Electronics project notes/Audio notes - microphones

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Contents

Choosing a mic for a purpose

Which type for which use

"I'm a streamer, ..."

"...what's the cheapest good mic?"

"...are USB mics any good?"

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

tl;dr: Some are, some aren't.

There are now more good choices than there used to be, but the crap is still there, so you still want to do your research - and test if you can.


Mics with USB connections have always existed, but were usually things of convenience, not quality.

We used to mostly have the two decisions "absolutely anything will do", and "I want to sound sultry and smooth and surely throwing money at professional gear is the way", and basically no demand inbetween, so no market. This niche area inbetween grew when vlogging and streaming became more of a thing, and even then only slowly.


With increasing needs to get decent quality (and the convenience of not spending a week reading up on pro gear), there are more and more USB mics of decent quality, sensitivity, and noise levels.

And also plenty of shit, so check reviews and do tests where you can.

...and remember about other effects (e.g. poor positioning can make any mic sound bad, and good positioning can make cheap mics sound a little better)


USB mics are audio interface and microphone in one.

If you only ever need one mic - which includes most streamers - this is smaller, less fuss, and a little cheaper
you can can spend something like 100 EUR/USD in total on audio, and have it sound pretty decent
...and at the same time feels inflexible to professionals who would rather have an audio interface they can plug any of their collection of mics into, and may talk about upgrade paths (=upgrading one part, without replacing the entire thing).

"...can I get an off-screen mic?"

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

tl;dr: Yes, but it may not be worth it.


Putting a mic closer to your mouth is the easiest way to keep the amplification lower, which is the easiest way to have it be stronger than environment noise.

That's mostly sound physics, that you cannot change with a different mic.

Two footnotes to that:

  • a directional mic essentially gives you stronger signal in one direction.
It means you might be a few db louder than the room sound. And not a lot more than that, again because physics
  • the mic's internal electronic noise, which is part of type and design quality


All that aside, you may like your mouth moderately close, for the warmer sound of having some proximity effect (more lower frequencies).


The above roughly why

decent quality vocal mic tend to be on screen.
boom mics on movie sets (directional) are still specifically held as close as possible without being in frame.
headphones with a built-in mic, even when cheap, can actually sound quite decent
they are close, meaning they can be cheap omnidirectional and still have your voice be stronger than environment noise (that said, other noise in the path is less predictable)
also, they are at a stable distance (consistent proximity effect), and can often be positioned to the side (don't have to think about pops and esses)
you see a lot of lav mics (the things you pin to a shirt) in things like interviews - it's closeish and stable
their downside is the wire (nice-quality wireless means expensive), and you need to learn to place them, because rubbing against them is very audible.


If you insist on an out-of-shot mic:

consider doing facecam rather than room cam - it means "just out of shot" is closer.
a directional mic, probably a shotgun mics (e.g. those specific for camcorders)
...but you can't buy away physics, so it only goes so far, and don't expect very much spending under 100-200EUR/USD (and you can get a somewhat nicer vocal mic for that price).
above 200 you start taking pro mics, like NTG2, which may also be directional enough to reduce room characteristics


Also, if your goal is actually an unobtrusive mic, you might also consider a halfway-decent lav. But those are some work on positioning, thinking about movement noise, and are still wired (wireless lavs are a bunch more expensive).


Using a mic well

Typical use and gain / How to set reasonable levels for any given mic

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)



Some (cheapish) quality-improving tips

Mic technique

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


A person's microphone technique is being aware of microphone behaviour enough to work around it or with it.


This includes things like

  • avoid popping and sibilance
a pop shield (e.g. sock on a wire) a dozen centimeters away works decenty too, and means few specific instructions to give (people adopt a decent distance without instructions, even)
note that if you speaking alongside a mic, not directly into it, you may not need a pop filter and can get closer (but may or may not want to get that close, for other reasons)
pop shields and such also help avoid dampness problems, which e.g. condensers are somewhat sensitive to over the long term
  • turn your head away when breathing, when possible
  • varying distance to even out volume, e.g. when going from quiet to louder singing
yes, you can fix this later to some degree, but that's a lot of work do smoothly, so if you can teach yourself to do this almost instinctively, that saves a lot of work.
also, depending on your range of loudness, you might make some inputs clip if you don't do this
also, an automatic filter may do weird things when presented with different loudness
  • knowing the type of mic well enough to know what distance and loudness are going to give good signal(-to-noise)
  • knowing about directionality
staying in the sensitive area so that you don't unintentionally vary volume and frequency content
  • know about the proximity effect, to
avoid hearing it (volume and frequency difference ), and/or
using it to your benefit (e.g. that bassy radio voice)
recording engineers may even consider using two mics, one close and one a little further away, to give more mixing options later.


