In audio circles, it is an article of faith that you cannot judge a microphones by its specs or frequency response graphs, but you have to hear it - mics with similar technical specs can (and do) sound quite different.
And that faith is well placed, because it's entirely true.
Two mics with very similar response frequency and polar pattern charts can indeed sound quite different. Sometimes people exagerate these difference a bit, but they are actually verifiably there, in the sense that the two mics can picked up in a blind test with relative ease (which is the only sense that matters from an audio perspective).
It's also well known that a given mic may work very well on a specific source (say, the voice of a specific performer or certain instruments) and not well at all on another, and a different mic may work exactly in the opposite way, even if the two belong to the same class (say, two LDC).
Certain renowned microphones, like the Neumann U87 and the humble SM57, are considered jack of all trades and prized for that, because they produce a mixable sound (if not a spectacular one) in most situations.
Now, in general people take this gospel and run with it, which is well and good because in the end all that matters is the sound. But we are a bit more curious.
The question lingers: why does that happen? How can it be that the technical specs we see do not capture the essence of the mic?
Or, in other words, why technically similar mics sound different?
Let's have a look!
Class differences
First of all, let's go quickly thru the usual stuff, that you probably know already.
The simplest and most well known distinction is the class of transducing technology.
Dynamic mics sound different to condenser mics, and within dynamic mics, moving-coil mics sound different to ribbon mics.
That's because, in general terms different transducing technologies are physically capable of capturing only certain amounts of energy, and do so with different degrees of efficiency.
That's due to mass (for example, a moving coil assembly in a handheld dynamic microphone is usually heavier than the thin diaphragm of a condenser), physical construction (for example a moving coil assembly vs. a ribbon motor) or even the basic way that air produces a voltage change (i.e. an audio signal).. for exemple piezo transducers use material that generates a voltage differential under pressure - including air pressure, aka sound.
In particular the heavier arrangements of dynamic mics will have more inertia and move less easily (or not move at all) for to weaker sound waves. Which means very quiet sounds or high frequencies. Which is why we can say that, in average, dynamic mics will capture less highs and less detail (that is, the lower-energy components of the air waves).
Of course it's always possible to find the odd microphone engineered to overcome the average characteristics of its class, but in broad terms these characteristics translate to differences in the microphone's ability to capture various frequency bands.
You can find a ton of Internet articles on the subject, so I won't go into any further details here.
Size matters!
Again, it's probably not big news for you, but with the diaphragm of condenser microphones, size matters. Which is to say, there is a difference between the sound produced by small and large diaphragm condensers.
Being physically... well, smaller, a small diaphragm has less inertia and can move more rapidly than a large one, so it reacts faster - which in sonic terms means that it register transients a little more accurately and will less smearing over time.
This results in a little more "detailed" transient pickup and a little more definition - again, the diaphragm has less inertia so bounces up and down faster and the recorded voltages changes faster. It can also be a little more sensitive to quiet sounds and low-energy high frequencies for the same reasons.
Whether or not that is significant depends on the source, and if transients are already "smeared" over time or not (for example, strong reflections will "prolong" a transient in the room, and the mic will pick up what is in the room).
For ribbon mics, the moving element is of a thickness a little greater or comparable to some condenser mics, which is why they can often capture quite a few details - even if they will be comparatively lower in level. The size and tension of the ribbon will also have an effect on the sonics, and so its corrugations, producing difference similar to the ones between SDC and LDCs - inertia, flexibility and capacity to react to lower energy will result in different recorded sounds, especially at higher frequencies.
Same as above, there's plenty of good material to read online and not, so little point to repeat things here.
Output electronics in condenser mics
A studio microphone always produces what's called "balanced signal" (which is actually three wires, two for the signal and any noise, and a ground reference) out of a "low impedance" output.
Without going into details, for condenser mics there's always a stage after the transducer (the capacitor capsule) that is used to lower the output impedance. This can be physically realized by using tubes and transformers, or "electronically balanced" circuits which do not require a transformer.
With transformers, the signal produced by the capsule may be augmented by some resonances on the bottom end and in or just beyond the "air" band of the audible spectrum, from 4 to 20 KHz.
So otherwise identical microphones which use a transformer as a final stage will sound a bit different to these which do not.
What the microphone actually picks up
All of the above, however, does not really explain why two technically very similar microphones (say two LDC with transformers) still manage to sound audibly quite different from each other.
Even as their physical characteristics and their frequency response charts are quite similar.
The key to understanding this is fairly simple: resolution.
Our ears (and brains) have an incredible ability to discern air waves in the audible range (give or take 20 Hz to 20 KHz). We do exceptionally well in the midrange but (especially at younger age) we don't do half bad even at higher frequencies, over 10 KHz.
