, you've almost got it!! Forget the condensor/phantom power part and you've got it.

A pressure gradient mic depends on delays getting to the back of the diaphragm, whether it's a ribbon mic, a dynamic moving coil mic, or a condensor mic.

A condensor mic works by the difference in voltage between the back plate and the diaphragm. The voltage can be either a permanent pre-charged voltage (an electret), or a capsule that has 48 volts across one side or more (some B&K mics use over 100 volts to charge the capsule). The phantom power is only one way to get a condenson mic to work - it has nothing to do with the patterns.

Other than that, you've got it!!! The delay from sound hitting the back side of the diaphragm of any mic results in the different patterns. If the back side of the diaphragm is sealed, it's strictly a pressure mic, and it's omni - always!

Very good!!!
 
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You also asked for some examples of different patterns:

Pressure (omni) - all calibration mics, Earthworks.
Cardioid - Shure SM-57, Neumann TLM103
Hypercardioid - Beyer M201, AT25
Figure 8 (Bidirectional) RCA 44BX

Condeser mics which use two back to back diaphragms can simulate several patterns by electronically combining the two diaphragms in different configurations (e.g., combining a figure 8 pattern and and an omni pattern results in a cardioid pattern).
 
Harvey Gerst said:
By George, you've almost got it!! Forget the condensor/phantom power part and you've got it.

Consider it forgotten.

A pressure gradient mic depends on delays getting to the back of the diaphragm, wheter it's a ribbon mic, a dynamic moving coil mic, or a condensor mic.


Two things:

a) I don't think I understand the difference between "Dynamic" and "Condensor" mic designs other than the fact that Condensor mics are powered and Dynamics are not. I have noticed that dynamics are often used for applications where the mic is going to be exposed to really high SPL's - like on snare drums played by musclebound rockers using tree trunks for sticks (ex - SM57), kick drums played by same (AT? 25 - the little stubby one), and diva singers (that was meant as a non gender-specific term BTW) belting out their dramatic sh*t with the mic halfway down their throat during live shows (SM58). I think I must have been mistakenly confusing "Dynamic" with what you describe as "Pressure Transducer". Until now, I always thought of condensor mics as being much more of an "ambient" sounding mic.

b) I just figured out during your last post that the word "Condensor" is spelled with an "or" at the end instead of an "er". DOH! Better late than never.


A condensor mic works by the difference in voltage between the back plate and the diaphragm. The voltage can be either a permanent pre-charged voltage (an electret), or a capsule that has 48 volts across one side or more (some B&K mics use over 100 volts to charge the capsule). The phantom power is only one way to get a condenson mic to work - it has nothing to do with the patterns.



Gotcha.


Other than that, you've got it!!! The delay from sound hitting the back side of the diaphragm of any mic results in the different patterns. If the back side of the diaphragm is sealed, it's strictly a pressure mic, and it's omni - always!

I think I get it. But then, how does an omnidirectional condensor work? And what is a common type of pressure mic that I can think of to try to visualize the difference between a pressure transducer and a pressure gradient mic? If I'm understanding you so far, most of the mics used in the recording industry are all pressure gradient mics, right?

Anybody else still following this thread, or have I become a class of one? Speak up guys! I don't want to be the only one asking dumb questions around here. ;)
 
Whoops, you answered my last question while I was posting.

Thanks.

P.S. - This is totally off topic, and I'm only including it here because it would be a total waste of bandwidth as an entire thread, but does anybody know how I can change my username to include my entire last name? I tried when I first registered by the last letter got chopped off, so I abbreviated.
 
We're here

Chris F said:


Anybody else still following this thread, or have I become a class of one? Speak up guys! I don't want to be the only one asking dumb questions around here. ;)

Hi Chris,

I just started following this thread, but I have a feeling that you are not at all alone in your interest here. ;) I am certainly enjoying learning something at once basic and new. Can you believe that we use mics ALL the time and yet might have no idea how they do what they do?

Also, thanks for being brave and asking those "dumb" questions- your q's are helping me get up to speed with the thread. I'd throw in my dumb ones, too, but too many questions might make it more difficult for Harvey to get where he wants to go.

Take care,
Chris
 
Yub, still watchin...

I'm especially interested in how B&K made a microphone with flat response. I'd imagine it's a mix of various patterns and electrical tricks... Also, how much did it cost and was it a big deal when they announced it?

Also, thanks, Harvey. Cool thread.
 
Still here and on the edge of my chair....

BTW Chris, you're questions are FAR from dumb :)


Keep up the excellent work guys

-tkr
 
Kelly Holdridge said:
I'm especially interested in how B&K made a microphone with flat response. I'd imagine it's a mix of various patterns and electrical tricks... Also, how much did it cost and was it a big deal when they announced it?

