Small diaphragm or large diaphragm microphones for acoustic guitar recording?

[Jean-Michel G]

Getting a good recording of an acoustic guitar is not easy, because many factors come into play. In my opinion, the most important factors to consider come in the following order of importance:

1. Getting a good sound from your guitar.
This means using a good guitar (and play it well!), but also that the guitar should be set up correctly. A bad guitar will never give a good guitar recording.

2. Room acoustics
If the room acoustic is bad, it will be much harder to get a pleasant recording. You will need to place the microphone closer to the guitar to eliminate as much of the ambiant sound as possible, and this isn’t usually an optimal mic placement.

3. Mic placement
There are various strategies depending on the sound you are after. But in general, the closer the mic is to the guitar, and/or the closer it is to the sound hole, the boomier the recording. The default starting position is at the 12th fret, about 30 cm away from the guitar.

4. Mic type
The subject of this post! But as you can see, this is not the most important factor (in my opinion). We will be discussing types of condenser mics, which you should definitely favor above dynamic microphones for home recording; but even a good dynamic microphone will be better than piezo pickups!

So, what is a small diaphragm (SD) or large diaphragm (LD) microphone?

First of all, this distinction only applies to condenser microphones, not to dynamic microphones. Conventionally, microphones with a recording capsule of 1 inch (25 mm) and above are qualified as LD, while those with a recording capsule of about 1/2 (12 mm) inch are said to be SD.
But in fact, many “SD” microphones have a capsule whose size is just below one inch (20 or 22 mm).

Historically, LD microphones came first. The capsule needed to be large to overcome the noise of the tube electronics. Large diaphragms pick up more sound pressure, so the S/N ratio is improved.
SD microphones came with transistor-based electronics in the 50’s and 60’s.

LD microphones usually have very little self-noise (a.k.a. equivalent noise), often less than 10 dB-A and usually much less than SD microphones (anything between 11 and 19 dB-A). They are also typically “warm”, so they are usually very good for recording vocals.

But for guitars (and other acoustic instruments), you’ll typically prefer a more faithful recording.
SD microphones:
1) have a wider frequency range and a faster transient response LD capsules (because small capsules are easier to set in motion)
2) support a higher pressure level before distortion than LD microphones (because of the stiffness of the membrane)
3) offer a much more consistent polar pattern across the frequency range than LD mics (also due to the stiffness of the membrane)

So SD microphones capture the most details in the high-end and avoid the overly boomy response of LD microphones. However, if the capsule is too small, the result may actually sound brittle, especially if the mic is not placed optimally.

Conclusion
Unless you are looking for a very specific, warm-ish sound, a well-placed SD microphone will provide a better, more accurate recording of your acoustic guitar than a LD microphone.
Almost all professional acoustic guitar recordings toady are done with SD microphones. Classical orchestras are also miked almost exclusively with SD microphones.
That being said, a LD microphone such as the Rode NT-1 will certainly give good results if that’s what you currently own.

[charjo]

J-M,
I often see recordings with SD microphones done in pairs, one directed toward the 12th fret and one toward the bridge the guitar. The purpose is to pan the tracks completely left and right as if the listener were sitting playing the guitar.
It could probably be the content of another post but there are so many other adjustments that could be made in the DAW such as emphasizing certain frequencies and eliminating offending ones, adding compression and reverb.
Paul Davids has done some interesting acoustic recording videos.
John

[Jean-Michel G]

Yes John, this is a very broad topic. I just wanted to pinpoint the fact that a SD condenser mic is usually better for acoustic recording than a LD microphone – I know that many people believe the opposite, and the reasons behind that.

You can certainly record with more than one microphone, and there are again many possibilities regarding the placement of the mikes. I tried many of them, but in the end I decided to usually stick with just one microphone and “make the sound big and lush” in the DAW. I just use busses to apply different plug-ins, or I duplicate the original track if needed.
And I do agree with Paul Davids: some EQ-ing, some compression and a touch of delay is usually all you need. For acoustic guitars, less is more!

[Andy N]

Many thanks for posting this J-M. Lots of useful info and food for thought here.
I’m assuming that if one did want to go down the stereo route John mentions, then a matched pair of SD mics would be required?
That would also require the issue of phasing to be addressed which is yet another topic.
In the past I’ve tried Paul’s suggestion of duplicating, delaying and transposing the left and right channels of a stereo signal and it certainly helps to enrich and enlarge the sound. But obviously that’s not going to compensate for any lack of quality in the initial recording.

[Jean-Michel G]

If you want to go down the stereo route then obviously you’ll need (at least) a pair of microphones. Whether the two microphones need to be matched or not, is another whole subject of its own!

