The use of ultrasonic noise — silent to the human ear — in mobile payments and rewards hasn't garnered as much attention as Near Field Communication chips and QR codes, but there are a few successful implementations.

Alipay and shopkick use an uncommon approach for mobile transactions. The most prominent mobile payment systems, such as Google Wallet and Isis, rely on NFC to make contactless payments at the point of sale. Others, such as Starbucks and LevelUp, display images called QR codes. Still others, such as PayPal and Square Wallet, use geolocation to detect a customer's presence.

Not every phone has an NFC chip, but every phone has a microphone. Alipay, which launched peer-to-peer payment using audio signals in January with its mobile wallet, announced recently that consumers can now use the ultrasonic noise for payments at vending machines in Beijing's subway stations. 

"The solution itself is interesting because it could work on any kind of mobile device, not only smartphones," says Jordan McKee, an analyst with The Yankee Group.  "Sound waves could be used in third world countries, similar to what M-Pesa is doing, because it works with feature phones."

Silicon Valley-based shopkick also relies on audio technology. Shopkick offers incentives to consumers for visiting stores and, through partnerships with MasterCard and Visa, it further rewards users for their purchases.

Sound wave-based mobile payments "are using equipment that comes naturally on a consumer's phone," says Dave Kaminsky, senior analyst at Mercator Advisory Group.

With shopkick's app, the phone's mic picks up a signal sent through a small nightlight-sized device plugged in at the store's entrance. Stores such as Macy's, which pipe in music from Mood Media Corp. (owner of Muzak), can also broadcast the ultrasonic shopkick signal over their speakers.

By broadcasting different signals in different parts of a department store, shopkick's merchant clients can reward shoppers for visiting a specific section. By comparison, GPS doesn't pinpoint the consumer's exact location well enough, says Aaron Emigh, co-founder and chief technology officer at shopkick.

The company's system "is the 21st century version of the Sunday circular or catalog," he says.

Sound wave-based payments should also work with most point of sale hardware, Kaminsky says. But the technology is "not as intuitive as QR codes," he says.

Sound is also potentially less secure, since audio signals could be picked up from a farther distance than the alternatives, he says.

Cryptographic protocols can deter fraud, Emigh says. Systems similar to Secure Socket Layer (SSL), an encryption tool banks use, could work for audio signals as well.

While there seem to be niche use cases, McKee is still skeptical of the overall success of the model.

"It took NFC and QR codes a long time to get where they are today and for something like sound waves to jump into the picture, it's going to be very difficult for it to dethrone NFC and QR codes," he says. 

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