Also, thanks, Harvey. Cool thread.
Brüel & Kjær has been making measurement mics for about 50 years now. They're omnidirectional, and flat (within a fer tenths of a dB) from about 5Hz to 40kHz, although the have some models that are flat down to 1Hz and other models that are flat to about 140kHz. Every mic manufacturers uses B&K to see what their own mics are doing; the test is very simple:

You point the B&K mic and the mic you wanna test at any sound source and record the two response curves. You subtract the B&K results from the mic under test, and any differences from the B&K response - well, that's the your mic's response curve.

They made two basic types of omni test mics - one for pressure field (on axis) and one for diffuse field (90 degrees off axis). Their DPA web site (DPA is their name for their studio type mics) contains a whole bunch of good, objective info about the differences between small and large diaphragm mics, but it's pretty techie oriented. (Bottom line: small diaphragm mics have higher noise, flatter response, greater dynamic range, large diaphragm mics have higher output, lower noise, and less dynamic range and frequency response.)

Dick Rosmini in California was a big champion of using B&K test mics for recording and we usta have long arguments about it, since I found them kinda boring.
 
not just boring,

I had a love affair with using Earthworks omnis for a while, then I got over it. I still love them for recording acoustic guitar, overheads and sometimes as ambient mics. But the IDEA of very flat, full freq omni is cooler than the reality. If I was doing more classical or location sessions instead of Rawk sessions, maybe I'd use them more. Flat is not necessarily the best IMHO.


Tom Cram
dbx Senior Technical Support
(801) 568-7530
tcram@dbxpro.com
 
The best recorded piano sound I've ever heard was on a couple of Lynne Arriale trio records from the 90's, and was recorded using a pair of Earthworks omni's.... I don't know the model number, but they were tapered down to about pencil diameter at the business end. Fantastic sound, but I don't know how they processed it. The beauty of the sound also had a lot to do with her playing, which is stellar, especially as regards tone.

I have heard that those mics do not color the sound in any way. This seems both intriguing and frightening at the same time, since crystal clear sound exposes all of the warts on both instrument and player. My Baldwin grand is a decent piano, but it has plenty of warts (the doctor wants about $3000 to remove them, and I've already spent about $1500). If I understand the meaning of "omnidirectional" correctly, it means that when you use them you had better use them with the intention of picking up every sound in the room, which means either a very controlled or very forgiving environment would be needed for best results. How many home reccer's can afford THAT luxury?

On the other hand, that piano sound is the stuff that dreams are made of....
 
Let's review what we've learned so far.

Okay, no pop quiz today, but let's review some of the stuff we've gone thru so far. For most applications, the 3 basic mic designs are:
1. Condenser mics
2. Dynamic (moving coil)
3. Ribbon mics (a special class of dynamic mics)

The basic Polar patterns are:

1. Omni- directional (pressure)
2. Uni-directional (cardioid)
3. Bi-directional (figure 8)
3. Hyper-cardioid

True omni-directional mics have a sealed back chamber and only allow sound to hit the front of the diaphragm. The other polar patterns are created by using "pressure gradient" techniques to delay and let some of the sound hit the back of the diaphragm.

Condenser mics can be made in small (1/2" or smaller), medium (5/8" to 7/8"), and large diaphragm (!" and larger) sizes. Small diaphragm condensor mics have these advantages:

1. Flatter, extended frequency response
2. Higher spl levels
3. Better off-axis response
4. Greater accuracy

They have these disadvantages:

1. Lower output levels
2. Higher self-noise

Large diaphragm condensor mics have these advantages:

1. Higher output levels
2. Lower self-noise

They have these disadvantages:

1. Poorer limited frequency response
2. Lower spl levels
3. Uneven off-axis response
4. Less accuracy

However, some of the resonances in a large diaphragm condensor mic can be very pleasing and musical, and can often compliment the voice and some instruments very well.

"Pure" pressure mics do not have proximity effect (bass buildup as you get closer) - all pressure gradient mics DO have proximity effect (dual diaphragm condensor omnis have the least, then cardioids, then hypercardioids, then figure 8, which has the most proximity effect).

You would use small diaphragm condensor omnis where you want the greatest accuracy or in high level situations where self-noise isn't a factor. Large diaphragm condensor mics are better used for quiet sources, or where you want a particular type of complimentary coloration.

We'll get into choice of mic, patterns and placements in the next installment.
 
i have been reading this forum passively so far, but decided to register to tell you that this thread is absolute killer. the "mikrofonbuch" from the schoeps site is one of the best resources i've ever found, and your explications are extemely helpful. Since this thread started i anxiously have been checking this forum about twice ore more times everyday, waiting for class to start (c:
thank you very much
harald kuhn
 
Harvey,

your last post summed things up admirably as far as I am concerned. I am eagerly awaiting the next installment of "Microphones 101".

Torpedo tubes flooded and ready, Cap'n....fire when ready.
 
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