Bangalore, September 24 2017: As the specifications of most speakers and music systems prove,  human hearing extends over a range of sound from 20 Hz to about 20 KHz. Dogs can hear higher frequencies -- up to 45 KHz  -- and cats even higher: up to 64 KHz.  But these non audible ( to humans) sounds  have their uses! The most recent application  is in e-payments. 
Last week, when Google launched its mobile payments app,  Tez ( or 'fast'), it  showed off a feature called Cash Mode where two persons could send or receive money by  holding their mobile phones to each other. Google calls its technology Audio QR ( for Quick  Response)  and it uses ultrasound to  carry data between the two phones that authenticates the transaction.
This particular technology has a compelling attraction today: it works with the most basic non-smart phones -- as long as the device  has a microphone and a loudspeaker --as which phone hasn't!  No  fancy technology needed -- like Near Field Communication , or Bluetooth or Infra Red.
The name of the game is Ultrasonic Authentication, and this is a  child with many claiming to be parent since 2011:  a Californian startup,  Naratte, called its version Zoosh.  Researchers at University College, London called their  'sound' solution, Chirp.   The French Copsonic crafted solutions that AirFrance uses to help its customers e-pay.  Chirp's first customer was the Indian bus booking app,  Shuttl that is popular around Delhi.  AliPay  the digital payment tool of ecommerce giant Alibaba, has installed  kiosks in the Beijing subway, where customers can make payments  using the same technology.
All of them work more or less similarly using sound at frequencies higher than  22.1 KHz to send bursts of data at 50-100 bits per second, protected by strong authentication software. This can be used to make financial  transactions or just to exchange small file securely.
The idea is  compelling because it is simple and relatively cheap. Ultrasound is emerging as the next big technology to fuel mobile transactions in India. Get ready to make a noise you can't hear!