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I ask this trying to get words to what I have experienced and have seen as standard practice but without having words to back it up.

What are the differences between big and small membrane microphones?
I often use small membrane for room sound capture and big membrane to more near sound source recording, but would like to know if the physical difference makes on of them better for room ambiance and the other better for near mic recording.

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2 Answers

It's true that LDCMs are more sensitive, but that's in practise not such a big concern – good SDCMs already have plenty of headroom, usually the noise floor is well below any ambient noise even for chamber music room applications. Of course, the diaphragm mustn't be too small, cheap mics with less than a centimetre usually won't do. But they still beat anything you could practically achieve with dynamic microphones. The commercially-available microphones with the highest sensitivity are actually HF-biased small diaphragm mics such as the Sennheiser MKH 40 P48.

One obvious difference is a mechanical aspect: condenser mics are always more vulnarable than dynamic mics, but LD ones are particularly fragile. Combined with their clunky heaviness, this already makes me hesitate to use them in any live setting.

But most important is the sound. I like SDC mics a lot because they cover the entire spectrum quite linearly, behave very predictable, have excellent phase response and can be aligned any way you want. LDCMs usually have already quite a shaped frequency response. Nothing dramatical and often it sounds just great, but if you like to start with all-linear it's not really nice. The transient and phase characteristics are still very Dirac-like – always much cleaner than dynamic mics – yet for percussive instruments and even guitars I sometimes feel it already lacks some of the precision that SDCMs offer.(Though actually I doubt the scientific significance of that observation, because even studio headphones can't really hope to match the phase response of a LDCM.)

Where the diaphragm size starts to matter really a lot is when doing close-field recording, like most vocal recording nowadays. With SD microphones or omnidirectional microphones, distance doesn't make to much of a difference sound-wise, you just get a higher amount of direct signal compared to the room components. But with the commonly used large-diaphragm cardioid microphones, there's quite a strong proximity effect; basically you get a natural bass boost without making the response muddy. This is highly sought-after for vocal recording and can sometimes be benefitial for drums etc., but most of the time it just adds frequency components you don't need and don't really want. When recording guitars with LDCMs, I generally leave a bit of space so the proximity effect won't be too pronounced. Given good room acoustics, this can sound very nice, but rather more often I opt for SDCMs and close miking, resulting in a rich yet clear and direct sound. You can still add room mics, internal mics, perhaps even piezo pickups or contact microphones for extra sound components, their influence will be much more controllable in the final mix.

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The main difference is Sensitivity (and Noise accordingly).

An LDC(Large Diaphragm Condenser) is more sensitive than an SDC(Small Diaphragm Condenser), and also tends to generate a higher output voltage, given the same input SPL.

Why the LDC is more sensitive?

Remember that a condenser mic is made of a conductive diaphragm next to a conductive backplate. Those parts are charged with a bias voltage across them, forming a capacitor.

When sound waves vibrate the diaphragm in and out, the capacitance varies in step with the sound waves, which in turn generates a signal voltage that varies in step with the sound waves.

The changes in capacitance due to the vibration are bigger for a large, high-capacitance diaphragm than for a small diaphragm, so the output signal voltage is higher for a large diaphragm. The greater diaphragm displacement in the LDC results in a higher signal voltage.

EDIT: Most LDC directional mics have a deeper low-frequency response than SDC directional mics. That’s because the resonance frequency of the diaphragm is lower in the LDC due to the diaphragm’s higher mass. So if you want to capture a deep tone (as from tom toms or a vocal), you might make an LDC your first choice. On the other hand, the smaller mass of an SDC’s diaphragm helps it respond better to extreme high frequencies (as with cymbals).

On the same site you can see an comparison graph of 2 mics (SDC and LDC). As you can see, there are some differences but nothing substantial.

The published frequency responses of a Neumann U 87 Ai, an LDC (above), and a Neumann KM 184 A, an SDC.

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Thank you Eugene for the answer and link. Can the SDC be more sensitive/effective in room capture because the diaphragm is smaller and can move/vibrate easyer (if it's possible to answer without specific mic model) ? –  Sergio Aug 8 '13 at 9:48
    
@Sergio I have edited my answer. –  Eugene S Aug 8 '13 at 11:09
    
I read the article, it was interesting as I wrote before. I think the question I asked in my comment it too broad, it depends on mic model, room and sound being recorded. –  Sergio Aug 8 '13 at 11:12
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"The changes in capacitance due to the vibration are bigger for a large, high-capacitance diaphragm than for a small diaphragm, so the output signal voltage is higher for a large diaphragm" – isn't quite right actually. It's the relative change in capacitance that matters. That is itself a bit higher for LDCMs, but more important is the "bigger sail" effect, and the point that a larger capacitance means lower impedance which means the preamplifier can have higher load, which usually means less noise. –  leftaroundabout Aug 8 '13 at 12:03
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there are some differences but something substantial: Did you mean nothing substantial here? –  chirlu Aug 9 '13 at 7:41
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