We often take wired headphones for granted: simply plug them in and voila, we have sound. As an audio engineer, I find it fascinating to understand exactly how audio is carried through headphone cables. I’ve created this article for those who share my curiosity and would like to better understand audio signal flow and headphones.
How do headphone cables carry audio? A headphone cable creates an electrical circuit that carries the audio signal (AC voltage) from a source (smartphone, amp, interface, etc.) to the headphones which turn the signal into sound. Various wiring schematics are used to carry mono or stereo and unbalanced or balanced audio.
In this article, we’ll talk about each of the common schematics in greater detail and discuss the basics of audio signal flow and headphone transducers.
What Is Audio?
Audio is loosely defined as an electrical representation of sound.
Sound is described as mechanical waves that vary the localized pressure within a medium. A sound wave travels through a medium, causing peaks of maximum compression and troughs of maximum rarefaction at varying locations within the medium.
Analog audio signals reproduce these waveforms as AC voltages (electrical signals). Digital audio signals represent the analog signals by sampling the amplitude (as bit depth) many times per second (as sample rate).
Audible sound waves and audio signals have a frequency range between 20 Hz – 20,000 Hz (Hz = Hertz which measures cycles per second). Sounds naturally have many frequencies (harmonics and other) that make up the sound’s timbre and pitch. These frequencies each typically have their own transients and amplitudes to make up the character of the sound.
Sine waves are single-frequency waves and are the easiest way to represent sound. Here is an illustration of a sine wave:
With sound, the trough represents the max rarefaction caused by the wave while the peak represents the max compression of the wave.
With analog audio, the trough represents the minimum voltage of the alternating current while the peak represents the max voltage of the alternating current.
With digital analog, the trough represents the lowest dBFS (decibels full scale) value of the signal while the peak represents the highest dBFS value.
To learn more about sound and audio, check out my article What Is The Difference Between Sound And Audio?
The audio that is sent to headphone drivers is always analog. Regardless of the driver type, it requires an alternating electrical current to convert into sound waves. Headphones are, after all, transducers that turn audio signals into sound waves.
The Headphone Transducer
Let’s elaborate on how headphones fit the definition of a transducer. A transducer is a device that converts one form of energy into another form of energy.
Headphone transducers convert electrical energy (audio signal) into mechanical wave energy (sound waves). The element responsible for this conversion is called the driver.
The vast majority of headphones have moving-coil dynamic drivers that convert audio to sound via electromagnetic induction.
The 5 main headphone drivers and their working principles are as follows:
- Moving-coil dynamic: electromagnetic induction
- Planar magnetic: electromagnetic induction
- Electrostatic: electrostatic principles
- Balanced armature: electromagnetic induction
- Bone conduction: piezoelectric
Regardless of the driver type, an electrical circuit must be made with the driver in one way or another. This requires at least two electrical lead wires to be connected to the conductive part of the driver.
With moving-coil dynamic drivers, the lead wires are connected to either end of a conductive coil of wire.
Planar magnetic driver diaphragms have embedded conductive material and so the electrical leads are attached to the diaphragm at either end of the conductive element.
Electrostatic drivers have parallel stators that act as a sort of capacitor. An amplified signal is sent via two electrical lead wires: one attached to each stator.
Balanced armature drivers also use a conductive coil. This coil is wrapped around the armature and has lead wires attached to each of its ends.
Finally, the piezoelectric crystal of the bone conduction driver has two electrical leads attached to it to complete the electrical circuit.
Explaining these driver types gives us a good idea of how audio signals interact with headphones. Beginning with the end in mind, let’s explore how audio signals are carried through headphone cables.
How Do Headphone Cables Carry Audio?
So far we know that headphones are transducers. In order to convert energy, their drivers must be part of an electrical circuit that has an analog audio signal (AC voltage) passing through it.
This AC voltage is typically carried from the connected audio device through the headphone cable to the driver.
That being said, wireless headphones are becoming more and more common (more on wireless headphones here).
But for this article, we’re focusing on how audio passes from the device through the headphone jack and headphone cable to the drivers.
The Headphone Jack
Audio signals sent from a device to a pair of headphones nearly always passes through a headphone jack.
