What Is A Microphone Audio Signal, Electrically Speaking?


A Capture Of A Digital Audio Signal In The DAW Logic Pro X

Before my career as an audio technician, it was magic to me that by speaking into a microphone, my voice could be recreated at a much louder level through a loudspeaker. The technical reasons why would come to me years later. It all starts with the audio signal created by the microphone.

So what is a microphone audio signal, electrically speaking? Audio signals are representations of sound as electrical alternating currents between 20 Hz – 20,000 Hz (range of human hearing). Microphone audio signals are signals produced by microphone transducers. Electrically, they are measured in millivolts (mV) or decibels relative to voltage (dBV or dBu).

That’s a basic answer, but there’s plenty more to explain about audio signals and specifically microphone audio signals. Let’s get into answering this question with more detail in this article!


What Is A Microphone Audio Signal, Electrically Speaking?

A microphone audio signal, as the name suggests, is an audio signal created by a microphone.

Microphones are transducers. They convert sound waves (mechanical wave energy) into audio signals (electrical energy).

So a microphone audio signal is an electrical signal that represents the sound waves the microphone has captured. Some microphones capture these sound waves more accurately than others. Although there are different styles of transducers within microphones, all mics are built for the purpose of converting sound into audio signals.

Audio signals are AC (alternating current) electrical signals. They are typically measured as AC voltages or as decibels relative to voltage (dBu or dBV). It’s important to note these values are rms (root mean square) rather than peak values. Rms values are useful in determining the overall “strength” of the audio signal (or any AC signal for that matter).

The strength of an audio signal varies greatly throughout the audio chain due to gain staging. Microphone signals are the weakest signals (lowest AC voltage values) while speaker signals are the strongest (largest AC voltage values).

Microphones are said to produce audio signals at mic level. Mic levels are nominally between 1 millivolt (−60 dBV) and 100 millivolts (−20 dBV). Of course, the actual rms value of a microphone audio signal depends on the microphone sensitivity and the loudness/proximity of the sound source.

These mic level audio signals are outputted from microphones and typically travel through balanced XLR cables to microphone preamplifiers. The job of the mic preamp is to amplify the microphone audio signal to line level for use in professional audio equipment.

Audio signals contain frequencies between 20 Hz – 20,000 Hz (within the range of human hearing). The frequency contents of a microphone audio signal depend on the frequency response of the specific microphone as well as the harmonic content of the sound source.

For everything you need to know about microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).

Audio signals are often recorded. We live in the era of digital recording, although analog recording practices still take place. The former entails converting the electrical AC signal to digital information (1’s and 0’s), while the latter often entails imposing the electrical AC signals onto magnetic tape.

Let’s recap the electrical characteristics of a microphone audio signal:

  • AC electrical signal
  • Produced by microphone transducers
  • Measured as either an rms AC voltage or in decibels relative to voltage (dBu or dBV)
  • Signals are at mic level which is nominally between 1 mV (−60 dBV) and 100 mV (−20 dBV)
  • Frequency content within the range of 20 Hz – 20,000 Hz
  • Travels in closed circuits (often through balanced XLR cables)
  • Can be stored by analog or digital means

Let’s discuss each of these points in more detail!


What Is An Alternating Current?

Alternating current (AC) is the flow of electrons in an electric circuit that periodically reverses direction.

To envision audio signals as “AC,” I like to think of a loudspeaker. We can see a loudspeaker’s diaphragm move inward and outward as it produces sound. We can say the “flow” of the loudspeaker diaphragm periodically reverses direction. Loudspeakers are actually controlled by AC voltages and act similarly to a microphone, only in reverse.

It’s the same back-and-forth idea for a microphone diaphragm. A mic diaphragm vibrates back and forth due to sound pressure variance. The diaphragm of the microphone, as we’ll discuss in the next section, coincides with the microphone’s audio signal.

Because the flow of electrons in AC changes directions, the values of current and voltage (among other electrical values) have both positive and negative values. This means their maximum values are at both positive and negative peaks. It also means that the voltage and current of AC signals are zero at certain instants within a cycle. For these reasons, AC voltage and current are most often measured as rms (root mean square) values rather than max values.

The simplest AC signal is a sinusoidal waveform. It should come to no surprise that the simplest audio tone is a sinusoidal waveform as well!