Basic mic positioning / mic technique: Just plain distance

Relevant for: everyone (from streamers to casual recorders to studios)


Too close: pops and proximity effect

Pops are sudden rushes of air from your mouth (from plosives - p and b sounds mostly). Those rushes disspiate quickly, but if you're basically eating the mic, they arrive at the capsure very efficiently. And if you're that close you also can't put anything inbetween that reduces this. You can maybe do it to the side, but would have to know about it.

The proximity effect basically points to a constructive effect that means bassy sounds work well when very close to a mic. Which you may like, for that deeper radio voice, yet it also means very little movement varies that bass, so you would have to know about it.

There is a simple and stupid way to avoid having to explain this to people: tell them to sit 10-20cm away. Or avoid having to, but putting a pop filter or something else in front. (I've seen radio stations use large foam windscreens, probably for this reason)



Too far: Consider the how intended signal strength, such as your voice, relates to environment noise.

If your environment is otherwise pretty quiet, and you want to pick up the environment (people, cats, dogs, whatnot), then you can sit at some distance so that everything is picked up.


You can absolutely amplify that, but you will necessarily be amplifying both you and that environment noise.

So consider your room being loud, or people or cars through an open window are, or the HVAC system has a low hum, your PC is loud and/or you'll be on your keyboard while recording, etc. If it's roughly as loud as you, you will hear it just as much, and there's no separating the two afterwards.


If you want to pick up just you, then yes, you can look for directional mics, but there is an even simple method: sit closer.

The louder your are (in absolute terms), the less gain you need.

The closer you are to the mic, the louder you are. That environment noise did not change.

If you turn down the gain to keep your voice output the same, the net effect is that you are only really turning down that environment noise.

Or, from another view, that you are isolating yourself better from that noise.

(...also from other people in the room. This is one reason radio stations that have guests tend to sit close to mics. And have them sit at some distance, but there's only so much room for that)


In fact, the better you can do this, the more that even cheap mics will sound pretty decent. This is one reason even cheap headset mics sometimes work quite well - you even avoid the proximity effect variations by being in a constant position, and also pops if you put it a little to the side.


If you get to play with a decent mic (negligible other noise sources such as internal noise and lower amplification noise, so the effect is clearer):

Sit at half a meter, turn up the gain until you are at decent level.
You're clear, and so is you snapping your fingers behind you.
Sit at a few cm and turn down the gain accordingly
That same snap behind you is now barely there.


(Note: in the moment you won't hear the environment noise as much, because we've had a lifetime of experience at tuning that out, and you're hearing it through the headphones at the same time as you're in the room. But if you record it and play it back, it's a lot more apparent.)



Now record both and listen to it later (because in the moment, you're good at thinking away the environment).

Listen to the difference in sound clutter and noise in the background, which probably includes your PC, and imagine a passing truck, neighbours shouting or walking down the hall, or even someone else on their own mic in the same room.


Notes:

  • This is one reason that a decent headset mic actually works quite well.
  • if your environment is louder than a mic's noise specs, those noise specs barely matter anymore
particularly for free-standing mics
  • in fact, if you're recording in your bedroom without sound isolation, this puts a serious limit on how much a fancy mic even can help
in some cases you may be just as well off with a EUR30 dynamic mic or decent headset mic (just because you'll use it closer) than a sensitive EUR200+ condenser


Beyond vocals:

  • Distance on acoustic instruments
again, closer makes for better isolation
for concurrent recording of live performances, this matters
also why pickups are nice
closer may catch some odd harmonic effects, more fingering sound, and such
further than necessary just loses volume (and isolation)
  • people point mics at guitar amps, rather than using the signal going to its speaker
arguments against micing an amp:
most of the internal tone and distortion processing is also present on the output
so using a DI is smaller, and not another another mic and stand to lug around.
you have to tend to gains to get decent on-stage isolation of sound
neutral arguments:
the physical driver is probably a little bassy, so it's not quite the same as the signal, though you can mostly EQ that
arguments for micing an amp:
it's an easy way to avoid ground loop noise (when you don't have enough DI boxes to do this properly)
the setup may introduce a little compression, because physics(verify) which is e.g. nice on bass guitars


Proximity effect

Accessories

Wind, shock, pop, reflection, and other noise protection

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


Relevant for: vocals, outside recording, preventing some environment rumble anywhere


Wind as in weather.