Remember how a drive requires at least two lead wires to function properly? Headphone jacks effectively prepare the audio signal into its separate components on different conductors (often referred to as poles).
Headphone jacks also very often have digital-to-analog converters (DACs) in close proximity to their designs. Headphones are inherently analog but many of our audio devices are digital (smartphones, digital consoles, laptops, audio interfaces, tablets, etc.).
A quality DAC will convert digital audio from the device into analog audio for the jack to pass on to the cable, which connects to the headphone driver.
Note that if the headphones are designed as a headset (they include a microphone), the DAC can effectively convert the analog signal from the microphone into digital audio for the digital device to process.
Poles: Tips, Rings And Sleeves
Getting back to the separate conductors of a headphone jack and its compatible plug, we must discuss tips, rings and sleeves.
If you have a pair of wired headphones nearby, have a look at the end of their cable. Chances are you’ll see a connector that resembles this:
This is referred to as a TRS (tip-ring-sleeve) connector or plug. The picture above is of a 3.5mm (measuring diameter) plug. The tip, ring and sleeve are well labelled.
It’s easy to remember the names of these conductive poles by visualizing their physical form on the connector:
- The tip is at the tip of the connector.
- The ring forms a ring around the connector.
- The sleeve is too long to be a ring and goes all the way to the bottom of the connector.
The female headphone port is referred to as the jack and the male headphone connection is referred to as the plug.
For more information on jacks and plugs, check out my article What Is The Difference Between A Microphone Plug And Jack?
Each conductive pole is insulated from its neighbouring pole and carries its own individual piece of the complete audio signal.
Though there are different wiring standards, most headphone jacks are compatible with headphone plugs and their associated headphones.
Note that not all headphones utilize headphone jacks. There are plenty of wireless headphones out there. It’s also the case that electrostatic and some planar magnetic headphones require different connectors to connect to their specialized amplifiers.
If your wired headphone connector did not resemble the above picture, it probably looks like this:
This TRRS connector has one more conductor than the aforementioned TRS connector.
The sleeve connector is generally the ground pole and often acts as a return for the audio signal. In other words, it connects the devices and headphones to the electrical ground but also completes the circuit between the headphone jack and headphone driver. This type of circuit is referred to as unbalanced audio.
The tip and ring connectors are typically the signal wires. This effectively means that the audio signals are carried through them and they are not connected to ground. It’s important to restate that the sleeve will often be required as the return wire for the tips and rings.
Some wired headphones have digital connectors such as Lightning or USB. These cables will be discussed in a later section of this article (skip to that section by clicking here).
With that primer on tips, rings and sleeves, let’s get into the various wiring standards for headphone cables:
Unbalanced Mono Headphone Signals
Unbalanced mono headphone signals are carried via two wires within a cable. These wires connect to one or two headphone drivers since mono headphones can have one or two drivers. In the case of dual-driver mono headphones, both wires are split and sent to each driver.
Mono headphones often have TS (tip-sleeve) connectors with the tip acting as the signal wire and the sleeve acting as the ground/return wire.
- Tip: signal wire
- Sleeve: ground and signal return wire
In the diagram below, the tip would be connected to the red line while the sleeve would be connected to the black line.
The connections of these headsets vary based on the features and capabilities of the headset but the mono headphone signal itself is still sent through two wires.
Headsets will always have more than two conductors (unlike the TS connector described above). This is because the microphone requires its own signal path with at least one designated signal wire (and a return but the return can be common to the headphones return/ground).
Unbalanced Stereo Headphone Audio Signals
The vast majority of headphones are wired for unbalanced stereo and, therefore, their cables carry unbalanced stereo signals.
The simple reason is that stereo audio recordings are the standard in most music and television. Headphone cables are also relatively short and so proper balancing is an added expense and complication that is not necessary for great headphone audio.
Typically this is achieved with a TRS (tip-ring-sleeve) connector at the end of the headphones cable that connects to an appropriate jack.
