Units Of Measurement & Prefixes In Sound & Audio Electronics


Sound and audio are regularly measured in order to improve our understanding of how audio equipment performs.

There are many units of measurement and prefixes that often accompany these measurements. In this article, we will go through the definitions of common units of measurement; how they work within the context of sound and audio, and the prefixes we will often find accompanying the units.

Here is a table of the common measurements and the units of measurement we come across in the study of sound and audio:

Unit/MeasurementQuantitySymbol
HertzFrequencyHz
DecibelLogarithmic Ratio
dB
OhmImpedance
Resistance
Reactance
Ω
VoltVoltageV
AmpereCurrentA
DegreePhase
Angle
º
PascalPressurePa
WattPowerW
SecondTimes
GramWeightg
Pound/Ounce (Imperial)Weightlb/oz
MeterLengthm
Foot/Inch (Imperial)Lengthft/in
PercentagePercentage
%
Degrees CelsiusTemperatureºC
Degrees Farenheit (Imperial)TemperatureºF
BitBit DepthBit
FaradCapacitanceF
CoulombChargeC
NewtonForceN
TeslaMagnetic Flux DensityT
HenryInductanceH

The following is a list of the prefixes used in our measurements:

Metric PrefixSymbolMultiplier/Exponential
TeraT1012
GigaG109
MegaM106
Kilok103
BaseN/A100
Decid10-1
Centic10-2
Millim10-3
Microµ10-6
Nanon10-9
Picop10-12

Hertz

Hertz (Hz), measured in cycles per second are units of frequency.

In audio, frequency is used to define the bandwidth and pitch of audio signals; the sample rates of digital audio resolution and the wireless carrier signals used in wireless audio transmission.

The range of hum hearing spans from 20 Hz – 20,000 Hz with ultrasound above and infrasound below this range. Most audio equipment aims to be capable of processing, producing and/or reproducing audio in this frequency range.

The sample rate of digital audio (how many times the audio signal is sampled per second) is typically measure in kiloHertz (kHz) and defines the resolution of the digital audio signal.

Common sample rates include:

  • 44.1 kHz
  • 48 kHz
  • 88.2 kHz
  • 96 kHz
  • 176.4 kHz
  • 192 kHz

Frequency also applies to the radio frequencies used as carrier signals in wireless audio signal transfer. Think of AM radio (about 550 to 1720 kHz) and FM radio (88 to 108 MHz). Bluetooth and many other personal wireless audio connections use radio frequencies in the range of 2.400 to 2.4835 GHz.


Decibel

Decibels refer to 1/10th of a Bel. I have only ever seen decibels be used in audio.

The decibel is a relative unit of measurement (a ratio). Decibels express the ratio of one value of a power or root power quantity to another on a logarithmic scale.

Decibels are somewhat tricky to understand and apply to plenty of audio applications.

This is because decibels apply to power quantities, root power quantities, and even to perceived quantities.

Power Quantities

Power quantities are directly proportional to power and energy.

Power quantities include:

Root-Power Quantities

Root power quantities are quantities that have square root values proportional to power. The term “root power quantity” has replaced the term “field quantity”.

Root-power quantities include:

  • Sound pressure
  • Voltage
  • Current
  • Electric field strength
  • Speed
  • Charge density

Perceived Quantities

Perceived quantities are defined by how we perceive power and root power quantities. We nearly always perceive less than the level of the power or root power quantity.

Perceived loudness is the psychoacoustic perceived quantity we’re concerned with in audio technology.

Decibels actually help simplify things by amalgamating a ratio for power, root power and perceived quantities. A specified decibel ratio will unify these various quantities but will mean different actual ratios for each type of quantity.

For example, a 20 dB difference means a:

  • 1:100 (or 100:1) for power quantities
  • 1:10 (or 10:1) for root-power quantities
  • 1:4 (or 4:1) for loudness quantities

So, let’s say we wanted to, theoretically, increase perceived loudness by 4x, we’d require a dB increase (a 10x increase in sound pressure level at the listening point and a 100x sound intensity increase at the listening point).