The Sinusoidal Waveform

So AC voltage is most easily shown as a sinusoidal waveform (sine wave). Here is a graph I drew demonstrating a sine wave:

We can see here that a sine wave has 2 fundamental factors that take up the x-axis and y-axis, respectively:

  1. Time
  2. Amplitude

Time And Frequency

Alternating currents and audio signals are functions of time. As we can see from our simple sine wave above, there are 2 patterns drawn.

The sine wave increases from zero to its positive peak, then decreases past zero to its negative peak, and finally increases back to zero to start over again. This is called a cycle and takes the amount of time called a period.

The number of cycles that fit within one second is known as the frequency of the AC signal. Frequency is measured in Hertz (Hz) which means cycles per second. Sound familiar?

The audible frequency range of human hearing is known to be between 20 Hz and 20,000 Hz. It’s worth noting that this is a continuous range rather than a discrete range.

Sound, by nature, is made up of many frequencies interacting with each other, creating constructive and destructive interferences and, ultimately, what we hear.

Sound waves do this by varying the air pressure positively and negatively.

Audio signals do this with alternating current.

Higher frequencies yield higher-pitched sounds. The way we hear frequencies in the audible frequency spectrum is logarithmic. For example, for each doubling of a frequency, we get an octave. 200 Hz is an octave above 100 Hz (they’re the same “note”). Similarly, 5000 Hz is an octave above 2500 Hz.

Amplitude And Signal Level

The other critical factor of sine waves, audio signals, and sound itself is the amplitude of the waves.

The amplitude of a sound wave is synonymous with how loud that sound wave is. Of course, sound waves dissipate quickly due to the inverse square law (−6 dB for every doubling of distance).

The amplitude of an audio signal similarly coincides with the strength of that signal. Remember that we measure electrical signals at root mean square (rms) values rather at peak values.

The easy equation to determine the rms value for a sine wave (with a known peak value) is as follows:

Vrms = 1/√2 × Vmax

or

Vrms ≈ 0.707 × Vmax

Root mean square values are much more accurate at telling us the effective voltage of an audio signal.

Frequency And Amplitude

The various frequencies of a sound will have differing amplitudes. This creates some really complex sinusoidal waveforms!

However, all sound can be reduced to sine waves if we dive deep enough. Even the most complex audio signals are “simply” made of multiple simple sine waves interfering with each other at various frequencies and amplitudes.

The basis of additive synthesis is even based on the stacking of many individual sine waves to create complex sounds! That topic is for another blog.

Other Notes On Alternating Current And Audio Signals

AC signals, as opposed to DC signals, have the ability to pass through capacitors and transistors which are two important components in microphone circuitry.

The positive and negative peaks of audio signals do not necessarily have the same absolute values as they do in a simple sine wave.

The electricity our societies run on is also AC. The power transmission in most countries runs at a utility frequency of 50 Hz or 60 Hz.
In North America, the standard is 120 volts AC at 60 Hz. This power transmission current can even find its way into our audio signals through electromagnetic interference (EMI), inducing the infamous “60 cycle hum.”
In Europe and other parts of the world, the same issue applies but at 50 Hz.

Now that we know what sine waves and audio signals are, let’s talk about how microphones create audio signals.


A Microphone Is A Transducer Of Energy

So we’ve discussed that a microphone’s audio signal is an electrical representation of a captured sound. How exactly do microphones capture sound? The answer depends on the microphone and the transducer principle the microphone works on.

A transducer is simply a device that converts one form of energy to another. In the case of microphones, the conversion is from sound waves (mechanical wave energy) to audio signals (electrical energy).

Nearly all types of microphones have a diaphragm. Diaphragms vibrate in reaction to external sound waves. Sound waves cause small air pressure differences between the front and back of a microphone diaphragm, causing it to move.

The way this diaphragm movement is then converted to an audio signal depends on the microphone transducer type.

There are 2 common types of microphone transducers:

  1. Electromagnetic transducer (moving-coil & ribbon “dynamic” microphones)
  2. Electrostatic transducer (true condenser & electret microphones)

Let’s touch on each of these transducer types in a bit more detail:

Dynamic Microphone Transducers

Dynamic microphones include both the common moving-coil and ribbon microphones.

Moving-coil microphones are most often referred to as simply “dynamic microphones.”

Dynamic microphones convert mechanical wave energy into electrical energy by means of electromagnetism. More specifically, they convert sound into an audio signal through electromagnetic induction. Let’s discuss moving-coil and ribbon microphones separately here.