In this configuration, the poles would be wired in the following scheme:
- Tip: left headphone channel wire
- Ring: right headphone channel wire
- Sleeve: common ground/return wire
So the tip’s connection would carry the left channel to the left driver, completing the circuit with the return wire (sleeve). The ring’s connection would carry the right channel to the right driver and also complete the circuit with the return wire (sleeve). This can be visualized in the diagram below:
This allows the headphones to effectively recreate stereo audio.
Balanced Stereo Headphone Audio Signals
Much less common than unbalanced stereo headphones are balanced stereo headphones.
These headphones (and their cables and connectors) are wired to accept balanced audio from a balanced stereo source (or two balanced audio sources).
Before we get into this wiring scheme, let’s quickly go over balanced audio.
First, we’ll restate that unbalanced audio has one signal wire and one return wire (which typically doubles as the ground/shield). With balanced audio, there are effectively two signal wires that carry the same signal at equal amplitudes but opposite polarities.
Balanced audio is typically used to transfer audio signals through long cable runs. Balanced inputs have differential amplifiers that sum the differences between the two signal wires. This not only improves the overall signal level but also eliminates any noise interference that is common to both wires.
In headphone design, there’s no real input to have a differential amplifier. Rather, the balanced signals are sent directly to the headphone driver.
To better envision this, let’s think of audio signals as the alternating electrical currents that they are:
So we have the same signal in opposite polarity on two conductors. This effectively means that as the current flows in one direction on the first wire, an equal amount of current flows in the opposite direction on the second wire.
These wires are connected to either end of the conductive element within the driver and, therefore, there is a doubling of the amp’s voltage slew rate and voltage swing range. This improves the efficiency of the headphone driver circuit.
Additional benefits are the reduction of THD (Total Harmonic Distortion) due to the lower requirements in signal strength and the cancellation of crosstalk due to the elimination of the common ground wire (which is required in unbalanced connectors).
With that out of the way, it is the TRRRS 5-pole connector that is typically associated with balanced stereo headphones. This tip-ring-ring-ring-sleeve scheme is wired as follows:
- Tip: left channel headphone audio (positive polarity)
- Ring: left channel headphone audio (negative polarity)
- Ring: right channel headphone audio (positive polarity)
- Ring: right channel headphone audio (negative polarity)
- Sleeve: ground/shield
The most common connector for TRRRS is the 4.4 Pentaconn (pictured below):
The signal flow of balanced stereo headphone audio is described in the following diagram:
Headsets And Headphones With Built-In Microphones
With the rising popularity of mobile voice and video calling, headphones with built-in microphones are being used more and more. Headsets are also commonplace in many applications such as telephone work, video gaming, and aviation.
Although there are mono headphones with built-in mics, like most basic headphones, most headsets and headphones/mic combos deliver stereo headphone audio. More specifically, they generally deliver unbalanced headphone audio.
For more information on the differences between balanced audio and unbalanced audio, check out my article
From what we’ve learned about unbalanced headphones, it would make sense that any pair of unbalanced headphones with a built-in microphone would require an additional conductor/pole to carry the mic signal.
That is exactly correct in most cases: headphones with built-in mics generally get (and give) audio via 4-pole wires and connectors.
In most cases, this is via a TRRS (tip-ring-ring-sleeve connector). Here’s the same picture again as a refresher:
Though there are different wiring scheme for the versatile TRRS connector, the most modern and common standard is the CTIA’s (Cellular Telecommunications and Internet Association AHJ (American Headset Jack) standard; The AHJ is wired as follows:
- Tip: left channel headphone audio
- Ring: right channel headphone audio
- Ring: common ground
- Sleeve: microphone audio
The typical TRRS headset can be described in the following diagram:
It’s important to note that these devices effectively send signals to and from the headphones/microphone. The signals flow within the cable in both directions.
When connected to a digital device, then, these headphones+mic combos act as both input and output devices.
To learn more about headphones and microphones and their roles as input and output devices, check out the following My New Microphone articles:
• Are Headphones Input Or Output Devices?
• Are Microphones Input Or Output Devices?
Most headphone jacks nowadays are wired as TRRS (CTIA standard) and can accept TS, TRS and TRRS plugs.