Microphones are manufactured within specific tolerances and two units from the same series usually have slightly different specs, particularly in their frequency response. But it just so happens that even small differences in frequency response can be detrimental to the stereo effect.
This is why matched pairs are manufactured within even smaller tolerances resulting in quasi-identical units. Of course, this means additional manufacturing costs and therefore (much) higher prices.

The question then becomes: do you really need that level of sophistication for your home recording?
Professionals use matched pairs, but they also have all the other parameters straight (professional players with top instruments, perfect acoustic environment, ideal placement).
If you use two unmatched microphones of the same series, a golden-ear studio engineer will possibly be able to tell that the pair wasn’t matched, but most people will not.

Yes, phase issues arise as soon as you use more than one mic. Microphone placement is the main parameter here. But again, small phasing issues may be audible in the DAW but inaudible in the whole mix!
After all, the stereo effect itself depends on (very small) phase differences!

[sunjamr]

Great discussion. Among my mics, I have a Shure SM58 (dynamic) and Rode M3 (SD condenser). What I noticed first of all is that the SM58 barely puts out enough signal to give a decent recording. So I bought a little pre-amp device that plugs into the output end of the mic, and now it works like a charm. The pre-amp is powered off the 48V line source, so you have to remember to turn it on to power the mic. Of course, the Rode M3 always requires the 48V input anyway. So, output-wise, the two mics are pretty much equal now. Then the next thing I noticed is that the Rode condenser mic picks up every little sound in the room, such as blinds blowing in the wind, or someone on the other side of the room coughing. And it picks up a lot of string squeak and random string noise caused by my less-than-perfect playing. The SM58 doesn’t do that. It gives a more mellow sounding recording, and mainly picks up sounds closer to the mic. If you watch a stand-up comedian holding a mic, it’s usually an SM58, and you will notice that the volume varies hugely depending on how far the comedian holds it from his mouth. Also notice that many singers are just about kissing the mic as they sing, and that’s to keep the volume consistent. So I sometimes use both mics positioned side by side about 200-300mm from the sound hole, then diddle with the equalizer to make the condenser mic sound less tinny. Of course, much of this is just personal preference, and everyone is different in terms of what kind of sound they like.

Sunjamr Steve

[Andy N]

Your timing is perfect Steve! I’ve been wondering the last day or so about whether I would need to budget for an amplifier to go with any new mic(s) to get a recordable signal. With my Shure SM57 I can get a decent signal plugging the mic directly into my Scarlett 2i2 but only if I turn the gain up to maximum and the mic is not to far from guitar (in line with your proximity observation). More frequently I plug the mic into my Boss looper pedal which seems to give it a little boost. The looper pedal output is a little more useful too if I want to add any effects into the signal chain, like reverb, from a pedal rather than in the DAW (I’ve got a strymon bigsky which has a nice plate reverb).
From what you say Steve, it sounds like you don’t need a mic preamp with the SD
Mic? (It’s already too sensitive 😬😂) but you do need to make sure the whatever you plug the mic into can provide the 48v
Phantom power?

[sunjamr]

Now that you mention it, whenever I’ve plugged my SM58 into my Boss RC-300 looper, the signal strength coming from the looper is perfectly fine. Maybe there’s a pre-amp in the looper that handles it. Definitely no pre-amp is needed for the Rode SD condenser mic. BTW, the plug-in pre-amp I use on my SM58 is called a “Klark Teknik Mic Booster CT 1”, and I bought it on Amazon. Works like a charm. All I have to do now is remember to turn on the 48V phantom power for every mic I have, but that’s not too difficult: No 48V, no volume.

Sunjamr Steve

[Jean-Michel G]

Well yes, SD condenser mics are MUCH more sensitive than dynamic mics; possibly TOO sensitive if the recording environment isn’t quiet enough. You don’t need any pre-amp with a condenser microphone.
Using a pre-amp with a dynamic microphone is a good idea and it’s done very often. You just need a good pre-amp that doesn’t color the sound.

The reason why singers kiss their microphone on stage is twofold:
– the microphone is usually a dynamic one (because dynamic microphones are much more sturdy) and since a stage is such a noisy environment, singers need to get closer to the microphone. The closer to the mic, the higher the sound intensity picked (wave amplitude) up by the microphone.
– the closer to the mic, the warmer the signal. That’s called the “proximity effect” and it’s caused by the fact that at close distances the front of the microphone picks up more bass frequencies than the rear of the microphone. This bass boost is actually flattering for the voice (but it also causes much more plausives!).

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