Previously, I wrote an article talking about the preliminary research I conducted before purchasing my first mic, the Rode N1-USB.
Since then, I’ve done a LOT more research, trying to understand all the different parts of mics, how they work together, and – most usefully – what manufacturers mean when talking about terms like impedance, sound pressure levels, transient responses, and so on.
If you’re just wanting to know the basics, like the most common types of microphones and their polar patterns (read: how they pick up the sound around them), check that article above. But if you’re ready to take a deep dive into the nuts ‘n bolts of studio microphones, strap yourselves in!
How Do Mics Work?
This will help you understand the later parts, so bear with me here.
Let’s say you’ve hooked up a dynamic mic near your kick drum shell. When the drum is struck, sound waves pass into the mic capsule, causing a small, VERY thin piece of material to vibrate. This is called the diaphragm.
Diaphragms can be made of anything from aluminum to mylar, but the key is they are either electrically conductive, or connected to something that is.
The forward and back movement of the diaphragm creates an electric charge, which runs into a preamp, which preps the signal for it’s epic journey away from the microphone itself.
The signal moves on to the next device, usually another amp, which boosts the signal again to what is called the “line level”. At this point, the heavy lifting is done – processing aside, that drum signal is ready for human ears!
Now let’s explore some of the more important factors that ensure that sound is just as awesome coming out your speakers as it sounded in the recording studio.
Impedance
Impedance – usually expressed as load/input or output impedance – refers to the resistance of an electric circuit or component to an electrical current. It literally “impedes” the flow of electrons, and is affected by the various capacitors, transistors, tubes, and other components that make up a microphone’s innards.
Think of impedance like a dam a beaver has built in a river.
When the river is wide, the “current” is slow, and the dam can more easily “impede” it’s flow. That’s why most studio mics include preamps that increase the current, resulting in a proportional decrease in impedance.
As a general rule, the output impedance of a mic must be low – ideally no more than 600 ohms (Ω), and no more than a tenth of the input impedance of the next device in the signal chain.
Higher impedances cause the signal to gradually degrade as it passes through the cable, and may result in loss of signal strength when it hits the preamp. Speaking of which…
Preamp Impedance
While high impedance on the mic itself is nearly always a bad thing, preamp impedances have a little more wiggle room. In fact, higher end preamps allow the user to switch between higher and lower impedances.
Why is this a good thing? Well, changing the preamp impedance also changes the quality of the sound, particularly with dynamic and passive ribbon mics. Lower impedances tend to be warmer and more bass-y, for example, while higher impedances emphasize the high end, resulting in a brighter sound.
An in-depth look at this is beyond the scope of this article, but suffice to say that some preamps can actually “weight” their load impedance towards the high or low-end of the frequency spectrum, which brings out more of the sound in those frequencies.
Frequency Response Charts
Microphones generally don’t produce the same volume of sound across the entire frequency spectrum. In particular, there is a “rolloff” of sound in the lower and higher frequencies, and some mics even intentionally accent certain ranges, to bring out the details in certain instruments, for example.
Manufacturers create detailed charts to give you an idea of where the mic has been boosted or attenuated. It should look something like this:
Okay, this mic has a pretty bad rolloff at both ends of the spectrum, but you get the idea. Frequency response charts can be extremely useful, depending on the type of sound you’re working with.
Signal-To-Noise Ratio
Signal-to-Noise Ratios (SNR) refer to the power of the signal leaving the mic, relative to the background noise that is present in that signal. SNR values are expressed in decibels, with at least 74 dB being acceptable for most professional recording.
Generally this only applies to “active” microphones – that is, mics that are powered externally using “phantom power”. But even the most state-of-the-art mic on the market produces a certain amount of self-noise.
And some produce a LOT of self-noise.
Self-noise is a kind of background static produced both by the internal components of a mic, or picked up in the surrounding air molecules. This can be a problem, because boosting a signal also boosts the noise.
Tech sheets may also refer to this as the equivalent noise level. It is expressed in dB-A, which is a sound volume weighted to simulate human perception of that sound.
As self-noise/equivalent noise levels are measuring background noise (not the signal itself), we want these values as low as possible, especially when recording quieter sounds. A rating of less than 10 dB-A is excellent for virtually all applications, while 20-23 dB-A is audible when recording anything below speaking level.
On a related note (last volume-related one, promise!) you should also pay attention to the…
Microphone Sensitivity
All sound exerts a small but significant amount of pressure on a mic’s diaphragm. If we subjected two different mics to the same “sound pressure level” (SPL), one would produce a slightly louder sound than the other.
This occurs because some mics can more efficiently pass and amp up a signal, with minimal loss of information. If you were paying attention earlier, you would realize this means less boosting of the signal is required, which means – you guessed it! – less self-noise.
You may also see the term “Max SPL”, which is the highest volume a mic can handle without significant distortion.
Tube VS Solid State (FET)
Back in ye olden days, condenser microphone capsules, used a tube circuit to amplify and transmit sound – similar to those used in some guitar amps, or landline phones.
While relatively cheap to make, tube mic signals accented certain frequencies, colouring the signal and giving it a certain “vintage” quality.
By comparison, solid state, FET, or JFET circuits are lauded for their “flat” responses, rugged design, and resistance to heat. Despite this, many artists still prefer the vintage sound of a good tube mic.
Transient Response
“Transients” in sound are small bursts of energy, often present at the beginning of a sound, that give it a “crack” or “smacking” sound. They are also essential to making low-end sounds, like that kick drum we started with, more audible.
Since these sounds are very fast, the microphone diaphragm must be able to react fast enough to pick them up. How well they do so depends on how heavy the diaphragm is, as well as how tightly it is tensioned.
Moving-coil dynamic mics and condenser mics with large diaphragms tend to respond more slowly to these transients, while ribbon and small-diaphragm condensers are more accurate.
While this is a lot to say about microphones, none of it will mean very much to the untrained ear. The only way to match the manufacturer specs to your own preferences is to hear the difference for yourself. So pick one (or more) up, and start recording!
Credits:
“Brown and White Animal on Brown Tree Trunk” by David Selbert from Pexels
“Confused Young Girl in Denim Shirt” by Skyler Ewing from Pexels
“Grey Condencer Microphone” by Pixabay from Pexels
“Happy Asian Woman in Sunny Summer” by Trần Long from Pexels
“Person Petting the Cat” by cottonbro from Pexels
“Two Man Hiking on Snow Mountain” by Flo Maderebner from Pexels