Wireless headphones have been around since the 1960s. However, in 2004, Bluetooth wireless headphones came onto the market and revolutionized personal audio listening, gaining great popularity among audio consumers and professionals alike. If you've ever wondered how your wireless headphones or earbuds play back the audio from your device, then this article is for you.
How do wireless headphones work? Wireless headphones work by receiving wirelessly transmitted signals from their paired audio sources. These signals are encoded by the source device and transmitted via radio frequencies (common) or infrared (less common) carriers. The headphones receive the RF or IR signal and decode it to audio.
This article is all about wireless headphones and how they function. We'll discuss wireless transmission methods and the designs of wireless headphones, along with the pros and cons of going wireless.
Table Of Contents
- How Do Wireless Headphones Work?
- Carrier Waves & Modulating Signals
- Part 1: The Transmitter
- Part 2: The Receiver
- How Does Radio Frequency Transmission Work?
- How Does Infrared Transmission Work?
- RF Vs. IR Wireless Audio Transmission
- How Does Bluetooth Transmit Audio From A Device To Headphones?
- Connecting Wireless Headphones To A TV, Radio, Or Other Device
- Wireless Vs. True Wireless
- The Pros & Cons Of Wireless Headphones
- Related Questions
How Do Wireless Headphones Work?
As the name suggests, wireless headphones receive their audio signals wirelessly rather than via headphone cables (often referred to as “hardwired” headphones).
Wireless headphones have built-in receivers designed to accept the wireless waves that carry the audio signal intended to drive the headphone drivers. This “wireless signal” is appropriately called the carrier wave.
Transmitters effectively encode the audio signal into a wireless format (the carrier wave) and propagate the wireless signal through the air.
The intended audio signal is referred to as the modulating signal. This modulating signal is encoded into a carrier wave that is transmitted wirelessly to the receiver.
The carrier waves used to transmit wireless headphone signals are either in the radio frequency range or the infrared frequency range. You can skip ahead to the sections of this article that explain these carrier waves in more detail by clicking the links in this paragraph.
It's important to note that the carrier wave is a single-frequency wave within the above-mentioned ranges. Audio signals are generally made up of frequencies within the audible range of 20 Hz – 20,000 Hz.
The wireless receiver reads the carrier wave and decodes the modulating signal (audio signal) from the wave. Wireless receivers must be tuned to accept the specified carrier wave frequency.
If the audio signal is digital, it is converted into an analog audio signal. The analog audio signal is then amplified to drive the headphone drivers properly.
It's important to note that regardless of the specific type of wireless headphones, being wireless means the headphone is active. In other words, they require power to function properly.
To maintain the “wirelessness” of the headphones, the power must be supplied by internal batteries. These batteries may be AA, AAA or batteries that are inserted into the headphones. However, it is more common to have built-in rechargeable batteries in today's wireless headphones.
For more information on active headphones, check out my article How Do Headphones Get Power & Why Do They Need Power?
So to recap, let's go through the chronological steps involved in wireless headphone audio transmission:
- The audio source sends its audio signal to a wireless transmitter.
- The wireless transmitter encodes the audio signal (modulating signal) into a carrier wave.
- This single-frequency carrier wave is propagated through space.
- The wireless receiver, tuned to pick up the single-frequency carrier wave, accepts the wireless signal and effectively decodes the audio signal.
- Digital audio signals are then converted to analog signals via a digital-to-analog converter (if the headphones are designed for it).
- An internal amplifier amplifies the analog audio signal.
- The amplified audio signal is sent to the headphone drivers.
- The headphone drivers (transducers) convert the audio signal (electrical energy) into sound (mechanical wave energy).
Other than the method in which the audio signal is transferred from the source to the headphones and the internal DAC/amplifier, wireless headphones act the same way as wired headphones.
For an in-depth guide on how headphones work, check out my article How Do Headphones Work? (Illustrated Guide For All HP Types).
For a quicker read on how headphones make sound, check out my article How Do Headphones Make Sound? (A Simple Beginner’s Guide).
With that, let's get into the more detailed sections of this article to really understand the inner workings of wireless headphones.
Carrier Waves & Modulating Signals
To fully understand wireless headphones and wireless audio transmission in general, we must understand carrier waves and modulating signals.
As their name suggests, carrier waves carry the headphone's audio signal from the transmitter to the receiver.
Carrier waves are electromagnetic waves that are modulated with an information-bearing signal for wireless transmission.