A microphone translates air pressure in voltage differences, which are very, very small. That's why you need a preamplifier to be able to actually do something with that signal.
This means that minuscule differences matter a lot and they get - literally - magnified by preamplification, up to a level which is very recognizable by our ears.
A typical frequency chart puts 19980 Hertz of audible band in the space of, say,15cm or 6 inches. It's like zooming on your city from space, or at least an airplane flying very high.
The necessary level of detail can't be possibly captured by a frequency response chart - at least not one that fits into an A4 sheet!
There's just not enough resolution - just as you can't really reproduce Mona Lisa in ASCII-Art. You get an idea, but just an idea.
You mix what is there
What does it mean in terms of mixing and a microphone "sounding good"?
When we mix, we mix what has been originally captured. And what has been captured depends on the small, "high resolution" differences we just talked about.
Let's take, for example a small diaphragm condenser mic. It's so light and flexible that absolutely every single little packet of energy that hits it makes it move. Every single detail will be there - including the myriad of room reflection, flutter echoes, quiet movements due to heat, the little fan in the recording computer two meters away.
If say you hit that signal with a high frequency shelf to change the timbre of your sound, you will bring out all of these things. It may be good, it may not - but it will be there.
Another SDC however, may have a diaphragm made with a material with slightly different elasticity, or a couple microns thicker, or circuitry that smooths out a specific small band just a little.
For whatever reason, a specific band, say around 1KHz, is made flatter by the second mic.
This "litte difference" will be amplified a lot by the preamp.. and since the region around 1KHz is a frequency to which we are usually quite sensitive (and our brain uses as an element to guess distance) this will likely make a significant audible difference between the two mics. The flatter the area, the more the sound will appear "far away" - which means that for example we'll use less reverb or no reverb on theat source, or we need to boost a little etc.
That level of detail could only be shown if we were given frequency response chart the length of a room! And even then, there would be way too much information to be able to interpret every single difference (now very visible) in terms of audible result.
It is possible to make general statements (just like you can talk of the approximate shape of a piece of land from an airplane), and make generalizations on how the zoomed-out characteristics of a mic will complement the average and zoomed-out frequency response of a type of source ("this mic is good for brass") but you won't get a feeling of how things are for real until you actually walk the land - that is, in microphone terms,you listen to the result thru a relatively neutral speaker.
How do people know their mics, then?
Given all of the above, how can you know when a mic suit a source?
Well, you can't.
Or better, you can have expectations, but you need to evaluate the specific situation and your expectations sometimes go out of the window.
And, you really need to consider the room.
If you use a mic repeadetly, in the same room in a similar position, the mic will "see" (well, hear) a similar front of air and, with repeated experiments and practice, you have a chance to establish a pattern.
So if you can, it's a good idea to stick to the same mics in the same room as much as possible. Once you've "learned" your mics that way, you might be able to discern some of the pattern in a different space.
But even using the same space, the source changes - so you really can't know in advance.
The source may have some particular characteristics that make a certain mic work better or worse than another even if in average it doesnt. Especially with the most important source of all - the human voice.
In conclusion, you always want to have with you a bunch of apparently similar but in average different sounding mics - mics that you can try out until you get the best compromise.
It's a bit of a game between average case and special situation. That's why, in order to evaluate it, you really want to audition a microphone - even more, audition it with the source and in the room you want to use it.
The data you have is usually at too low resolution to tell you very much.
The Great Microphones
The "great mics" of the past were used for years in great recording studios by people doing it day in and day out.
These engineers were subject to precisely the process above: they got to know the mics and the rooms so well, that they could recognize patterns, and the main variable was only the source.
And for engineers working with the same artists for a long time, even this was less of a variable.
If you record in different rooms and with different artists, working a little more to find the "right" microphone is perfectly normal.
Sure, it's still possible to give a subjective impression of a mic and, so long who gives the impression has enough experience with a mic in a room, these impression will kind of represent an "average" and be a valuable guide.
It's also so that certain characteristics, for example very "hyped" mics in certain frequency ranges, will be me more likely to not sound so good in average.
But you really never know. That super-hyped inexpensive mic can be just the thing for a dull room with a bassy singer. And that very expensive C214 may leave you unimpressed with a bright female vocalist in a resonant room.
You've got to try.
In the end, it's all averages vs. special cases. The differences that matter to the final result are really too small to be seen - they can only be heard, in combination with the specific source.
Using a workflow that enables testing and monitoring several mics quickly at the start of a session is usually a hallmark of a good recording engineer - even if the session is you recording yourself.
Microphones are as different as people. You may have an idea of the ones with whom you're more likely to hit it off, but sometimes you'll be surprised!
Happy recordings!
