Travellers on Beijing's subway have begun trialling a system that uses sound waves to let them pay for goods at vending machines through their mobile phones.
According to Xinhua News (translation), e-commerce giant Alipay installed two 'acoustic payment' vending machines on Metro Line 4 in Beijing last week, with more to follow shortly.
To use them, commuters pick their purchase, open up their Alipay mobile wallet app, choose the relevant payment option and hold the phone next to a sensor on the machine.
The handset makes a noise, communicating the payment data to the machine through a unique pattern that is valid for five minutes.