How do I enable the 'Hey Siri' voice command in iOS?

The iPhone, iPad and iPod touch can listen for the words "Hey Siri" to start the personal voice assistant. How can I turn on Hey Siri on my device?

Hey Siri settings

To enable the "Hey Siri" voice command option in iOS navigate to Settings > Siri & Search and tap the Listen for "Hey Siri" slider to turn it ON (green). You will now be able to activate Siri by saying "Hey Siri" when your device is nearby.