It's a lot of physical movement, which if it reaches the capsule directly would often overpower most other sounds.

So you want to reduce passing wind, while reducing vibrations much less. This is relatively easy to do at all, though hard to do well.

Mics tend to come with a little wind-style protection built in, because it's universally useful.

But they won't do it much because it will also reduce the amount of useful sound that arrives as well, so it's better as an option you can add only when you know you need it.


Wind and people:

Whistling can also be windy, if you do it with a mic straight in font of your mouth.

One vocal-specific issue is pop, the sudden ejections of wind from your mouth that you get from plosives like p, b, and others. When this easily reaches the capsule, it's the same as the wind problem, though a lot more instantaneous.



Another vocal problem is sibilance, the ess sounds (s, t, ch) that sound harsh - and easily louder than other parts of the vocals.

This one's harder than pop, in part because it's less directional. It's a good idea to record less of it to start with.


Shock refers to hitting whatever the mic is standing on / handing by (and anything hard-coupled enough, like your desk and keyboard, your floor and your foot-tapping and the passing truck and neighbours rumbling or walking down the hall). If the mic is mounted to avoids hard coupling, most of that sound won't make it in via this route.


Reflection here refers to the fact you probably have multiple walls nearby, meaning you record direct and reflected sound - effectively a little reverb on everything. For live use this isn't much of an issue (it just sounds like a person in a room, which we are used to hearing), yet lessening this reflection gives you more leeway and options when mixing later. (Note also that this can be less relevant when you're closer to the mic)



Solutions designs, and products


  • Pointing the mic at your mouth from the side
Helps: vocal pops
But: positioning of yourself now has a little more effect on frequency content (and volume, due to the pickup shape),

so it's often easier (and a little more controlled) to explain and use pop filters.


  • Pop filter are primarily for reducing pop in vocal use.
Helps: vocal pops
It's typically just any thin piece of fabric suspended in front of the mic.
One design is a few nylon layers to reduce wind speed - which is easy enough to DIY with some coathangers and pantyhose.


are a specific foam stuck over all the inlet of a microphone (which for many mic designs works out as blobs, though longer for e.g. shotgun mics).
Helps: vocal pops, wind
These work against gentle wind, and also act as a pop filter.
They are not the best at either, but decent, cheap, and typically supplied with microphones.


are furry variants of windscreens, that tend to be be a little better at reducing wind than just foam
Helps: wind, vocal pops
They cost a little more, and come with some practical details like fluff varying with air moisture, and that you may have to clean them more often.
  • Softies (initially a brand name, but later a much more generic one)
is a vaguer term but frequently refers to a larger synthetic-fur thing large enough you can stick various microphones's business end in them.
Helps: wind, vocal pops
Helps: wind, vocal pops
similar to softies, but are larger, and will often contain the entire mic with a a bit more air between microphone and boundary, and usually use a mesh material (regularly with thin foam on the inside) to stop rushing air.
Seen e.g. on boom mics. They work better, but are heavier.
These may also have a removable synthetic fur cover. (This seems to be where the 'dead cat' name originated)


  • Shock mounts are elastic suspensions, which reduce physically coupled rumble.
Helps: shock, rumble
Basic versions are easy enough to improvise from, say, elastic bands. The characteristics of what frequencies they work best for varies, but halfway decent for no effort
Things like tension do matter matter to how well they work a little, and studios and other permanent setups will probably invest in something less fiddly and more durable.



Reflection filter is often a acoustic foam opposing the sound source, creating a small stall.

The intent is to control and reduce reflections from hard surfaces that may be in that direction, and in studios can help isolate the source from other sources somewhat.

Mostly for vocal work, though note that it does only half the job (at best) that e.g. a vocal booth would.



