Electronics project notes/Audio notes - microphones

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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, fix, or tell me)

Attempt at being brief

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, fix, or tell me)


By mic type

  • Very cheap mics (especially the PC microphones with 3.5mm plugs) are often electrets
cheap mics are often cheap electrets. Better electret exists, but are more expensive.
held close, these are quite fine. Phones used these for a long time before they moved to something more miniaturized
  • Sturdy mics are often dynamic mics
The bulb-headed stage mics for vocals (and more) are dynamic mics
They are less sensitive, but for most louder things that's actually what you want
  • studio recording mics are more sensitive and typically condenser, sometimes dynamic, sometimes ribbon
and many are designed to be directional, because that's rather practical for many uses


On directionality

  • you want directionality
on stage, when you want to lessen picking up the audience
on stage or in the studio, when you want to lessen picking up other artists on a channel
when there's environment noise (that is directional - only some of it is) that you can lessen by pointing away from it, and/or
when you get back what you put in (e.g. stage monitors), to avoid feedback
  • you don't need directionality
when the mic is close
e.g. lav mics are usually not directional - they they would be hard to aim. They work well enough because they're fairly close (though don't work as well in groups)
e.g. on-face mics (and headset mics) are right next to your mouth, and typically unidirectional, and not so sensitive, because they don't need to be - the signal they get is much stronger than the environment anyway.


Some (cheapish) quality-improving tips:

  • Isolate the microphone's body from hard-surface-to-hard-surface contact
this lessens hand fumble, environmental rumble
A cheap DIY microphone spider mount can be made from
a few hair ties, rubber bands, or other elastic materials [1]
Even just putting it on folded up towels works well enough (on top of something else, for height)
Don't forget that the microphone cable also physically carries shock
(e.g. if you have in-ear headphones, consider you'll hear fiddling with the wire)
  • closer means more sound energy, so lets you lower amplification (of environment noise that's always there)
...but you run into the proximity effect, roughly means more to excessive bass response
...and this varies a bunch with a small amount of distance, so isn't easy to EQ away
argues that a headset mic is actually decent tradeoff
  • For subtler (voice) quality, learn a few microphone positioning basics - it helps...
    • control what frequencies are picked up most (...and only touch the equalizer after positioning)
    • isolation between sources
    • noise levels
    • spatial feel (TODO: elaborate)


By use:

  • If you're a gamer or streamer
...if you don't use facecam, any headset-with-mic that isn't much cheaper than a week's groceries should be okay. Being close means even moderately cheap ones are often decent enough, it's consistent volume, and as long as you position it slightly to the side of your mouth you avoid most pops and esses.
...and do use facecam, consider a lav mic.
They're the ones you clip to your shirt or such, so are moderately close, are still moderately cheap.
It does mean a wire (making them wireless is probably not)
...if you don't mind a large mic near or in frame, there are halfway professional ones in the EUR100..200 range.
there are some choices in the EUR50..EUR100 range that are quite decent to start.
though this takes a bunch more searching - there's also a bunch of crap in that range
which are perfect if you're still testing whether you like streaming.
though if you do, you may well end up upgrading to something a little better


  • If you want home recording
environment noise is often a thing, so until you're fixing that, a closer mic is often the easiest first step. Takes a little attention, but works.


  • to record loud things, you don't need expensive mics.
In particular for punk and the likes, you actually want less-sensitive.
Dynamics and even those 20-dollar chinese pseudo-condensers might be perfectly okay choices for this specific case
  • to record things that are sometimes quiet and sometimes loud (a.k.a. more dynamics), you do need somewhat fancier mics. Think
pianos
singers that can be gentle and not
acoustic guitars played into microphones (i.e. without their own pickups)
and more
  • drumkits are a story of their own.


  • acoustic foam (or DIY) is there mostly to deaden the sound of room reverb, which gives you more options in mixing
and is less important if you're always close to your mic - but convenient if you don't want to think about that




Using a mic well

Microphone 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, fix, or tell me)

Just distance

Consider the how intended signal strength, say your voice, relates to environment noise.


The further away you are from the mic, the more that the signal resembles environment noise levels.

You can absolutely amplify that, and with nice gear you can do it with negligible electronic noise, but that's usually worth little, as you will necessarily amplify environment noise just as much.

The closer you are to the mic, the stronger your voice sounds relative to environment noise, the less you have to amplify, so the lower the environment signal is in the recording.


If you get to play with a decent mic (lower internal noise so the effect is clearer):

Sit at half a meter, turn up the gain so you are at decent level. You're clear, and so is you snapping your fingers at an arm's length further away.
Sit at a few cm and turn down the gain accordingly. That same snap is barely there.

Also, record both and listen to it later, and listen to the difference in sound clutter and noise in the background.

Now imagine your PC two meters away, a passing truck, neighbours shouting or walking down the hall, or even someone else on their own mic in the same room.


Note that this also means that mic sensitivity and internal noise specs only matter once your environment noise is accordingly low.

Frankly, if you're recording in your bedroom without sound isolation, this puts a serious limit on how useful a fancy mic even can be. In this case you may be better off with a EUR30 dynamic mic than a sensitive EUR200 condenser, just because you'll use it closer.