Here is a table relating dB ratings to power quantities, root-power quantities and perceived quantities:

dB Change
(Ratio)
Power Quantity Multiplier
• Acoustic Power
• Electrical Power
• Sound Intensity
Root Power Quantity Multiplier
• Voltage
• Current
• Sound Pressure Level
Perceived Quantity Multiplier
• Loudness/Volume
+60 dB1,000,000x√1,000,000
1,000x
64x
+50 dB100,000x√100,000x
316x
32x
+40 dB10,000x√10,000x
100x
16x
+30 dB1,000x√1,000x
31.6x
8x
+20 dB100x√100x
10x
4x
+10 dB10x√10x
3.16x
2x
+6 dB4x√4x
2x
1.52x
+3 dB2x√2x
1.414x
1.36x
0 dB1x1x1x
-3 dB1/2x
0.5x
1/√2x
0.707x
0.816x
-6 dB1/4x
0.25x
1/√4x
0.5x
0.660x
-10 dB1/10x
0.1x
1/√10x
0.316x
0.5x
-20 dB1/100x
0.01x
1/√100x
0.1x
0.25x
-30 dB1/1,000x
0.001x
1/√1,000x
0.031,6x
0.125x
-40 dB1/10,000x
0.000,1x
1/√10,000x
0.01x
0.0625x
-50 dB1/100,000x
0.000,01x
1/√100,000x
0.003,16x
0.03125x
-60 dB1/1,000,000x
0.000,001x
1/√1,000,000x
0.001x
0.015625x

Change in power quantity:

∆P = 10 • log10x

Change in root power quantity:

∆PR = 20 • log10x

Change in loudness (psychoacoustics):

∆L = 10 • log2x

Another, less common way of looking at decibels is relative to the true linear ratio of power, root power and perceived quantities:

Ratio (X:1)
(Linear)
dB Change in Power Quantity
• Acoustic Power
• Electrical Power
• Sound Intensity
dB Change in Root Power Quantity
• Voltage
• Current
• Sound Pressure Level
dB Change in Perceived Quantity
• Loudness/Volume
40+16.02 dB+32.04 dB+53.22 dB
30+14.77 dB+29.54 dB+49.07 dB
20+13.01 dB+26.02 dB+43.22 dB
15+11.76 dB+23.52 dB+39.07 dB
10+10 dB+20 dB+33.22 dB
5+6.99 dB+13.98 dB+23.22 dB
4+6.02 dB+12.04 dB+20 dB
3+4.77 dB+9.54 dB+15.58 dB
2+3.01 dB+6.02 dB+10 dB
1± 0 dB± 0 dB± 0 dB
1/2-3.01 dB-6.02 dB-10 dB
1/3-4.77 dB-9.54 dB-15.58 dB
1/4-6.02 dB-12.04 dB-20 dB
1/5-6.99 dB-13.98 dB-23.22 dB
1/10-10 dB-20 dB-33.22 dB
1/15-11.76 dB-23.52 dB-39.07 dB
1/20-13.01 dB-26.02 dB-43.22 dB
1/30-14.77 dB-29.54 dB-49.07 dB
1/40-16.02 dB-32.04 dB-53.22 dB

Change in power quantity:

x = 10∆L/10

Change in root power quantity:

x = 10∆L/20

Change in loudness (psychoacoustics):

x = 2∆L/10

Decibels are regularly used to purvey audio equipment data. Let’s look at the ways in which decibels are used in audio:

  • Gain
  • Signal level
    • Maximum input level
    • Dynamic range
    • Transformer insertion loss
  • Noise
    • Common-mode rejection ratio
    • Crosstalk/channel separation
    • Self-noise (equivalent noise)
    • Signal-to-noise ratio
  • Power transfer
  • Sound pressure level
    • Sensitivity
    • Maximum sound pressure level
  • Passive attenuation device (pad)
  • Tolerance
    • Frequency response
    • Power bandwidth
    • Driver matching
    • Polar response
    • Coverage angle
  • EQ
    • Boosts/cuts
    • Filters/crossovers (dB/octave)
  • Noise cancellation
    • Passive noise cancellation
    • Active noise cancellation

Gain

Decibels are perhaps most commonly used to define gain. Gain is technically the ratio of an amplified output signal to the input signal (pre-amplification).