Mic switches

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Low cut / roll-off (bent-line symbol) - removes low frequencies with a filter

when recording vocals, frequencies below 50 or 100Hz or so are likely to be nothing but rumble (also for home use; think passing trucks), and maybe some wind
knee frequency varies. Some mics have two positions for this, varying knee frequency
can't be changed, so doing this filter in an EQ down the line is sometimes more useful (and largely the same)


Pad switch - basically just lowers amount of signal - attenuation on the order of 5dB, 10dB or 20dB

useful when the input is structurally very loud, e.g. putting mics on a guitar cab.
You generally don't want this for softer instruments, softer vocals
for very loud things it's better to do this in the mic (to avoid it getting overloaded) than later (a loud but perfectly in-range signal can also be turned down later)
There are sometimes also pads down the line, which is more about gain staging - comparable levels and e.g. not forcing sliders/knobs down to their first 5%
(not directly related: pad is also sensible to have on DIs, for very hot signals)

Other mic tools

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


Fancier mic positioning

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Stereo/soundstage effects

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Relevant for: fancier serious instrument recording, studios

...and arguably also e.g. recording groups of voices. It turns out that e.g. the hand-held recorders with two mics on front tend to record a binaural-like effect that we humans can use well to isolate sources, making it easier to listen to and e.g. transcribe.


These are mostly techniques that let you get a spacious recording of something live, without synthesizing that effect in mixing.


  • XY pair
Two directional microphones, inlets/capsules very close, at a 90 degrees angle
proxmity means no time-of-arrival ambiguity, (so) stereo image comes from directional pressure differences.
less impression of space/depth than most other setups, but more stable
no issues mixing down to mono
small amount of high frequency loss in the plane between the mics, which is why they are usually placed above each other (means this rejection is above/below, not left/right)
if the mics touch, this may ruin the effect (or the recording, if there's rattling)
  • Blumlein pair
XY pair using bidirectional microphones
tends to give a nicely realistic soundstage


  • AB pair
Two omnidirectional microphones in parallel, some space apart
tweaking the distance changes amount of directionality that is picked up (verify)
a little bassier, because omnidirectional mics tend to be (verify)
Not ideal to mix to mono by adding the two, as that tends to show comb-filter-like effects. Yet often, using just one channel is perfectly fine.
  • Jecklin disk
AB, at 36cm distance, and with a disk inbetween that increases the apparent separation
Easier to mix to mono because of side rejection (side tends to arrive more in one mic)
  • Decca tree
three mics, at least 1.5m distance
seems to ask for moderately dictional mics, at least at higher frequencies (verify)
resembles AB with a center fill
wide stereo image, mostly used for orchestras and choirs and anything else large. Does not work so well for smaller areas(verify).
There seem to be variants with five (extra left, extra right, look for 'outrigger')
takes care to position, takes care to mix
https://en.wikipedia.org/wiki/Decca_tree


  • near coincident are setups with effects between XY and AB, usually decent ambient and decent directional, and most are named for institutions that thought up each specific setup, like ORTF (French television), NOS (Dutch television), DIN (German standardization)
ORTF: cardoid, pointed outwards, 110 degree angle between them, capsules 17cm apart (roughly a head's width)
https://en.wikipedia.org/wiki/ORTF_stereo_technique
NOS: cardoid, 90 degree, 30cm
https://en.wikipedia.org/wiki/NOS_stereo_technique
DIN: cardoid, 90 degree, 20cm


  • Mid/Side
seems to refer to a two-mic setup, with a cardoid or omni facing the sound source, and a figure-eight mic acting pointed perpendicular
...but also frequently imitated with three mics (two imitating that figure-eight) (verify)
Outputs are generally:
Mid as-is
Left = Mid + Side
Right = Mid − Side
The main reason seems to be flexibility: you can tweak the depth while mixing (something you can't do with most of the above)
mixing to mono is simple: just use the mid mic, with less thought about phase


Related tricks

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


Relevant for: stage, studios


Differential microphone is a noice-canceling arrangement useful in live setups on smaller stages, where crowds and things like guitar amps are nearby:

use two identical microphones, one trained on the sound you want, the other not, and probably nearby each other
invert one (i.e. reverse phase) (fancier consoles tend to allow this in the mixer)
anything that shows up equally at both mics is likelier to cancel out
which is likely to include lowish frequency crowd noise, guitar amp bleed, drums, backline speakers, etc.
anything that shows up at one mic (e.g. the singer) barely so.
sometimes leads to some odd phasing effects, though(verify)

Not to be confused with differential microphone arrays, which use beamforming from multiple mics to isolate in a direction, thereby suppressing background noise and some reverb.