The vibration of an electric charge creates electromagnetic waves. These electric charge vibrations have electric and magnetic components. These waves carry energy from one place to another. This can be the heat and light from the Sun to the Earth or the wireless audio from a transmitter to a headphone receiver.
Unlike sound waves, which are mechanical waves, electromagnetic waves may travel through a vacuum and do not interact directly with the molecules of a medium (though the atoms within a medium will absorb some of the electromagnetic wave energy).
Wireless headphone carrier signals are generally either radio waves (common) or infrared waves (rare).
Radio frequencies (RF) span the large range of 30 Hz to 300 GHz (300,000,000,000 Hz). The infrared (IR) frequency range is from 300 GHz to 430 THz.
Whether the headphones utilize RF or IR, the carrier wave is a sine wave with a signal frequency. The transmitter is tuned to propagate this single-frequency carrier wave, and the receiver is tuned to accept this single-frequency carrier wave.
Hertz is the measure of cycles per second.
Wireless headphones generally operate near 2.4 GHz (radio frequency), which offers a great wireless range of up to 91 m (300 ft).
The modulating signal, as its name suggests, is used to modulate the carrier signal. This modulating signal is then effectively carried by the carrier wave from the wireless transmitter to the receiver.
In the case of wireless headphones, the modulating signal is the audio signal intended for the headphone drivers.
There are several ways that the modulating signal may modulate the carrier wave.
Wireless Analog Audio Transmission
For wireless analog audio signal transmission to headphones, it is frequency modulation that is most common.
Yes, the same transmission is used in FM radio, which essentially makes our RF FM headphones like a mini radio station!
Frequency modulation (FM) works by having the modulating signal modulate the frequency of the carrier wave. If we were to send a simple audio sine wave, the resulting frequency modulated signal would resemble something like this:
So the “single-frequency” carrier wave actually must operate across a range of frequencies once modulated by an audio signal. The receiver is designed to accept the bandwidth of the modulated carrier wave.
To keep the variation in the carrier wave frequency low and concise, the audio signal is only amplified once the headphones receiver demodulates it.
Headphone audio signals are nearly always stereo. Fortunately, FM carrier signals may be used to carry stereo audio. This is done with multiplexing and demultiplexing before and after the frequency modulation process.
Multiplexing is effectively the combination of multiple mono signals or stereo signals being combined into a single signal.
With proper multiplexing and demultiplexing, the actual FM modulation and demodulation processes are identical in stereo and mono processes.
Wireless Digital Audio Transmission
With the rise of digital audio and digital audio devices, many headphones are now designed to accept digital audio wirelessly.
Digital audio is essentially a digital representation of analog audio.
Analog audio is made up of continuous waves of alternating current. Digital audio basically takes instantaneous snapshots of the amplitude of the audio signal and represents them digitally.
The quality of digital audio can be defined by its sample rate and bit-depth.
Sample rate refers to how many individual audio amplitudes are sampled each second. Common sample rates include 44.1 kHz and 48 kHz. In this case, Hz refers to samples/second.
The bit-depth refers to how many bits are used to represent the amplitude of any given sample. Bits refer to the number of binary digits (1s and 0s) chained together to represent a value. Common bit-depths include 16-bit (which has 65,536 distinct values) and 24-bit (which has 16,777,215 distinct values).
The higher the sample rate and bit-depth, the greater the resolution and, in theory, the higher the quality of the digital audio signal. Note that higher sample rates and bit-depth also require more processing power and that different sample rates may not be compatible with one another.
When it comes to sending digital audio wirelessly to headphones, Bluetooth is the most common method. We'll get to Bluetooth in greater detail in our section How Does Bluetooth Transmit Audio From A Device To Headphones?
For now, though, we'll discuss the actual digital transmission method used to transmit audio from a device to a pair of Bluetooth headphones since Bluetooth is so popular.
Of all the different methods of transmitting digital audio wirelessly, pulse-shift keying (PSK) modulation is the most popular.
PSK conveys digital data by modulating the phase of a single-frequency carrier wave. The modulation is accomplished by varying the sine and cosine inputs at a precise time.
PSK transmits digital data by modulating the phase of a single-frequency carrier wave. The modulation is accomplished by varying the sine and cosine inputs at a precise time referenced to the binary code of the digital signal.
PSK would look something like this:
To recap our section on the modulating signals and carrier waves of wireless headphone transmission, let's go over a few points:
- The intended audio signal acts as the modulating signal and can be either analog or digital.
- The carrier signal used with wireless headphones is typically a radio signal around 2.4 GHz. Carrier signals can be in the radio frequency or infrared frequency ranges.