Gain is a unitless measurement that is technically defined as a ratio linear. Decibels provide a logarithmic scale for this ratio.

Amplification of audio signals is standard and gain is, too. Decibels are the conventional way to measure and state gain in audio amplifiers.

Related articles:
What Is Microphone Gain And How Does It Affect Mic Signals?

Signal Level

Decibels are very common in defining the level of an audio signal.

The two most common audio signal level decibel measurements are:

  • dBV: decibels relative to a voltage of 1 volt.
    • 1 volt = 0 dBV
  • dBu: decibels relative to a voltage of 0.7746 volts.
    • 0.7746 volt = 0 dBu
  • dBFS: decibels relative to the digital ceiling of 0 dBFS.

To put these two ratings into further perspective:

  • Consumer nominal line level = -10 dBV (0.3162 volts)
  • Professional line level = +4 dBu (1.228 volts)
  • Digital audio clipping happens above 0 dBFS

Here are some formulae for defining the analog audio signal levels (voltage):

Level (in dB) = 20 • log (V / V0)

Voltage (in volts) = V0 • 10L (in dB) / 20

Where V0 = 1 V to measure dBV and V0 = 0.7746 to measure dBu.

These decibel units are used extensively to measure signal level.

dBV, dBu and dBFS can be found in the following audio specifications:

  • Maximum Input Level: the maximum signal strength that can drive an input without significant distortion and overload.
  • Dynamic Range: the range between the quietest possible signal (noise floor) and the loudest possible signal.
  • Transformer Insertion Loss: any loss of signal when connecting distributed speakers into a distributed system.

Noise

Decibel values are also often used to specify noise in a signal.

Common noise specifications for audio equipment include:

  • Common-Mode Rejection Ratio: the amount of noise/interference rejection that happens as a result of the differential amplifier in a balanced input.
  • Crosstalk/Channel Separation: the amount of signal that will spill from the right channel into the left channel and vice versa.
  • Self-Noise (Equivalent Noise): the inherent noise produced by the electronics of an active audio device.
  • Signal-To-Noise Ratio: the ratio of the intended signal to the unintended noise in an overall audio signal.

Related articles:
What Is Microphone Self-Noise? (Equivalent Noise Level)
What Is A Good Signal-To-Noise Ratio For A Microphone?

Power Transfer

Power transfer refers to the amount of power that is transferred between audio devices (how much power is dissipated at the load). It is another way to tell us the strength of an audio signal.

Audio power is often measured in watts when it applied to power amplifiers and loudspeakers.

In other audio devices, it is often defined in dBm (decibels as referenced to 1 milliwatt).

0 dBm = 1 mW.

Technically speaking, this is a reference to power only and does not consider voltage, current or resistance/impedance.

That being said, conventions allow us to assume dBm to be referenced to 1 milliwatt dissipated into a 600Ω load.

With a 600Ω load, 0.775 V (0 dBu) will produce 1 mW (0 dBm).

dBm is not overly used anymore because of this assumption.

The equation to calculate dBm is as follows:

dBm = 10 • log (P / P0)

Where P0 = 1 mW

Sound Pressure Level

Sound pressure level (SPL) measurements will tell us a lot about the strength of the sound waves. SPL can be measured linearly in Pascal (SI unit) or in pounds-per-square-inch (imperial unit).

However, SPL is more often measured in dB SPL. That is decibels of sound pressure relative to the threshold of human hearing (20 x 10-6 Pa or 20 µPa).