On active noise reduction

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


Noise, in this context, is primarily about what you hear other than talking.


After-the-fact noise suppression can help with any mic.

Not by much, because doing so afterwards will not make the quality of the signal any better, yet it may make the whole recording less distracting.

'May', because something as simple as noise gating is potentially more annoying than leaving the noise in. It depends a little on context.


Options:

  • gating assumes you only need to hear loud sounds, and anything low-level is only ever noise
sometimes this is hard gating, which basically toggles between as-is and basically-muted
but many implementations try to shift more smoothly between the strong attenuation and no attenuation, because this sounds less annoying
upsides:
when that threshold is well tuned
it's perfectly quiet when people are not talking
the noise is there when they are, but it's probably low enough to not bother communication
downsides:
when the threshold is set wrong
set too high and it removes to much, e.g. cutting off every first word
set too low and it removes no noise at all, or seems to cut in and out randomly
the noisier the mic is by nature, the harder the threshold is to set - it may be more distracting than not having it.
if input levels are not very consistent (e.g. varying distance to a stationary mic a lot), that threshold will be wrong over time
The later a gate sits in your audio chain, the more awkward it may be to tune (particularly if it's after a compressor(verify))


  • noise suppression based on an example of noise
Basic 'noise removal' features (e.g. try the one in audacity) often ask you to provide an example of just noise
typically what they do is
determine the frequency content in the noise, then later reduce those frequencies
often using an envelope detector to reduce it more strongly in weaker parts of the signal, so that it bothers actual signal less
upsides
great at removing anything constant - hum, AC rumble, whistle, microphone bias, steady device noise
downsides
as a moderately aggressive EQ (e.g. 12dB reduction in narrowish bands), so the more you remove frequencies, the more easily it introduces little artifacts. You can reduce that, but mainly by reducing how much noise is removed.


buzzword compliant due to neural nets
what they do is typically actually much like the previous (a little fancier, but not much),
partially trained beforehand on what kinds of spectra to respond to
but adaptive, so it doesn't require an example of noise, and can deal with change in noise
upsides:
magically more selective - when it works, it works well.
limitations
magically more unpredictable - the training hides a lot of assumptions, and you don't get to control them. These seem to be trained for average vocals over typical noise, so if you sing, shout, have low noise levels, have unusual noise, the result often sounds weird
usually litte to no way to tune or control any of that - unless you know how to train neural networks and you chose something not proprietary (most are)
you still have to think about your audio chain, e.g. a compressor after probably works better than before(verify).
RTX Voice
only runs on Nvidia cards, specific RTX and some GTX, and gamers may note a small framerate drop
proprietary(verify), free
can present a noise-filtered device based on another (practical to use it for voice chat without requiring plugin style integration)
RNNoiseRNNoise
Used by: OBS (its "Noise Suppression" audio filter)
open, free
KrispKrisp
Used by: Discord
proprietary
paid-for (free version is time-limited)
can present a noise-filtered device based on another (practical to use it for voice chat without requiring plugin style integration)

More technical

Microphone directionality

Sensitivity, noise performance, and some further stuff that influences quality (specs)

Self-noise



Noise

Noise, if high, is more important than sensitivity.

That said, in practice noise and sensitivity are somewhat entangled: cheaper mics are both noisier and less sensitive, fancier mics are less noisy and frequently more sensitive.


Self-noise is the amount of noise a mic produces even when no sound is present. This is often largely determined by the noise within the electronics (including unavoidable thermal noise), and thermal agitation of air.

In practice, this tends to combine to order of 10 to 20dBA. Which given typical environment noise is quite enough.



Mic sensitivity

We can dig into the theory of sensitivity later, and take a wider view first.

Say, there is a practical reasons that different microphones are meant to deal with for different sound levels.

Some are intended to be shouted into or be used on a drumkit.
Some are meant to be spoken into softly at close range.
Some are meant to pick up very subtle things even at some distance.

This is more of a question of whether you are on a stage, at a radio station, recording instrumentals in a studio.