- Analog audio is typically transmitted via radio frequencies using frequency modulation.
- Bluetooth typically transmits digital audio via pulse-shift key modulation.
- FM and PSK can transmit stereo audio.
Part 1: The Transmitter
The wireless transmitter is responsible for embedding the audio signal into a wireless format. In the section above, we learned that this involves modulating a carrier wave with the audio signal.
Transmitters can be standalone devices that plug into audio sources. These standalone wireless headphone transmitters are common for use in the home and with professional in-ear monitoring systems.
Alternatively, transmitters can be designed directly into audio devices. This is the case with most devices marketed as being wireless, and it is always the case with Bluetooth devices.
Transmitters are generally designed to send either analog or digital audio signals wirelessly and typically work within a limited range of frequencies that match a compatible receiver.
In some transmitters, the carrier signal frequency can be altered or “tuned” to a specific frequency by the user.
Part 2: The Receiver
Wireless headphone receivers are built into the headphones themselves. There are also standalone wireless receiver units that can be connected to wired headphones though these systems aren't completely “wireless.”
A receiver is designed to pick up the carrier wave transmitted by the transmitter effectively. Its role is then to decode the audio signal from the carrier signal.
As we've alluded to earlier, the receiver and transmitter must be tuned to the same frequency for wireless transmission to take place properly.
Receivers are designed to decode specific types of modulation. For compatibility with transmitters, analog receivers generally work on decoding FM signals (in the radio frequency range). Conversely, digital receivers generally work on decoding PSK signals (also in the radio frequency range).
Bluetooth headphones, for example, have PSK receivers that accept RF frequencies between 2.400 and 2.4835 GHz.
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How Does Radio Frequency Transmission Work?
To reiterate, radio waves are electromagnetic waves in the frequency band of 30 Hz to 300 GHz.
These electromagnetic waves carry energy with them and cause oscillations of electric and magnetic fields in their path. They naturally emit in all directions and, in a vacuum, travel at the speed of light. In mediums like air, electromagnetic waves travel more slowly due to interference from the molecules of the medium but are still incredibly fast.
Remember this definition of electromagnetic waves when we discuss infrared wireless headphones.
Back to RF transmission.
Although the RF range is from 30 Hz to 300 GHz, most RF headphones operate within the range of 900 MHz to 3.2 GHz, depending on the model. The Bluetooth wireless standard operates between 2.400 and 2.4835 GHz.
RF wireless headphones often receive analog audio signals wirelessly. As we've mentioned previous, these signals are sent wirelessly via frequency modulation:
- The audio signal (typically covering the band between 20 Hz and 20 kHz) acts as the modulating signal.
- The carrier wave is a sine wave in the range of 900 MHz to 3.2 GHz.
- The amplitude of the audio signal modulates the frequency of the carrier wave.
- The headphones receive the modulated carrier wave, and the audio signal is retrieved out of the carrier wave.
The Sennheiser RS 175 headphones are an excellent example of RF wireless headphones:
The picture above shows the RS 175 transmitter with the RS 175 headphones (with the built-in receiver) sitting on top.
To send stereo audio signals (which can be thought of as two separate mono signals), the transmitter must have a multiplexer to effectively combine the left and right channel signals into a single modulating signal. Similarly, the receiver must have a demultiplexer to split the signal back into the stereo signal to drive the left and right drivers.
Radio frequencies are commonly used to transmit audio and other signals because they are so effective.
Their relatively long wavelengths (compared to other electromagnetic waves, not to sound waves) allow them to travel long distances and penetrate through solids (walls, floors, etc.). A typical 2.4 GHz RF wireless transmitter can send its signals 91 m (300 ft) or more.
RF transmission waves can also be picked up by a virtually unlimited number of RF receivers if they are all tuned to the proper frequency. The limit is only theoretically a result of the physical space a receiver would take up relative to the volume of the distance range of the RF carrier wave.
The popularity of RF wireless causes a notable disadvantage, though. This disadvantage is that RF headphone signals are susceptible to interference from all the other devices that send audio on RF signals at or around the transmission frequency.
It's important to note that the increasingly popular Bluetooth standard transmits information on radio waves. However, this standard is commonly dissociated from RF, and so it's worth discussing Bluetooth headphones separately from RF headphones.
How Does Infrared Transmission Work?
Again, infrared (IR) waves are electromagnetic waves in the frequency band of 300 GHz to 430 THz. IR waves have higher frequencies than RF waves and, therefore, have shorter wavelengths. IR waves behave differently than RF waves, so let's talk about them.