The equation to calculate dB SPL from typical Pascal pressure measurements is as follows:

dB SPL = 20 • log (P / P0)

Where P0 = 20 x 10-6 Pa

Here is a table relating dB SPL values to Pascal values and common sources of the various levels.

dB SPLPascalSound Source Example
0 dB SPL0.00002 PaThreshold of hearing
10 dB SPL0.000063 PaLeaves rustling in the distance
20 dB SPL0.0002 PaBackground of a soundproof studio
30 dB SPL0.00063 PaQuiet bedroom at night
40 dB SPL0.002 PaQuiet library
50 dB SPL0.0063 PaAverage household with no talking
60 dB SPL0.02 PaNormal conversational level (1 meter distance)
70 dB SPL0.063 PaVacuum cleaner (1 meter distance)
80 dB SPL0.2 PaAverage city traffic
90 dB SPL0.63 PaTransport truck (10 meters)
100 dB SPL2 PaJackhammer
110 dB SPL6.3 PaThreshold of discomfort
120 dB SPL20 PaAmbulance siren
130 dB SPL63 PaJet engine taking off
140 dB SPL200 PaThreshold of pain

As an additional resource, here is a table listing out the safe exposure times for humans at various SPL listening levels (as defined by the National Institute for Occupational Safety and Health and Occupational Safety and Health Administration):

NIOSH Standard (dBA)Equivalent Sound Pressure Level (at 1 kHz)Maximum Exposure Time LimitOSHA Standard (dBA)Equivalent Sound Pressure Level (at 1 kHz)
127 dBA127 dB SPL
44.8 Pa
1 second160 dBA160 dB SPL
2.00 kPa
124 dBA124 dB SPL
31.7 Pa
3 seconds155 dBA155 dB SPL
1.12 kPa
121 dBA121 dB SPL
22.4 Pa
7 seconds150 dBA150 dB SPL
632 Pa
118 dBA118 dB SPL
12.6 Pa
14 seconds145 dBA145 dB SPL
356 Pa
115 dBA115 dB SPL
11.2 Pa
28 seconds140 dBA140 dB SPL
200 Pa
112 dBA112 dB SPL
7.96 Pa
56 seconds135 dBA135 dB SPL
112 Pa
109 dBA109 dB SPL
5.64 Pa
1 minute 52 seconds130 dBA130 dB SPL
63.2 Pa
106 dBA106 dB SPL
3.99 Pa
3 minutes 45 seconds125 dBA125 dB SPL
35.6 Pa
103 dBA103 dB SPL
2.83 Pa
7 minutes 30 seconds120 dBA120 dB SPL
20.0 Pa
100 dBA100 dB SPL
2.00 Pa
15 minutes115 dBA115 dB SPL
11.2 Pa
97 dBA97 dB SPL
1.42 Pa
30 minutes110 dBA110 dB SPL
6.32 Pa
94 dBA94 dB SPL
1.00 Pa
1 hour105 dBA105 dB SPL
3.56 Pa
91 dBA91 dB SPL
0.71 Pa
2 hours100 dBA100 dB SPL
2.00 Pa
88 dBA88 dB SPL
0.50 Pa
4 hours95 dBA95 dB SPL
1.12 Pa
85 dBA85 dB SPL
0.36 Pa
8 hours90 dBA90 dB SPL
0.63 Pa
82 dBA82 dB SPL
0.25 Pa
16 hours85 dBA85 dB SPL
0.36 Pa

dB SPL is important when defining audio transducers (the devices that convert sound waves or vibrations into audio signals or audio signals into sound waves).

Audio transducers include microphones, headphones, loudspeakers and more.

Of these devices, dB SPL values are used to describe a few things, mainly:

  • Sensitivity
  • Maximum Sound Pressure Level

Sensitivity

Microphone sensitivity tells us how much signal level the microphone will output for a given sound pressure level at its diaphragm.

Headphone sensitivity tells us how much sound pressure the headphone will produce (at the ear of the listen when the headphone is worn as intended) when a given signal level (typically measured as 1 mW) is applied to it.

Loudspeaker sensitivity tells us how much sound pressure level, at a given distance (typically 1 meter), a loudspeaker will produce when a certain signal level (generally measured at 1 watt or 2.83 volts) is applied to it.

Related articles:
Full Guide To Loudspeaker Sensitivity & Efficiency Ratings
The Complete Guide To Headphones Sensitivity Ratings
What Is Microphone Sensitivity? An In-Depth Description

Maximum Sound Pressure Level

The maximum sound pressure level of a microphones tells us how much SPL the microphone can effectively convert into an audio signal without distorting.

The maximum sound pressure level (often referred to as maximum output level) for headphones and loudspeakers refers to the maximum SPL (measured at a given distance) the device will produce without significant distortion.