Design-wise, mics for louder sounds have a heavier membrane, which won't move as much until you have louder sounds, nor break as easily. Mics for softer sounds have lighter membranes, which usually distort and break more easily.


This is a "choose the best tool for the job" thing. As well as a "know what your tool is most comfortable with".


Sensitivity seems measured by producing a 1 kHz sine a 94 dB SPL, and seeing what comes out of the mic.


For analog mics (which is most)

This at an electrical level, that same thing means they output less voltage for the same amount of sound.

This sensitivity is often given in dBV / Pascal (at a specific frequency, with a frequency curve alongside).

Which is a fairly direct measure - energy comes in, energy comes out. It tells you how much signal to expect for the same amount of sound (SPL).



Something similarly important, closely related, but often much fuzzier from producers, is noise.

Sure, there's noise that you can introduce later, and avoid introducing later. But the noise in the mic is the most unavoidable part of the process, because there's nothing to swap out there.

Because there will be non-zero noise electrical noise coming from the same microphone, so the lower the amount of output, the closer that noise is to the signal - an SNR thing.

Or, from a different view, the harder it is to design a mic with electronics that avoids doing that, and the harder it is for you to find a mic that actually bothered doing that, or finding it cheaply.


Sensitivity and self-noise go sort of hand in hand, but only to a degree.


Say, most dynamic microphones are less sensitive, but there are a few with such low self-noise that amplifying them actually gives them great signal at a lot of different levels


As mentioned, producers are less forthcoming about this, and seem to take joy out of reporting in in many different ways.

Equivalent Noise? SPL Self noise?


It turns out it's not actually that hard to get something decent for $1 or so. Good electrets have existed for a long time, in phones now replaced by MEMS microphones.


For digital mics (which are now typical in cell phones)

Digital mics are mostly named for digitising close at the capsule. Which they do in part for lower sensitivity to interference, but it has the side effect of changing how dynamic range is expressed, (roughly because for all digital signals this is better expressed as dbFS), and thereby also affects how sensitivity and noise are(verify).


https://www.analog.com/en/analog-dialogue/articles/understanding-microphone-sensitivity.html




💤 sensitivity is often open-circuit

For microphones, sensitivity is often mentioned for open circuit - with no load connected.


This is part of the "we do impedance bridge these days", which says a factor 5 to 10 more, so that the load won't alter what the mic does.

XLR mic inputs are designed for an input impedance between 1kOhm and 2kOhm, which makes it unsurprising that modern XLR mics output impedance is are around 200Ohm.


1/4" TS mic inputs are a different story -- but also fairly rare.





-->

Mic design and specs

SNR in use

Mic dynamic range

Directional behaviour

Directionality means a microphone picks up sound coming from some directions much more than from others.


More directional mics make it easier to train a mic one a specific sound source , to isolate some environment noise (e.g. the PC opposite you, though not the rumbling truck outside), to get somewhat isolated recordings when you're playing together (less need to record separately), (therefore) more mixing choices later, avoid feedback on stage (with stage monitors), to have speakers on their own mics in a radio studio or podcast even when they're fairly close together, and more.


Notes:

  • a bunch of these things are also served by putting mics closer (and dialing down the amplification), but with some footnotes.
  • frequency response will differ between directions
...which is one reason why, in well-controlled environments, omnidirectional designs can be useful - they sound more consistent and neutral. And why they sometimes have use in mixes.
  • even highly directional designs (shotgun, parabolic) rarely give more than 20dB of reak difference between what they focus on and what they don't.
Depending on your needs, this may be more than enough (e.g. when mics are closer) - or disappointing when your expectations came from spy movies and mic cost.


There are a bunch of words that are shorthands for typical shapes on the polar chart [1].

These include:

  • omnidirectional, a.k.a. non-directional
sound from all directions (more or less) equally.
any mic that does not use cavities or surfaces tends to be relatively omnidirectional.
truly omnidirectional response is actually hard, more so when it has to do so for higher frequencies well (but there is rarely a need for such purism)
Prone to feedback.
  • Subcardoid
Like cardoid, but without the rear rejection.
You could think of it as omnidirectional that was sort of biased to one direction after all.
More prone to feedback
  • Cardioid
The polar plot is shaped roughly like a heart, hence the name.
Fairly directional, which makes it useful for
voices, in that it's often close to and pointed at a person
stages in general, because lower sensitivity at the back lessens the likelines of feedback
  • Supercardoid
narrower than basic cardoid, effectively making it more directional towards the front
but also adds pickup directly behind
  • Hypercardoid
Basically the superlative of supercardoid: reject side better, pick up more in front - and directly behind.
...to the point they resemble bidirectional a bit.