A critical difference between IR and RF is that IR is line-of-sight. If any physical objects are eclipsing the IR transmitter from the IR receiver, there will be no signal transmission.
This is largely due to the short wavelengths and relatively weak infrared waves.
Another effect of the short infrared waves is that the transmission range is only good up to about 10 m (32 ft) as opposed to the 91 m (300 ft) or more that is available to RF.
This short-range line-of-sight is a huge disadvantage for many applications but can be a great benefit if privacy is wanted. IR headphones are generally used in small rooms and connect wirelessly to televisions and other sound sources in movie theatres, boardrooms, and courtrooms.
To further our discussion on privacy, infrared wireless technology can only make a single connection between a transmitter and one receiver.
The Unisar J3 TV920s are an example of infrared wireless headphones:
Because there's much less going on in the infrared wireless space than in the RF space, infrared headphones are less susceptible to interference and often offer higher audio fidelity.
High-end infrared wireless headphones may very well outperform RF headphones in terms of audio quality but only within a limited range from the transmitter.
RF Vs. IR Wireless Audio Transmission
The differences between RF and IR wireless headphones are summed up in the following table:
|RF Wireless Headphones
|IR Wireless Headphones
|Penetrate through solids (walls, floors, etc.)
|Susceptible to interference (radios, phones, radio transmission towers, etc.)
|Low risk of interference
|Multiple connection capabilities
|Longer distance range
Over 300 ft (~91 m)
|Shorter distance range
About 32 feet (~10m)
|Bluetooth technically uses RF
|Bluetooth does not use IR
How Does Bluetooth Transmit Audio From A Device To Headphones?
Now for the fun part. Bluetooth has become the standard for the vast majority of wireless headphones, so it's critical that we discuss it here.
First off, the Bluetooth standard is continuously being improved up, and there are many different versions. The Bluetooth Special Interest Group and the engineers behind the development of Bluetooth technology ensure cross-compatibility between versions. However, when pairing two devices with different BT versions, we'll only get the benefits of the oldest version.
Confusingly, there are lots of different standards within the overarching Bluetooth standard. If you're a Bluetooth expert, please bear with me as I focus on the standards generally used for Bluetooth headphones. For everyone else, know that there are other standards out there, and any given pair of Bluetooth headphones may not fit the exact description I'll be laying out here.
With that, let's get into how Bluetooth headphones [generally] work!
Bluetooth works by pairing devices. The headphones must pair to a device with Bluetooth technology for information (audio) to be transferred.
The Bluetooth network connection is referred to as Piconet, and the transfer of information is one-way. The audio device essentially controls the headphones.
Bluetooth transmits digital information via short-range radio frequencies in the frequency band between 2.400 to 2.485 GHz. In the case of Bluetooth headphones, this information is the digital audio signal from a paired device.
Bluetooth uses 79 distinct frequencies within the 2.400 to 2.485 GHz to transmit information. It can change this frequency 1600 times per second to avoid interference with other Bluetooth connections.
It is unlikely that two transmitters will be on the same frequency at the same time. This minimizes the risk of interference between Bluetooth devices since any interference on a particular frequency will last only a tiny fraction of a second.
The Beats Solo Pro are a great example of Bluetooth wireless headphones:
As mentioned above, BT sends digital info wirelessly. Bluetooth audio is sent via pulse-shift keying modulation (PSK).
The PSK modulating signal is the digital audio of the paired device. This modulating signal modulates the phase of a fixed-frequency carrier wave. It does so by varying the sine and cosine inputs at a precise time.
For Bluetooth headphones, this means that the digital audio signal is carried on one of the 79 radio wave frequencies that change 1600 times per second. This seemingly choppy wireless connection actually works just fine to send audio wirelessly.
Most Bluetooth headphones are either Class 1 or Class 2 devices. This means that the signal transfer between them and their paired device has a maximum permitted power of 100 mW (Class 1) or 2.5 mW (Class 2) and a signal range of about 100m (330 ft) for Class 1 or 10 m (33 ft) for Class 2.
This low power consumption allows for longer battery life.
The SoundPeats Q30HD is an example of Bluetooth wireless earphones:
Bluetooth offers a standard for connectivity between headphones and digital audio devices so long as both devices have the technology. Constant improvements are being made to better the audio quality though Bluetooth is not quite at the same quality as wired connectivity just yet.
Note that Bluetooth will also drain the battery of the paired device more than a wired pair of headphones would.