Related article:
What Does A Microphone’s Maximum Sound Pressure Level Actually Mean?

Passive Attenuation Device (Pad)

A pad is a passive switchable circuit that is featured in some audio equipment that works to drop the level of the signal by a defined amount. The amount of attenuation is predetermined in the pad design and is generally measured in decibels.

More specifically, pad values are defined in negative dB values (as they compare to the attenuated output signal level to the input signal level).

Related article:
What Is A Microphone Attenuation Pad And What Does It Do?

Tolerance

Tolerance is the “margin or error” or “margin of fluctuation” that helps give other specifications meaning.

The tolerance is typically measured in decibels (as a comparison to the average value or the on-point value). It is the measurements after the “+/-” or “±” signs.

Having a tolerance gives us a much better idea of any ranges in audio. It can commonly be seen in the following audio device specifications:

  • Frequency Response
  • Power Bandwidth
  • Driver Matching
  • Cutoff Frequency
  • Polar Response
  • Coverage Angle

Frequency Response

Frequency response refers to the frequency-specific sensitivity of an audio device.

In other words, how well (and evenly) will an audio device produce, reproduce, or process the audio signal. Will the audio device colour the signal by cutting out certain frequencies while reducing or boosting some others?

Note that the range of human hearing is 20 Hz to 20,000 Hz.

Frequency response is best shown with a graph:

  • Frequency (in Hertz) along the X-axis
  • Sensitivity (in decibels) along the Y-axis

Here is the Shure SM57 (link to compare prices at select retailers) frequency response graph as an example. We can see that:

  1. The SM57 is incapable of producing 20 Hz to 20,000 Hz.
  2. Not all frequencies that the microphone will output will be equally represented in the output signal.

Frequency response, however, is generally only defined as a range between the lowest frequency the unit is capable of handling and the highest frequency the unit is capable of handling.

The ranges are fairly useless without some sort of tolerance value that will tell us the points at which the unit’s frequency-dependent processing/sensitivity will fall off.

As an example, 20 Hz – 20,000 Hz is much less descriptive as 20 Hz – 20,000 Hz ± 3 dB.

Related articles:
What Is Headphone Frequency Response & What Is A Good Range?
Complete Guide To Microphone Frequency Response (With Mic Examples)

Power Bandwidth

This specification is pretty much the same as frequency response. It refer to the bandwidth (frequency range) than an amplifier can effectively output.

Though a graph would best for comprehension, a tolerance value (measured in dB) is very useful to understand power bandwidth.

Driver Matching

This is a headphone specification that tells us the maximum room for error for the relative output levels of the two drivers (left and right).

Polar Response

The polar response (also known as the polar pattern) of a microphone refers to the directionality of that microphone.

In other words, it tells us, relative to the on-axis direction of the microphone, how the microphone will pick up sound from all other directions.

This microphone specification is best defined with a graph, like that of the aforementioned Shure SM57 cardioid microphone. We can see from the graphs below that:

  • Polar pattern varies with frequency (becoming more directional at higher frequencies).
  • The most sensitive point is on-axis (0º) and is set at 0 dB. All other angles are relative to that 0 dB reference.

Aside (or instead of) a graph, a microphone may have a qualitative polar pattern title (such as the cardioid pattern mentioned above). It may also have an angle of acceptance with a “tolerance” or cutoff frequency.

For example, the Shure SM57 could have a pickup pattern acceptance angle of 60º ± 3 dB

Related articles:
The Complete Guide To Microphone Polar Patterns
What Is An Omnidirectional Microphone? (Polar Pattern + Mic Examples)
What Is A Supercardioid Microphone? (Polar Pattern + Mic Examples)
What Is A Hypercardioid Microphone? (Polar Pattern + Mic Examples)
The Lobar/Shotgun Microphone Polar Pattern (With Mic Examples)
The Hemispherical Boundary Microphone/PZM Polar Pattern
What Is A Cardioid Microphone? (Polar Pattern + Mic Examples)

Coverage Angle

Coverage angle refer to the output of a loudspeaker. Most speakers are at least somewhat directional (especially at mid and upper frequencies) due to their relatively large drivers and enclosures.