Beyond vocals:

  • Distance on acoustic instruments
closer makes for better isolation (less muddled for concurrent recording)
also why when pickups are nice
closer may catch some odd harmonic effects, and more fingering sound
further than necessary just loses volume (and isolation)
  • people point mics at guitar amps - rather than using their outputs
arguments against:
most of the internal tone and distortion processing is also present on the output
so it's just another mic and stand to lug around. Using a DI is smaller.
you have to use lower volume to get decent on-stage isolation of sound
neutral arguments:
the physical driver is probably a little bassy - but you can EQ that
arguments for:
it's an easy way to avoid ground loop noise (when you don't have enough DI boxes)
the setup may introduce a little compression, because physics(verify) which is e.g. nice on bass guitars


Stereo/soundstage effects

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


  • XY pair
Two directional microphones, inlets/capsules very close, 90 degrees angle
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
tiny 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)
and no issues mixing down to mono
don't let them touch or mechanical noise may run the recording.
  • 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; tends to show comb-filter-like effects. But often enough just using 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 ar just one mic)


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


  • Mid/Side
bidirectional to the left and right, cardioid (or omni) facing front
Outputs are generally:
Mid as-is
Left = Mid + Side
Right = Mid − Side
(in part because you can separate out the mid and side channels from just that)
If recorded separately, this allows depth tweaking while mixing
mixing to mono: just use the mid channel


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, fix, or tell me)


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) at the mixing console (fancier consoles tend to do this)
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.


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.

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, fix, or tell me)


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 perfectly. Mics tend to come with a little wind-style protection built in, because it's universally useful. But but not much, basically because you will reduce the amount of useful sound that arrives as well, so it's useful to add more only when you know you need it.

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


Pop refers to 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 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. Distance is a good start. And because high frequencies are more directional, angling the mic away from facing you directly also helps here. A pop filter helps only a little, but is often there anyway. The pencil trick works somewhat.


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)



Note that pointing the mic at your mouth from the side also helps, though 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.


Solutions designs, and products


Shock mounts are elastic suspensions, which reduce physically coupled rumble. Basic versions are easy enough to DIY from, say, elastic bands. 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.


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


Foam windscreens are 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).

These work against gentle wind, and also act as a pop filter. They are not the best at either, but decent, quite cheap, and typically supplied with microphones.


Wind muffs (sometimes 'dead cat') are furry variants, that tend to be be a little better at reducing wind than plain foam. 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.


Blimps and zeppelins are 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)


Wind muffs, softies, and blimps/zeppelins aren't as easy to DIY, in that finding something that blocks wind well but is also quite transparent to sound is quite a specific property, and the trial and error you need may not be worth your time over getting a decent product.


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 technique

Proximity effect

Some of the more practical stuff, in specs or use

Sensitivity, noise performance, and some further stuff that influences quality

Mic design and specs

SNR in use

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 [2].

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.

By purpose

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, fix, 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)

"I'm a streamer, ..."

"...are USB mics any good?"

On noise reduction

"...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, fix, 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. When your voice is stronger than the environment noise due to distance, you also need to amplifying environment noise less.

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, and somewhat better isolation
  • 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 tends 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).


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).


Some more glossary

On the technical side

Powering mics

Batteries

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, fix, or tell me)

Puts 12V DC between XLR pins 2 and 3 (the differential pair). Note: This is not phantom.

Upsides:

  • Avoids shield-related issues

Downsides:

  • accidentally mixing this with (the now typical) phantom-power gear can damage things
  • any power impurity is on the same wires as the audio signal, and therefore audible


See also:

Phantom power / P48 / IEC 61938 / DIN 45596

Phantom power is a voltage placed equally on pins 2 and 3, which means that the receiving side (the differential amplifier) shouldn't see it 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. This is sort of a bad idea in generic use of XLR for interconnects (see the Pin 1 problem) yet is fine on inputs that have a single floating or isolated device on the other end (verify).

That notably includes putting a single mic on an XLR input.

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

You probably want to turn phantom off on any inputs used for interconnects.


The exact way way phantom power is used on the mic side (or even its source side) can vary a little.


Upsides:

  • Lets you supply power to the mic which can then output higher signal levels
giving better signal-to-noise, and/or allows longer cables before noise is relevant.


Downsides / keep in mind:

  • mics that require phantom power will barely work without it, or not work at all
most notably condenser mics
  • 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 is the largest reason
applying this power on some unbalanced microphone designs (most aren't, but still) can be trouble
Earth lift, sometimes necessary to work around the pin 1 problem, will also disconnect phantom power
and some other details, see e.g. [3]
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 damage, so just keep this in mind


Mixer panels can regularly do phantom power.

Many active DI boxes can also use it.



Technical notes:

  • Voltage:
Technically three variants: 48V, 12V, and later 24V
in practice typically 48V
the 48V is purely for historical reasons, and actually somewhat impractical (9..12V is enough for almost all circuits, and microphones actually have to step it down to that)
  • Current:
early phantom power would only supply 2mA, modern phantom should be capable of 10mA-15mA
early microphones used only ~2mA (enough for a single FET)
modern microphones often use ~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, fix, 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 usually means electret mics, and some voltage is required for most electrets (or rather the FET typically in there) to function.


Bias voltage is mostly ignored by 3.5mm mics that don't use it.

The bias is often roughly ~2-3V DC, but this has varied with designs and over time.

Note that you can test for the presence of DC bias with a plain LED.

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

Ribbon mic

MEMS

Piezo microphone

Historic or exotic

Carbon

Liquid

Ribbon

Fiber optic

Laser