Moving-Coil Dynamic Microphone Transducer

A moving-coil microphone has a coil of conductive wire (typically copper) attached to its diaphragm. As the diaphragm moves back and forth from the rest position according to sound waves, the coil will oscillate with it. Hence the name “moving-coil.”

This moving coil is suspended in a gap between magnets within the microphone capsule. This means the conductive copper wire coil is moving through a magnetic field.

Through the process of electromagnetic induction, a voltage is created across the conductive coil as it moves through the magnetic field. Because the coil oscillates back and forth, this voltage is AC!

A lead is taken from each end of the moving-coil, which is effectively the audio signal! This microphone AC voltage audio signal is most often sent to a step-up transformer which effectively boosts the signal for a stronger microphone output voltage.

For more information on moving-coil dynamic mics, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.

Ribbon Dynamic Microphone Transducer

A ribbon microphone’s diaphragm is also the conductive material involved in the electromagnetic induction. The diaphragm (otherwise known as the ribbon) is long and thin (like a ribbon) and is often corrugated. A typical conductive ribbon material is aluminum foil.

The ribbon is positioned closely between magnets inside the microphone baffle (capsule). As sound waves cause the ribbon diaphragm to vibrate, it moves within a magnetic field.

A lead is taken from each end of the ribbon diaphragm, which is effectively the audio signal! This AC voltage is very small and often completes a circuit with a step-up transformer to boost the signal at the microphone output.

Alternatively, with active ribbon mics, the microphone audio signal is sent through an active preamplifier circuit before being outputted.

For more information on dynamic ribbon mics, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.

Note that no external power is needed for the electromagnetic transducers!

Condenser Microphone Transducers

There are two general types of condenser microphones: the true condenser and the electret condenser. Both these microphones work based on an electrostatic principle and have capacitor style capsules.

The capsule of a condenser microphone consists of a parallel plate capacitor. The front plate is movable and acts as a diaphragm while the backplate is stationary. As the diaphragm plate vibrates in reaction to sound waves, the capsule’s capacitance changes. If the capacitor has a fixed charged (which it needs to operate), the voltage of the capacitor will vary proportionally to the variance in capacitance. In other words, as the diaphragm moves back and forth across its resting position, an AC voltage (audio signal) will be outputted.

True condenser microphones require an external voltage in order to charge their capacitor diaphragms. The majority of the time, this external voltage is supplied by phantom power (+48 V DC).

Electret condenser microphone capsules have a permanently fixed charge due to their electret material. Electret materials have quasi-permanent electric charge since they’re created with dipole polarization. This is all to say that they provide the constant charge between the diaphragm and backplate, allowing their condenser capsule to work properly.

So both types of condenser mics work with a fixed charge “Q” on the capsules. As the capacitance “C” changes with the movement of the diaphragm, an output AC voltage “V” (audio signal) is created. This is according to the following equation

V=Q/C

Condenser microphones are active microphones. The microphone signal produced from the capacitor style capsule must undergo processes to lower the signal impedance as well as boost the amplitude of the signal. This is generally accomplished with an active circuit that includes op-amps and/or JFETs.

To learn more about microphones as transducers, feel free to check out the following My New Microphone articles:

How Do Microphones Work? (A Helpful Illustrated Guide).
Microphone Types: The 2 Primary Transducer Types + 5 Subtypes.
Differences Between Dynamic, Condenser, & Ribbon Microphones.


Mic Level Versus Line Level And Speaker Level

Since this article is more specifically about microphone audio signals, I figured I’d run through a typical path a microphone audio signal may take. Let’s discuss a microphone’s audio signal from its beginning inside a microphone to the point it cause a loudspeaker to produce a sound.

Mic Level

So a microphone converts sound into an audio signal. This audio signal is very low in level and is referred to as mic level. Within the microphone itself, there are ways to boost this level slightly.

The passive way to boost a microphone’s output is with a step-up transformer. A step-up transformer works on the principle of electromagnetic induction to boost the electrical signal at a microphone’s output. Many dynamic microphones utilize step-up transformers at their outputs.

The active way to boost a microphone’s output is with an operational amplifier (opamp) within the active circuitry. These amplifier circuits are typically capable of boosting a microphone signal more than a step-up transformer before distortion. This is why active ribbon and condenser microphones generally have higher sensitivity ratings than passive ribbon and dynamic moving-coil microphones.