  • Bi-directional (figure eight)
roughly equal pickup on one side and the opposite
also meaning better side rejection than most other things


Design-related

  • shotgun - actually a mic design, but it turns out to have a relatively unique polar patterns
...and vary between different designs. so this means "look closer"
but probably in the area of supercardoid, sometimes figure-eight-like (but more focus on one side, and rejects side less)
  • Parabolic
The nature of a parabola is that parallel incoming things are focused on one spot (or, in the other direction, things originating from that one spot end up sent out in parallel beams)
this makes it useful for dish microphones. (and for many non-sound things. Consider solar cooking, spot lighting, dish antennae)
the fact that higher frequencies are more directional is pretty clear in this design
below 2kHz you get relatively poor pickup. A larger dish helps, but only so much. (Apparently a parabola with a shorter focal length also helps(verify))
  • Laser
Laser mics aren't sound transducers themselves. They reads the vibrations off a remote surface,
which often makes it an extremely directonal pickup -- of a there-relatively-omnidirectional surface. So categorize how you prefer.

Surface microphones

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)


A surface microphone is one made to be attached to a surface, and mostly picks up that surface vibrations, rather than air vibrations.

This particularly makes sense for instruments.

However a surface microphone picks up most things more or less equally, and it is surprising how much you don't actually want that for many uses, or at least have to now think about things like handling that instrument.


They are often piezo elements, regularly with a simple amplifier circuit. See Electronic music - pickups.

On preamps

(rewriting)


Powering mics

Note that a lot of mics don't need power. Of the microphones in common use, it is primarily condensers that do.


Batteries

Pros: Simple, avoids need for all of the below


Cons: Batteries can be empty, which is awkward to deal with.

For real shows, people tend to swap them out a lot faster than necessary just to be sure.

T-powering / 12T / AB powering / Tonaderspeisung / DIN 45595

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Supply power to a mic via the XLR cable that also carries audio

by putting 12V DC between XLR pins 2 and 3 (the differential pair).


Note: This is not phantom, it is incompatible with phantom, and some combinations will damage the mic


Upsides:

  • Avoids shield, so avoids shield-related issues

Downsides:

  • accidentally mixing this with now-typical Phantom can damage things
  • any power impurity is on the same wires as the audio signal, and therefore audible (but you'll probably be DIYing neither this or Phantom, so...)


See also:

Phantom power / P48 / IEC 61938 / DIN 45596

Phantom power can supply power to a mic via the XLR cable that also carries audio.

by putting a voltage equally on pins 2 and 3
...and using shield (pin 1) as ground for this circuit
which is a bad idea for interconnects, which is why phantom should only be used/enabled for mics (and other phantom-powered things) and it is good habit to turn it off until you need it


Audio interfaces with XLR inputs often supply phantom on it.

Mixer panels can regularly let you enable phantom power on all their inputs.


Many active DI boxes can be powered by phantom, often as one of the options (Passive DI boxes do not need power).


Anything non-XLR does not do phantom power.



Upsides:

  • Lets you supply power to the mics that need it (mostly condensers) without needing extra wiring, replacing batteries, etc.
mics that need it send a stronger signal, so the net effect is that you can use longer cables before noise is relevant.
  • should not affect signal quality


Keep in mind:

  • mics that require phantom power will probably barely work without it, or not work at all
most notably condenser mics
  • For mics that don't support it, it makes no difference
  • There are a few reasons to keep phantom power supply turned off until you know you need it, roughly:
the pin 1 problem in interconnects (probably the largest reason)
Earth lift, sometimes necessary to work around the pin 1 problem, will also disconnect phantom power
applying this power on some unbalanced microphone designs (most aren't) can be trouble
and some other details, see e.g. [2]
Generally none of these are an issue, since you'll generally only plug balanced mics (or mics via DI boxes) into XLR-with-phantom sockets - but there is the odd case where you can introduce noise or even damage (mostly in stage settings), so it's something you want to eventually know


Technical side:

Phantom power is

  • a voltage placed equally on pins 2 and 3
which means that the receiving side (the differential amplifier on the audio lines) shouldn't see it on the signal at all (hence 'Phantom'), as as power should flow equally through both balanced-pair wires.
  • ...and using shield (pin 1) is now used ground for this circuit.