Decibels relative to the on-axis response are useful for determining a set cutoff point/threshold at which the speaker’s directionality can be defined by.

EQ

Decibels are used to define the frequency-dependent cutting and boosting that happens with audio equalization.

Filters (including those used in speaker crossovers) can also be defined using decibels. More specifically, filters are largely defined by the roll-off (decibels/octave) in which they reduce the level of the audio.

Noise Cancellation

Noise cancellation is a headphone specification that tells us how much the headphone will block out external noise. This is typically measured in decibels relative to the “noise” we would otherwise hear had we not worn the headphones.

This is true of passive and active noise cancellation.

Passive noise cancellation is the simple mechanical blockage of sound waves from entering the ear canal.

Active noise cancellation refers to the use of complex circuits complete with microphones; feed-forward and feedback circuitry; phase and volume adjustments, and speakers to inject the anti-noise sound waves into the headphone output.

Related articles:
Passive Vs. Active Noise-Cancelling Headphones
How Do Noise-Cancelling Headphones Work? (PNC & ANC)
Do Noise-Cancelling Headphones Work With Or Without Music?


Ohm

The ohm is the unit of measurement for several important electrical quantities that have to do with audio devices. These quantities are:

  • Electrical Resistance
  • Electrical Impedance
  • Electrical Reactance

Electrical Resistance

Electrical resistance (R) is the opposition to current flow in an electrical circuit. More specifically, it is the opposition to direct current (DC).

1 Ohm is defined as the resistance of a conductor with a potential of 1 volt applied to the ends through which 1 ampere of current flows.

Though analog audio signals are AC (alternating current), resisitance is still important.

Most audio devices have resistors in their circuits. If they don’t, there’s at least natural resistance that happens in the components and wiring of the circuit.

Resistance also acts as a sort of placeholder for impedance to help simplify equations in the world of audio.

Electrical Impedance

We mentioned that audio signals are alternating currents. Therefore, electrical impedance (Z) is a factor in audio signal transfer.

Electrical impedance essentially extends the idea of resistance to AC circuits. Impedance, therefore, has both magnitude and phase components and is frequency-dependent. It is made up of electrical resistance (real) as well as electrical reactance (imaginary).

Impedance is actually one of the most important (and misunderstood) concepts for audio devices. It applies to all audio equipment.

The input and/or output impedances are of particular importance to audio equipment.

The internal impedance of the circuits is also critical but is typically designed so that the device will work as it’s supposed to.

Input and output impedances are of special importance because inputs and output are the connections that connect one audio device to another. Sending an audio signal from a source’s output to a load’s input requires appropriate source (output) impedance values and load (input) impedance values.

A general rule of thumb for decent voltage/signal transfer from one device to another is to have a load impedance at least 10 times greater than the source impedance. This Zload > Zsource condition holds true of all audio connections where the source is sending audio signal to the load.

We can have a look at a simple voltage divider to understand why:

VL = VS • [ZL / (ZS + ZL)]

So we can see that the larger ZLoad is relative to ZSource, the closer VLoad will be to VSource. Put in a different way, this means better voltage/signal transfer.

The damping factor of an amplifier-loudspeaker connection is defined as the ratio of ZLoad to ZSource. Higher damping factors allow for more amplifier control and cleaner sound, up to a point.

Electrical Reactance

Electrical reactance (X) is the opposition of electrical current due to the inductance or capacitance of a circuit element.

Reactance affects the phase and amplitude of an audio component’s impedance.

This can be seen in loudspeaker and headphone drivers, where inductive reactance increases the impedance in the high frequencies.

Related articles:
The Complete Guide To Speaker Impedance (2Ω, 4Ω, 8Ω & More)
What Is Amplifier Impedance? (Actual Vs. Rated Impedance)
The Complete Guide To Understanding Headphone Impedance
Microphone Impedance: What Is It And Why Is It Important?
What Is Damping Factor Between An Amplifier & Loudspeaker?


Volt

The volt is the unit of voltage or electrical potential. It is a measurement of the difference in electric potential between two points.

