Electronics project notes/Audio notes - microphones

From Helpful
(Redirected from Electret mic)
Jump to: navigation, search
This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · · · · Shorter-range wireless (IR, ISM RF) · RFID and NFC · bluetooth · 802.15 (including zigbee) · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

Audio notes: microphones · device voltage and impedance, audio and otherwise · amps and speakers · basic audio hacks · digital audio · noise reduction · multichannel and surround ·

Less sorted: Common terms, useful basics, soldering · Arduino and AVR notes · ESP series notes · PLL · signal reflection · pulse modulation · electricity and humans · resource metering · Microcontroller and computer platforms · SDR · Unsorted stuff

See also Category:Electronics.

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 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
when there's a lot of environment noise, and/or
when you hear what you put in (e.g. on stage) to avoid feedback, and picking up what the people next to you
  • lav mics are usually not directional - they they would be hard to aim, and work well enough just because they're close.

Some (cheapish) quality-improving tips:

  • Isolate the microphone's body from hard-surface-to-hard-surface contact, to avoid a lot of hand fumble and environmental rumble
A cheap DIY microphone spider mount can be made from a few hair ties, rubber bands, or other elastic materials [1].
even something as simple as putting it on a towel folded over a few times already helps.
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 less noise
...but you run into the proximity effect, roughly means excessive bass response
...and this varies a bunch with distance, so isn't easy to EQ away
  • For subtler (voice) quality, learn a few microphone positioning basics - it helps...
    • control what frequencies are picked up most. (...so only touch the equalizer after positioning)
    • spatial feel
    • isolation between sources
    • noise levels

By purpose:

  • If you're a gamer or streamer
...and don't use facecam, go for any headset-with-mic that isn't in the bottom-most tier. Being close means even moderately cheap ones are often decent enough, it's consistent volume, and as long as you keep 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.

  • 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 may 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 fancier mics. Think
singers that can be gentle
acoustic guitars played into microphones (i.e. without their own pickups)
and more
  • drumkits are a story of their own.

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.

This makes it easier to use 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 fairly 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.


  • 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


  • 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.

Proximity effect

Microphone positioning

Related tricks

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 movement, which if it reaches the capsule directly would often be very loud. It's relatively easy to reducing passing wind while reducing vibrations much less. Mics tend to have a little wind-style protection on them because it's fairly universally useful. But but not much, basically because you will reduce the amount of useful sound that arrives as well, and it's more practical for you to choose to add more when necessary.

Pop refers to your mouth. When you lean closer you get a stronger and more isolated signal - but also get the ejections of wind (plosives like p, b, and other) directly reaching the mic for, basically short wind sounds. Or actual wind, e.g. when whistling. Pop filters let you sit closer without these being big issues.

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.

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. 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)

Product / design-wise:

Shock mounts are elastic suspensions, aimed at shock as mentioned, and at rumble that makes it in the same way. Also moderately simple to DIY from, say, elastic bands. Though things like tension matter to how well they work, having this at all is already a good start.

Pop filter, for vocal mics use, are 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 foam stuck over all the inlet of a microphone (often blobs, though longer for e.g. shotgun mics).

These work against gentle wind, and act as a pop filter. Not the best at either, but quite cheap and typically supplied with microphones, and if you need it for wind anyway it's nice to have pop filtering for free.

Wind muffs (sometimes 'dead cat') are furry variants, that tend to be better at wind than plain foam, while costing little more. Note there are some minor practical details like fluff varying with air moisture, and that you may have to clean them more often.

Softies, initially a brand name so a somewhat vaguer term in general, often 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 often larger containers with a bit more air between microphone and boundary, and uses a mesh material (regularly with thin foam on the inside) designed to stop rushing air. Seen e.g. on boom mics. They work better, but are heavier. (do they reduce directionality?(verify))

These may also have a removable synthetic fur cover. (This seems to be where the 'dead cat' name comes from)

These 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 it may not be worth the time you'll invest over getting a decent product.

Reflection filter is often a semicircle of acoustic foam placed around a microphone, creating a small stall. Mostly for vocal work.

This lessens e.g. the hard reflections that various rooms will easily add, and helps isolate the source from other sources somewhat, though only do half a job that e.g. a vocal booth would.

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. This particularly makes sense for instruments.

They are often piezo elements -- see Electronic music - pickups.

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.

On preamps

"I'm a streamer, ..."

"...are USB mics any good?"

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


A less sensitive mic, or putting it further away, means you have to amplify the signal more. Any electronic noise, from the mic itself and any devices it goes through, will be amplified right along.

But usually more important than that is environment noise: The further away, the more your levels resemble that of your environment. Even if you can amplify with absolutely zero electronic noise, your environment is going to be much more noticeable. Any housemates, trucks, mouse clicks are going to be noticeable.

Implied is that you'll also pick up room's reverb. (Acoustic foam on nearby surfaces can help - but as a fix this too is pricy)

The easiest solution to most of that is "if you are much closer, then you are louder than what you don't want".

If you want less environment noise at some distance, you want a strongly directional mic. This refers to using physical design to be more sensitive in one direction than another.

The further away, the more that physics makes it hard for things to really be directional enough, so you tend to win no more than a meter or two at best.

Most mics that specialize in vocals are not highly directional, so putting them a meter away, even if you can gain the signal with near-zero electronic noise (which is usually pricy) will still fundamentally pick up more environment noise.

Some more glossary

On the technical side

Powering mics


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.


  • Avoids shield-related issues


  • 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)).

And in most designs, if you turn phantom off you'll be fine even when using it for interconnects.

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


  • 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
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)

Bias voltage is required for electrets, and in practice is common for any (non-powered) mic you plug into a 3.5mm jack - PCs, video cameras, DSLR, phones, voice recorders, minidisc)

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

The bias is 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 roughly 200 ohm, to impedance-bridge with the ~1.2kOhm on the mixer side.

Higher or lower mic or amp impedance (particularly if swithchable) changes the amount of load, which mostly just changes the frequency response.


Isolation, DC removal


Types of microphone - workings

Dynamic microphone


Electret microphone

Circuit use

Related hacking

Ribbon mic


Piezo microphone

Historic or exotic




Fiber optic