Connecting Wireless Headphones To A TV, Radio, Or Other Device
Connecting wireless headphones to devices can be as easy as pairing two Bluetooth devices together.
If the devices have other wireless connection means (RF or IR), we can connect them simply by turning the transmitter and receiver on and tuning them to the same frequency.
However, if the devices do not have Bluetooth or another compatible built-in wireless system, we'll need to connect a standalone transmitter to the device and/or standalone receiver to the headphones.
Ideally, we'd have wireless headphones, to begin with, and so they'd have a built-in wireless receiver. However, there are receivers available that wired headphones can connect to that will make at least part of the signal path wireless.
Fortunately, both RF and IR wireless headphones will almost always come with their own dedicated transmitter. This makes it easy to connect the headphones to the transmitter and simplifies our issue by ensuring the transmitter can connect to our device (TV, radio, etc.).
The transmitters will typically connect to the device via an analog connection like a left/right RCA audio out or 3.5mm TRS stereo out.
Wireless Vs. True Wireless
Wireless headphones and true wireless earbuds both work on the same principles of wireless audio transmission.
However, there is one major difference. True wireless earphones have two independent earpieces that are not connected physically.
That means that each earphone has its own receiver, and that receiver must decode its proper channel (left or right) from the transmitted carrier signal.
Each earphone also has an amplifier to boost the audio signal to properly drive the drivers and a DAC if digital-to-analog conversion is required.
This is different from most “normal” wireless headphones, which have a single receiver and decode the stereo audio signal from the carrier signal before sending the appropriate channels to the appropriate drivers.
The Bang & Olufsen Beoplay E8 are an example of true wireless Bluetooth earphones:
It's worth noting that true wireless earbuds are more likely to use Bluetooth than their “regular” wireless headphone/earphone counterparts.
Wireless Vs. Wired Headphones
The most obvious difference between wired and wireless headphones is in the names. Wired headphones receive their audio signals via wires or cables, while wireless headphones receiver their audio signals wirelessly.
The other differences are summed up in the following table:
|Physically connect to headphone jacks
|Connect wirelessly to the audio source
|Connect to external amplifiers
|Have internal amplifiers
|Better audio connection
|Spottier audio connection
|Active or passive
|May or may not require batteries
|Always requires batteries
|Has wires and is bound to the audio device
|Different wireless standards have different distances from audio source
I would also add that in-ear monitors for stage use pretty much have to be wireless to allow freedom of movement during live performances.
Other than these key differences, wired and wireless headphones work pretty much the same. They can both take on any form factor (headphones; earphones; closed-back; open-back; over-ear; on-ear, etc.) and can have any driver type.
The key components of any headphones are the transducers/drivers. Wired and wireless designs change the method in which audio gets from the source to the drivers but does not alter the transducer elements at all.
To learn more about the signal transfer in wired headphones, check out my articles How Do Headphone Jacks And Plugs Work? (+ Wiring Diagrams) and An In-Depth Look Into How Headphone Cables Carry Audio.
The Pros & Cons Of Wireless Headphones
To continue on from the above section on wired vs. wireless headphones, let's quickly discuss the pros and cons of wireless headphones.
The pros of wireless headphones include:
- The common Bluetooth standard (which is now used in most new wireless headphones) is compatible with many of the popular audio devices today.
- Not being tethered to the audio device.
- Bluetooth (Class 2) generally allows up to 10 m (32 ft) range.
- RF (at about 2.4 GHz) allows 91 m (300 ft) range or more.
- IR allows up to 10 m (32 ft) range.
The cons of wireless headphones include:
- Requiring batteries to function, which require regular replacement or regular recharging.
- The Bluetooth standard also drains the battery of the paired device.
- A higher price point due to the built-in receiver and amplifier (and potential DAC).
- It can be a bit frustrating to connect quickly compared to wired headphones.
Can you listen to TV through wireless headphones? Yes. Many modern televisions have Bluetooth capabilities to connect to Bluetooth headphones easily. Alternatively, RF (radio frequency) and IR (infrared) wireless systems allow a transmitter to be connected to the TV to send the audio wirelessly to compatible headphones.
How do wireless microphones work? Wireless microphones work by modulating a sine wave carrier wave with the microphone's audio signal and sending the modulated carrier wave wirelessly to a compatible receiver. The receiver then decodes the audio from the carrier signal and sends it to a mic preamplifier.
To learn more about wireless microphones, check out my article How Do Wireless Microphones Work?
Choosing the right headphones or earphones for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Headphones/Earphones Buyer's Guide. Check it out for help in determining your next headphones/earphones purchase.