The basic idea here is that the headphone jack is somewhat universal (though its size/diameter is somewhat restrictive without adapters). A TRS plug, for example, will connect as follows:
- Tip to tip: left headphone audio connected
- Ring to first ring: right headphone audio connected
- Sleeve to second ring: common grounds connected
- Sleeve to sleeve: common ground to microphone results in an electrical short sent to ground
This system isn’t perfect and complications can arise (especially when plugging in and unplugging due to electrical shorts) but for the most part, it’s a fairly versatile system.
To learn more about headphone jack sizes, check out my article Differences Between 2.5mm, 3.5mm & 6.35mm Headphone Jacks.
Note that active noise-cancelling headphones also have built-in microphones. However, these mics are not for telephony but rather for recording the environmental noise so that the headphones can effectively produce “anti-noise” signals to cancel out the noise at the listener’s ear.
To learn more about headphones and noise-cancellation, check out my articles Passive Vs. Active Noise-Cancelling Headphones and Do Noise-Cancelling Headphones Work With Or Without Music?
Digital Headphone Cable Connections
Thus far we’ve been discussing how analog audio signals travel from an audio source through a cable and to a headphone driver. These cables generally have TS, TRS, TRRS or TRRRS connectors though other analog connectors are also used (XLR, etc.).
But what about the headphones with digital connectors? These are the wired headphones with Lightning and USB connectors that have become more popular since smartphone manufacturers stopped designing their products with 3.5mm headphone jacks.
One example of a digital headphone connection is the Lightning connector at the end of the popular Apple Earpods (link to check the price on Amazon):
So the digital connector (Lightning, USB, etc.) makes a digital connection with a digital device (smartphone, computer, etc.). This connection passes digital audio between the digital “jack” and “plug.”
However, directly after the connector in the headphone cable (the Lightning connector in the case of the Apple Earpods), is a digital-to-analog converter.
This converter effectively changes the digital audio signals to real AC voltages that are capable of properly driving the headphones’ drivers.
So these digital connectors really pass audio through their cables in the same fashion as the headphones with analog connectors.
How Do Wireless Headphones Work?
Now that we understand how wired headphones receive their audio signals, it’s important to discuss how wireless headphones receive their audio signals. After all, wireless devices are becoming more prominent on the market and headphones are no exception.
Rather than travelling through conductive wires, wireless headphone audio signals are sent without any physical connection between the audio device and the headphones.
So how is audio transmitted wireless? Well, we can’t simply project the audio signal through the air since it’s confined as an electrical signal. We could transduce this signal to a mechanical wave but then we’d have sound being transmitted through the medium which would defeat the purpose.
Audio signals, therefore, are encoded into carrier signals that are imperceptible to humans. Carrier signals used in wireless headphone audio transmission are typically in radio frequencies or infrared frequencies.
What happens is the wireless headphone transmitter, whether that is your smartphone using Bluetooth or a physical transmitter device, encodes the audio into a wireless signal format with a specific frequency and projects that signal into the air.
The wireless receiver in the headphones is tuned to accept this wireless frequency and acts to decode the signal, taking the original audio signal from it.
Note that analog audio and digital audio can be encoded into a carrier signal by analog or digital transmitters, respectively, and decoded by a compatible analog or digital receiver. However, the carrier signal itself must be a continuous analog signal.
To learn more about wireless audio transmission between devices, check out my articles:
• How Do Wireless Microphones Work?
• How To Connect A Wireless Microphone To A Computer (+ Bluetooth Mics)
How do headphones work? Headphones are transducers that convert audio signals (electrical energy) into sound waves (mechanical wave energy). There are 5 main transducer elements known as drivers: moving-coil, planar magnetic, electrostatic, balanced armature, and bone conduction. Working principles include electromagnetic, electrostatic and piezoelectric.
To learn more about how headphones work, check out my article How Do Headphones Make Sound? (A Simple Beginner’s Guide).
How do headphone jacks work? Headphone jacks work by producing the proper audio signals on separated pins/conductors. These conductors then connect to headphone cables and send compatible audio signals properly to the headphone drivers. Many headphone jacks have digital-to-analog converters built-in close proximity to effectively change digital audio to the analog audio required of headphones.
For more information on headphone jacks, check out my article What Is The Difference Between A Microphone Plug And Jack?