Still, though, the audio signal at a microphone’s output is very low in level. Its AC voltage (remember this is an rms value) is likely only a few millivolts. This level of an audio signal is referred to as mic level.

Mic level is generally agreed to be between 1 millivolt (−60 dBV) and 100 millivolts (−20 dBV). These are nominal values. Of course, the actual output voltage of a microphone depends highly on the mic sensitivity and the loudness/proximity of the sound source. It’s quite possible to have voltages well below 1 millivolt and well above 100 millivolts.

Line Level

In order for a microphone audio signal to work well within professional audio equipment, it must be amplified to line level.

Professional line level, nominally, is generally specified as +4 dBu. To keep consistent units, this is roughly 1.78 dBV.

dBV and dBu are both decibels relative to a voltage. However, their reference points are different:

  • 0 dBV = 1 volts
  • 0 dBu = 0.775 volts

Consumer line level is nominally −10 dBV, but we won’t talk about that…

So line level (~1.78 dBV) is roughly 22 to 62 dB greater than mic level. This means the AC voltage of a line level signal is roughly 12.6 to 1260 times that of a mic level signal.

This is where microphone preamplifiers come into play. A quality preamp will deliver the necessary clean gain to a mic level signal to bring it up to line level.

Line level is used in nearly all professional audio equipment. Mic level signals are those signals produced by microphones.

However, to drive large speakers, we need to boost the audio signal even more to what is known as speaker level.

Speaker Level

Speaker level varies from one loudspeaker to the next. It really depends on the size and capacity of a loudspeaker to recreate sound.

Speaker levels can range from nearly the same as line level (1 volt) to 100 volts and above.

The amplification required to bring our audio signals to this level needs to be very clean to reproduce quality audio. The amplifiers may be standalone (to feed passive loudspeakers) or built-in (to active loudspeakers).

For a detailed read on the various audio signal levels, check out my article Do Microphones Output Mic, Line, Or Instrument Level Signals?

Amplification Of A Microphone Audio Signal

So there are a few different gain stages to bring a microphone audio signal from the mic capsule to the speaker. In the most basic setup, there may be the following:

  • A slight boost in the microphone itself.
  • Some gain at the mic preamp.
  • Some more gain at the speaker amplifier.

It’s awesome to think an audio signal could potentially be brought from 0.001 volts to 100 volts and still retain its sound quality!

For more information on microphone amplification and gain, check out my article What Is Microphone Gain And How Does It Affect Mic Signals?


Balanced Audio

So now we understand what a microphone audio signal is and where it should go. But we don’t exactly know how the signal travels from device to device. Let’s talk about the professional way of transferring microphone audio signals.

Professional microphones typically have XLR output connections. The 3 pins in the XLR-style audio cable carry balanced audio.

To learn more about balanced audio, check out my article Do Microphones Output Balanced Or Unbalanced Audio?

Let’s discuss how microphone audio signals travel from the microphone to their preamps.

XLR Cables And Connections

As mentioned, XLR cables have 3 pins to carry audio:

  • Pin 1: Ground/Shield
  • Pin 2: Hot/Positive
  • Pin 3: Cold/Negative

Nearly all professional analog microphones have XLR output connections and utilize the Ground, Hot, and Cold pins to transfer audio signals.

Pin 1 is chassis ground. The voltages on Pins 2 and 3 are relative to the ground pin. Pin 1 also provides a cable shield for professional XLR cables, which aids tremendously in reducing electromagnetic and radio frequency interference.

Pin 2 is the positive polarity terminal while Pin 3 is the negative polarity terminal. When a microphone outputs its audio signal through XLR, it sends the signal down Pins 2 and 3. Pin 2 has the “regular polarity” audio signal and Pin 3 has the same audio signal, only in reverse polarity.

So if we were to sum up the two AC voltages on pins 2 and 3 in the XLR cable, we’d have zero volts along the entire cable. However, any possible interference (electromagnetic, radio, or other) will show up on both pins 2 and 3 (relative to pin 1).

When the microphone audio signal reaches the preamplifier, a differential amplifier boosts the signal. A differential amplifier takes the difference between pins 2 and 3. This effectively sums the audio portion of pins 2/3 while eliminating any interference.

This elimination of interference is known as “common mode rejection” and is highly effective.