1 volt is equal to 1 joule of work per 1 coulomb of charge.

Voltage is required for many reasons in audio technology.

First, audio signals themselves are defined by AC voltage. Signal strength is often measured in volts (or millivolts); dBV (decibels relative to 1 volt), or dBu (decibels relative to 0.7746 volts).

Second, active audio devices require a power source and, in many cases, a power supply to provide proper biasing voltage to work properly.

Here are a few examples:

  • Power amplifiers require both positive and negative voltage rails.
  • Condenser microphones require DC voltage to drive their impedance converters and, in some cases, to polarize their capsules (often via +48 VDC phantom power).

This voltage is part of powering the active circuitry.

Related article:
What Is Phantom Power And How Does It Work With Microphones?
Why Do Audio Amplifiers & Preamplifiers Need Power To Work?


Ampere

The ampere (often shortened to “amp”) is a measurement of electrical current.

1 ampere is equal to 1 coulomb of charge going past a given point per second.

There are essentially two types of electrical current flow:

  • Direct current: electrical current that flows in only one direction.
  • Alternating current: electrical current that alternates its direction of flow.

Analog audio signals are made of alternating current.

This alternating current is in the form of a wave that will generally have frequencies in the range of 20 Hz – 20,000 Hz (though a wider or narrowed range is certainly possible). Remember that human hearing only extends from 20 Hz – 20 kHz.

An easy way to visualize the fact that audio signals are AC is to think of a speaker driver. The speaker drive must oscillate (move back and forth) relative to the audio signal in order to produce sound waves.

If a direct current was applied to the speaker driver, it would shoot outward (or inward, depending on the polarity) and stay there until the DC was stopped (or adjusted in level).

Applying an AC to the speaker driver will cause the driver to move inward and outward and produce sound in doing so.

Direct current is still required in audio devices as it pertains to power supplies and biasing voltages.


Degree

Degrees are used to represent the phase of audio signals and the angles of sound propagation.

Phase typically refers to the timing of waves (whether they be sound waves of audio signals). A difference in phase between two identical signals means a shift in where each waveform begins relative to where the other begins.

Phase issues arise when two or more waves begin cancelling each other out (one wave is at its peak positive while the other is at its peak negative, for example).

180º would mean that a signal is completely out of phase with another. This is typically bad but it is a key component to balanced audio, where the balanced input has a differential amplifier to sum the differences between the two out-of-phase but otherwise identical signals.

Related articles:
Microphone Polarity & Phase: How They Affect Mic Signals
Do Microphones Output Balanced Or Unbalanced Audio?

Phase is also used in impedance graphs of headphone and loudspeaker drivers to show us whether the driver is “pulling” power from the amplifier or “pushing” power back to the amplifier.

Angles are useful when determining the polar response and angle of acceptance of a microphone or the coverage angle of a loudspeaker.


Pascal

The Pascal (Pa) is the SI unit for pressure.

1 Pascal is equal to one Newton of force per square meter.

Sound pressure level, as we’ve discussed, is typically measured in decibels but is ultimately defined by pressure. SPL is an important factor to consider with audio transducers (microphones, headphones, loudspeakers, etc.).


Watt

The watt is a measurement of power. Audio equipment is concerned with electrical power and, sometimes, with acoustic power (though acoustic power is not typically discussed).

Electrical power is the transfer of electrical energy and 1 watt is equal to the transfer of 1 joule per second.

In some small signal audio devices (microphones, preamps, etc.), decibels relative to 1 millivolt (dBm) may be sued to define signal strength.

In larger signal audio devices (power amplifiers and loudspeakers), we are more concerned with power transfer, defined in watts.

Loudspeakers will have a power handling rating that states the amount of power they are designed to receive from an amplifier.

Similarly, power amplifiers will have power ratings that define the amount of power they are capable of providing a loudspeaker.

Active audio devices also need power to function. However, the power requirements are typically specified in voltage and drawn current rather than in watts.

Related articles:
Complete Guide To Speaker Power Handling & Wattage Ratings
Why Do Loudspeakers Need Power & How Are They Powered?
How Do Headphones Get Power & Why Do They Need Power?
How Are Microphones Powered? (7 Mic Powering Methods)


Second

Time and audio are very intertwined. The second is the building block of time.