Using shield as ground is not advisable in general, but primarily because it is a bad idea when using XLR for interconnects (see also the Pin 1 problem - and you want to turn phantom off on any XLR inputs used as interconnects), yet is fine on inputs that are a single microphone (which is floating/isolated).

On DI boxes there are some extra footnotes (mostly to their design(verify)).


  • Voltage:
Technically three variants: 48V, 12V, and later 24V
in practice often 48V (though is apparently allowed to be 10..52V(verify)
the 48V is purely for historical reasons, and actually somewhat impractical now (9..12V is enough for almost all circuits, and microphones have to step it down to that)
  • Current:
early phantom supplies might only supply 2mA, enough for a single FET
modern phantom supplies should be capable of 10mA-15mA, and modern mics usually use something like ~5mA

See also:

  • http://en.wikiaudio.org/Phantom_power
  • mention in IEC 61938 (1993) ("Multimedia systems - Guide to the recommended characteristics of analogue interfaces to achieve interoperability")
  • mention in DIN 45596 (1973, 1981)

Plug-in power / Bias voltage

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

In practice, bias voltage is a mostly a thing on mics connected via 3.5mm TRS, like PCs, video cameras, DSLR, phones, voice recorders, minidisc)


...because that context usually means electret mics, and some voltage is required for (the FET typically in) most electrets to function.

Note that not all mics with 3.5mm need (or can use) bias voltage. Mics that don't need it are likely to be designed to ignore it.


The bias is often roughly ~2-3V DC on consumer hardware, but this has varied with designs and over time (and on specific equipment might be between 1.5V and 10V?).

Expect it to supply very little current (order of 1mA or less).


The voltage, and low current capacity, means there is a quick and dirty test for the presence of DC bias on a mic input with a plain LED (...ideally one with known low voltage forward bias).

Wiring microphones

Things to keep in mind:

On impedance

See Electronics_project_notes/Audio_notes_-_device_voltage_and_impedance#On_microphone_impedance

But basically: most pro mics are order of 200 ohm, because they impedance-bridge with approximately 1.2kOhm on the mixer side.


Higher or lower mic impedance and higher or lower amp impedance both exist. These are mostly special cases, and special uses you'll probably know about.

If either side's impedance is switchable, that mostly changes the amount of load, which mostly just bends the frequency response a little.


Offset or rectify

Amplification

Isolation, DC removal

Types of microphone - workings

Dynamic microphone

Condenser

Electret microphone

Circuit use

Related hacking

MEMS

Piezo microphone

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Piezo microphones, a.k.a. crystal microphones.


Piezo elements are more general type of sensor, usable to sense stress, vibration. They are ceramic capacitors that are designed to be bent a little. This bending creates the voltage. (due to the piezoelectric effect: stress on a crystal leads to voltage across it).

They are often discs, presumably because it's an easy design and it makes it easier to couple to a larger surface. There are other designs, e.g. the flat ones in piezo guitar bridge pickups -magnetic.


Reacting to being bent means they are good at picking up sound that carries through a solid material, and quite poor at picking up airwave sound directly.

This "it hears only what it's pressed onto' is a feature in applications like as in vibration sensors, impact sensors (you can DIY a drumkit with these), instrument pickups. Contact microphones are often piezo elements plus a little amplification.


One limitation of piezos is relatively narrow frequency response, which also comes in part due to the physical size - each disc design has a significant physical resonance frequency and its sensitivity falls off beyond it.


Acoustic-guitar pickups are often piezos under the bridge so that string vibration makes it to the piezo fairly directly. (There may also be a body pickup as well, piezo or electret, to get the soundbox's sound).

Piezos are not used on electric guitars, which typically use magnetic pickups. Magnetic pickups work only with metal strings, which isolates to pick up just the string movement, and will e.g. ignore slapping and some other things you might do playing acoustic guitar (except whatever movement that makes it to the strings).

See more notes at Electronic music - pickups


Ribbon mic

Historic or exotic

Carbon

Liquid

Ribbon

Fiber optic

Laser

Some more glossary

Measurement microphones