For more information on microphones and XLR cables/connectors, check out my article Why Do Microphones Use XLR Cables?


Wireless Audio Transmission

There is another way to send balanced audio from a microphone: wirelessly!

Wireless microphones have radio transmitters that “package up” and send their audio signals via RF frequencies to compatible radio receivers. The output of the receiver is often an XLR connection that may then be plugged into the mixing console.

For more information on wireless microphones, check out my article How Do Wireless Microphones Work?


Storage Of Audio Signals

When it comes down to recording the audio signals, we must have some way of storing them. Recording audio allows us to manipulate, mix together, reproduce, and audio signals without having to recreate them live. We hear stored/recorded audio every day in the music we listen to and the videos we watch. If it’s not live in front of you or a live broadcast, chances are it’s “stored!”

There are 2 general ways of storing recording audio:

  1. Analog storage
  2. Digital storage

Analog Storage Of Audio Signals

Magnetic tape was the basis for nearly all commercial audio recordings from 1950 to 1980 and is still used today for “that analog sound.” Common storage methods of analog audio on tape include tape reels (older acetate versions and newer polyester versions), compact audio cassettes, 8-tracks, and, of course, the vinyl record.

Magnetic tape recording utilizes amplified electrical audio signals (microphone signals or otherwise) to produce corresponding variations of the magnetic field of the tape head. The tape head is stationary and the tape itself moves across the tape head as recording takes place. The varying magnetic field at the tape head impresses analogous variations of magnetization on the moving tape.

This is similar to the electromagnetic principle of the dynamic microphone transducer, only in reverse. With magnetic tape recording, we’re applying an electrical signal to the tape head which then creates a varying magnetic field. With a dynamic microphone, we’re varying a magnetic field to create an electrical signal.

The big idea behind the recording and storing of audio is, of course, to play it back! In “playback mode,” the signal path of the magnetic tape recording setup is reverse. The tape, now with recorded magnetic variations (audio signals) impressed upon it, moves across the tape head. The tape head now acts as a miniature electric generator, turning the magnetic variations into corresponding electrical audio signals!

As with any physical storage, and especially on anything as fragile as tape, care must be taken with storage.

Another drawback of analog storage of audio lies in the recreation. Each time tape is rerecorded (from one tape to another) its magnetic imprint gets weaker. This is known as generation loss. It’s also an issue that each time we record on a tape, it gets weaker as a recording medium.

Digital Storage Of Audio Signals

Digital audio recording became feasible in the late 1970s and is now the standard for recording audio. Audio signals are encoded as numerical samples in a continuous sequence. Basically, digital audio is “a bunch of 1’s and 0’s” in a computer. The common storage methods of digital audio are compact discs (CDs), hard drives, and “online” through servers.

So how do electrical AC voltage audio signals get recorded and stored as digital information? The conversion happens within an analog-to-digital converter (ADC) typically through the process of pulse-code modulation.

USB microphones have built-in ADCs. External ADCs must be used to convert electrical audio signals into digital audio signals. Any audio interface is an ADC.

Once the audio is encoded as digital information, we may manipulate it in many ways to get desired (or undesired) results.

Unlike analog audio editing, digital typically has the handy “undo” function. Another huge benefit of digital audio over analog audio is that there’s no generation loss in digital. A digital audio recording can be duplicated any number of times without any worsening of quality.

When playing back digitally recorded audio, a digital-to-analog converter must be utilized. As the name suggests, the DAC converts the digital “1’s and 0’s” into an electrical signal that may then feed loudspeaker devices!

For more information on connecting microphones to computers, check out my article How To Connect A Microphone To A Computer.


Related Questions

Why is phantom power DC rather than AC? Phantom power has developed as DC to easily separate it from the AC audio signals in the circuitry of a microphone. Transistors and capacitors allow AC signals to “pass” but “block” DC signals. The components are common in mic circuitry, allowing phantom power to do its job safely.

For more information on phantom power, check out my article Do Microphones Need Phantom Power To Work Properly?

Is there such a thing as a digital microphone? Technically no. USB mics come close but are actually analog mics with built-in analog-to-digital converters. A “digital microphone,” by definition, would convert mechanical wave energy (sound waves) into digital information. There is currently nothing on the market to do that.

To learn more about the intricacies of analog mics and “digital” mics, please consider reading my article Are Microphones Analog Or Digital Devices? (Mic Output Designs).

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