Here are just a few of the ways in which time is used to measure factors in audio and audio equipment:

  • Slew rate (typically measured in volts per microsecond): measures the rate at which an amplifier can react to an input signal.
  • Hertz (measures in cycles per second): relates to audio, sample rate and radio carrier wave frequency.
  • Warranty (measured in years): the warranty of audio equipment.

Related article: What Is Amplifier Slew Rate & Does It Affect Performance?


Gram

Audio equipment has weight. The gram is a metric measurement of weight.

Pound/Ounce

Imperial units for weight.


Meter

Audio equipment has dimensions and cables have run lengths. The meter is a metric measurement of weight.

Feet/Inches

Imperial units for length.


Percentage

Percentage is used in certain measurement that pertain to audio equipment. Most notable among these measurements are total harmonic distortion, intermodulation distortion and relative humidity.

Total Harmonic Distortion

THD is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

It is calculated with a simple sine wave (a single frequency) and is stated as a percentage.

Related article: What Is Total Harmonic Distortion In Audio And Microphones?

Intermodulation Distortion

Intermodulation distortion (IMD), like THD, is measured and represented as a percentage of the total output at specified testing circumstances.

IMD occurs when two or more signals are mixed in the amplifier. Tones interact with each other and often produce modulated non-harmonic “side-band” frequencies that are not actually part of the input signals. These additional frequencies are amplified by the amplifier and add distortion to the output signal.

Relative Humidity

Audio equipment is electrical. Electrical components are not always built to withstand extended periods of high humidity.

The relative humidity specification of a piece of audio equipment refers to the maximum suggested humidity the gear should be expected to perform in, measured in percentage.


Degrees Celsius

Audio equipment will only perform as it’s supposed to within a defined temperature range, whether that range is specified or not.

Degrees celsius is a metric unit for the measurement of temperature.

Amplifiers and speaker drivers are the typical casualities that come to mind when overheated.

Tube gear is the typical type of gear we think of malfunctioning when under-heated or heated too quickly from a cold temperature.

Related article: Loudspeaker Blow-Out: Why It Happens & How To Avoid/Fix It

Degrees Farenheit

Imperial units for temperature.


Bit

Digital audio, as we’ve discussed is defined, in part, by the sample rate (the number of times per second the digital signal is sampled).

The bit-depth refers to the potential amplitudes that each of these samples could potentially have.

Common bit-depths are:

  • 16-bit: 65,536
  • 24-bit: 16,777,216

Farad

The Farad is a measurement of capacitance. Capacitance is the ability of a body to store an electrical charge.

A capacitance of 1 farad would cause a potential difference of 1 volt across a capacitor when charged with 1 coulomb.

A very large number of audio devices have capacitors in their circuits to block DC and filter signals.

Capacitance is especially important to condenser microphones, which have capsules that effectively act as large parallel-place capacitors.

Related article: What Is A Condenser Microphone? (Detailed Answer + Examples)


Coulomb

The Coulomb is the SI unit of electric charge. Audio is electrical and so electric charge is inherent.

That being said, we typically don’t see specifications with Coulombs nor do we discuss Coulombs when dealing with audio.

1 Coulomb is equal to the amount of electric charge transported by a constant electric current of 1 ampere in 1 second.


Newton

The Newton is the SI unit for force.

Sometimes headphone manufacturers will list how much force their headphones will exert over the listener’s ears when worn correctly. This is the only time I’ve ever seen Newtons in a specification for a piece of audio equipment.


Tesla

The Tesla is a measurement of magnetic flux density.

Moving-coil type transducers (particularly speaker drivers) may have a specified Tesla value for the magnetic flux density produced by their magnets.


Henry

The Henry is an SI unit of measurement for electrical inductance.

Conductive coils, which are regularly used in moving-coil-type transducers, have self-inductance, which plays a role in how they acts when electricity (audio signals) as passed through them.

A coil with a self-inductance of 1 henry will produce a flux of 1 weber with a current of 1 ampere flowing through it.

Recent Content