How does Bluetooth connect nearby devices?

1. Zara's music is stored on the phone on the table, but she hears it in her headphones across the room. No wire joins them. So how does the music travel?
2. A tiny antenna in the phone sends out radio waves. They are invisible and very weak, like a whisper instead of a shout. That saves the battery, but the waves only reach about ten meters.
3. First the gadgets must pair. The headphones call out, 'I am here!' The phone hears them and you tap to connect. Then they swap a secret key, so they can recognize each other next time.
4. The phone does not send the song in one go. It cuts the music into tiny pieces called packets and sends them one after another. The headphones put the pieces back in order and play them.
5. Other gadgets use the same kind of radio waves. So the pair keeps hopping between many channels, like lanes on a road, both jumping at the same moment. If a channel is busy, they stop using it.
6. As Zara walks away, the signal gets weaker, and a wall soaks up even more. Some packets get lost, so the music stutters and then stops. When she comes back, the paired gadgets join up again by themselves.

This ear cup is cut open. Tap each part to see what it does.

• Antenna. A tiny metal track, shorter than your finger. It catches the radio waves from the phone and sends waves back.
• Radio chip. A small chip that does all the Bluetooth work: listening, sending, hopping between channels and putting the packets in order.
• Secret key. When the gadgets pair, both save a secret key in their memory. That is how they know each other next time, without asking you again.
• Battery. A small rechargeable battery. Because Bluetooth signals are so weak, the battery can run the headphones for many hours.
• Speaker. The speaker turns the electrical music signal into sound by making a thin cone move back and forth very fast.
• Pairing button and light. Hold this button and the headphones start calling out so that a phone can find them. The blinking light shows they are ready to pair.
• Phone. The phone has its own antenna and Bluetooth chip. It holds the music and sends it as packets. It can stay paired with several gadgets.

Pair the headphones with the phone: tap the headphones, then tap the phone. Then drag Zara farther away and watch the packets. Try a wall too.

Experiment: Zara walks farther and farther from the phone. What happens to her music?

1. Tap the headphones, then the phone, to pair them. The music starts.
2. Drag Zara slowly away. Watch the signal bars and the flying packets.
3. Find the distance where the music starts to stutter, and where it stops.
4. Switch on the wall and find those two distances again.

What we found: The music does not get quieter. It is sent as packets, and each packet either arrives or is lost. Nearby, they all arrive. Farther away some are lost and the music stutters. A little farther and the link breaks. A wall soaks up part of the signal, so all of this happens closer to the phone.

Turn on JavaScript to try it yourself.

Three gadgets are waiting for music. Walls weaken the signal. Drag the phone to a spot where all three get a strong signal, then press Play music.

Bluetooth is a way for gadgets to talk using radio waves, the same kind of invisible waves that carry radio stations and wireless internet. The difference is strength. A Bluetooth signal is very weak, like a whisper instead of a shout. That is on purpose: a weak signal needs very little electricity, so tiny batteries in earbuds and watches can last a long time.

Two gadgets cannot just start talking. First they pair. One gadget calls out, 'I am here!' The other one listens and finds it. When you agree, they swap a secret key and save it. From then on they recognize each other and connect by themselves whenever they are close and switched on.

Once connected, they send information in tiny packets, thousands every second. Lots of other gadgets nearby use the same kind of radio waves, so a Bluetooth pair does a clever trick: both hop from channel to channel at exactly the same moments, many times a second. If a packet is spoiled on one channel, it is sent again on another.

The whisper fades quickly. Across a room it is fine. About ten meters away, or behind a couple of walls, packets start to get lost and the sound stutters. Then the link breaks. The sound does not fade like a voice does, because a packet either arrives or it does not. Our note-passing comparison breaks down a little: Bluetooth notes travel at the speed of light, and nobody has to get out of their seat.

Key words

Radio waves

Invisible waves that carry signals through the air at the speed of light.

Pairing

Introducing two gadgets so that they swap and save a secret key and can recognize each other later.

Packet

A tiny piece of data sent in one short radio burst.

Channel

One narrow slice of radio frequencies, like one lane on a road.

Frequency hopping

Jumping from channel to channel many times a second, both gadgets together, to dodge interference.

Range

How far apart two gadgets can be and still keep their link.

Check your thinking

1. You carry your headphones far from the phone. What happens to the music?

   Show the answer

   It stutters and then stops, because packets get lost. The signal gets too weak, so pieces of the music go missing. The pieces that do arrive are still full volume.

2. What happens when two gadgets pair?

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   They find each other and save a shared secret key. The saved key is why your headphones connect to your phone, and not to a stranger's, without asking each time.

3. Why is a Bluetooth signal made so weak?

   Show the answer

   A weak signal uses little battery, though it cannot reach far. It is a trade: low power means long battery life but a short reach.

4. Why do Bluetooth gadgets keep hopping between channels?

   Show the answer

   To dodge other signals that are using the same radio waves. If one channel is busy, only a tiny piece is spoiled, and it is sent again on another channel.

5. There is a thick wall between the phone and the speaker. What does it do to the Bluetooth signal?

   Show the answer

   It soaks up part of the signal, so the range is shorter. Walls absorb and scatter some of the radio energy. Less signal gets through, so the link fails at a shorter distance.