What Is Bluetooth and How Does It Work?
Bluetooth is a short range wireless standard for connecting devices directly to each other, without a network in between. Headphones to a phone, a keyboard to a laptop, a watch to a handset.
It differs from Wi-Fi in purpose rather than principle. Wi-Fi connects devices to a network, carries much more data and reaches further. Bluetooth connects two devices, uses far less power and works over a few metres.
How it works
Bluetooth uses the 2.4 GHz band, the same crowded space as many Wi-Fi networks and microwave ovens. To cope, it hops between channels many times a second, so interference on one frequency affects only a fraction of the transmission.
Pairing exchanges keys so the two devices recognise and trust each other in future. After the first pairing they reconnect automatically, which is why a new set of headphones needs setting up once and then simply works.
Classic Bluetooth and Low Energy
There are two distinct technologies under one name, which explains several confusing behaviours.
- Bluetooth Classic carries a continuous stream and is used for audio. It uses more power.
- Bluetooth Low Energy (BLE) sends small bursts of data and sleeps between them. It is used by fitness trackers, sensors and item finders, and can run for months on a coin cell.
Most modern devices support both and choose based on the task.
What the version numbers mean
| Version | Main change |
|---|---|
| 4.0 | Introduced Low Energy |
| 4.2 | Better speed and privacy |
| 5.0 | Roughly double the speed, four times the range of 4.2 in low energy mode |
| 5.2 | LE Audio foundation, including the LC3 codec |
| 5.3 and 5.4 | Efficiency and reliability improvements |
Both devices have to support a feature for it to work. Pairing a Bluetooth 5.3 phone with older headphones gives you what the headphones can do.
Audio codecs, and why quality differs
Bluetooth audio is compressed, and which codec is used decides how it sounds and how much it lags.
- SBC. Supported everywhere, the fallback, acceptable rather than good.
- AAC. Used by Apple devices, better than SBC at the same bitrate.
- aptX and aptX HD. Common on Android, lower latency.
- LDAC. Higher bitrate, needs a strong signal.
- LC3. Part of LE Audio, better quality at lower bitrates.
Both ends must support a codec. Expensive headphones connected to a device that only supports SBC will sound like SBC.
Why video and audio sometimes fall out of sync
Compression, transmission and decoding all take time, typically between 100 and 300 milliseconds. Video players usually compensate automatically, which is why films are fine.
Games and video calls cannot compensate, because the audio has to respond to something happening now. This is why wired headphones remain common for gaming.
Fixing common problems
It will not pair
- Put the device into pairing mode. Powering on is not the same thing, and it usually means holding a button until a light flashes.
- Move closer and remove obstacles. Your own body blocks 2.4 GHz surprisingly well.
- Remove the old pairing from both devices and start again.
- Check it is not still connected to something else. Many headphones connect to only one device at a time.
It keeps dropping
- Check the battery on both ends. Low power causes drops before it causes shutdown.
- Move away from the Wi-Fi router, which shares the band. Our guide on Wi-Fi interference covers the same congestion from the other side.
- Carrying your phone in a back pocket puts your body between it and your headphones.
- Update firmware on both devices.
Poor sound quality
- Check which codec is in use in developer settings on Android.
- Make sure the device is not using a call profile, which heavily reduces quality for microphone use.
Frequently asked questions
What is the range of Bluetooth?
Around ten metres for typical devices, further in open space with Bluetooth 5 and less through walls and bodies.
Does Bluetooth drain battery?
Modern Bluetooth, especially Low Energy, uses very little. Leaving it on is not a meaningful battery cost on current phones.
Is Bluetooth secure?
Modern versions encrypt connections and are reasonably secure. Risks come mainly from old devices, unpatched firmware and accepting pairing requests you did not initiate.
Can I connect two sets of headphones at once?
Many recent phones support audio sharing to two outputs. Older devices generally do not.
Why does it connect to the wrong device?
Devices reconnect to whatever they last used and find first. Removing unused pairings from the list prevents it.
Does Bluetooth need the internet?
No. It connects devices directly, and works with no network at all.
