How do Bluetooth headphones work?


Before you buy a pair of Bluetooth headphones, it’s worth knowing about the upsides and downsides of wireless audio.

The issues around Bluetooth stereo have taken front stage due to the persistent suggestion that Apple’s upcoming iPhone 7 will omit a headphone jack entirely, making it the first smartphone to proactively push people to Bluetooth audio. But in order to understand the benefits and drawbacks of Bluetooth headphones, it’s useful to know a little about Bluetooth itself.

What is Bluetooth?

Bluetooth is a defined wireless data standard for short range wireless transmissions at its core. It’s designed to be the lower-power alternative to technologies such as Wi-Fi, which can handle larger distances and transmission rates, but at the cost of higher power draw. As such, Bluetooth is a good match for low-data usage data transmissions on mobile devices, where battery life is often at a premium.

When Bluetooth was first introduced, the technology behind it only supported very simple low bitrate mono audio, which was enough for the simple use of Bluetooth headsets beloved by city types everywhere, but lousy for anyone wanting to listen to actual music. As the Bluetooth specification was refined and updated, a separate profile known as A2DP was added to deal with the challenges of delivering stereo music in decent quality to wireless headsets.


What is A2DP?

A2DP is the specific profile within the Bluetooth specification that deals with the supply of stereo audio over Bluetooth. There are other Bluetooth profiles within the specification that apply to Bluetooth headphones, such as HSP (headset profile) and AVRCP (Audio/Video Remote Control Profile) which allows you to remotely control playback functions. Most of these functions are very well established in the smartphones you’re likely to buy today, as well as the headphones that you’ll pair with them. If you’re rocking a particularly ancient smartphone you may find some features not present in your device.

A2DP defines the codecs (the core encoding mechanisms) used to send the audio from your phone to your headphones. These are designed to balance the needs of power usage, processing and reasonable data rates, but will vary by devices. For example, Apple has a version of its AAC codec for use over Bluetooth that maxes out at 250kbps, which is solid for a streaming service but still a step below CD quality and well below the demands of high quality audiophiles. Other codecs may offer slightly higher quality bitrates, and it obviously depends on the source quality of your media as well.

What is Bluetooth Low Energy?

Introduced in the Bluetooth 4.0 specification, Bluetooth Low Energy allows for very low power data transmission, expanding the possible devices that could be built with Bluetooth in mind. The bad news here is that for most Bluetooth headphones, the demands of stereo audio transmission means that they stay using the "classic" power profile even on Bluetooth 4.0 and better devices.


What are the advantages of Bluetooth Stereo?

The key advantage for Bluetooth Stereo is identical to the reasons why you’d opt for Wi-Fi instead of a cabled ethernet connection for your computer. There are no wires from your device to your headset to get tangled, caught or damaged over time, which means that they can be a great partner for fitness enthusiasts, with many bluetooth headsets built light and specifically with the needs of the heavy sweating crowd in mind. All the best phones today include Bluetooth functionality as standard, so there's fairly ubiquitous support for Bluetooth headphones as well.

Most Bluetooth stereo headphones are also built with AVRCP support for music controls as well as HSP for call answering purposes. This means that you can answer your phone and take calls (as long as there’s an embedded microphone in the headphones) without having to dig your phone out of your pocket or purse.

What are the disadvantages of Bluetooth Stereo?

The key disadvantage of using Bluetooth Stereo headphones as distinct from a wired solution is the requirement for power on both ends. Most wired headphones (excluding certain noise-cancelling models) will work fine with no power on their end, but for Bluetooth stereo to work, both your headphones and your smartphone have to be charged. As you listen, that power draws down, which means that eventually you’ll run out of juice. On the headphones side that’ll stop them in their tracks for most headphones, although some models do offer optional wired connections to dodge that.

On the phone side, however, while Bluetooth is designed to only sip at power, it’s still running a communications radio consistently to send the audio data to your headphones. As such, the difference in battery life for a user with Bluetooth stereo headphones and one using standard wired headphones at the same volume could be quite marked. If you’re constantly running out of power on your phone, Bluetooth headphones could be a poor fit.

While the use of optional codecs such as aptX for increased audio fidelity have improved the performance of Bluetooth headphones over time, there’s still a ceiling to the data quality presented over A2DP that may not suit if you’re a total audiophile. Current codecs come very close to CD quality, but not quite there and certainly not within the kind of scope of high resolution audio files. If you’re notably audio-obsessed, Bluetooth Headphones won’t appeal to you anywhere near as much as standard wired headphones.

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