For years, banks, cell-phone carriers, and tech companies have been experimenting with technology that enables mobile phones to connect securely over short distances and make payments in stores. Although Google and other big companies say they’re committed to the radio-based technology known as near-field communications (NFC), few phones have the necessary hardware built in.

Now startup Naratte, based in Sunnyvale, California, claims it can deliver the same experience on almost any existing phone, establishing a secure link by generating a sound too high-pitched for the human ear. “All you need is a speaker and microphone, which you already have on your device,” says Brett Paulson, Naratte’s chief executive and cofounder. “We’ve built everything in software so you just download an app to get a contactless experience.”

Using the technology, known as Zoosh, involves briefly holding a phone within six inches of either another handset with a Zoosh-enabled app or a dedicated reader connected to a store’s checkout terminal. The devices exchange short ID tokens encoded into blips of ultrasound to identify each other, a process that takes less than one second. Then users can make debit or credit transactions of points or even cash, or let devices swap data such as contact info.

Naratte has spent two years developing the audio-processing technology needed to make the approach secure enough for payments data and robust enough to work even in noisy environments, says Byron Alsberg, the company’s other cofounder and chief development officer. “Only in the last couple of years has it become possible to do the audio processing needed on a phone without adding a specialized chip,” he says. As phones have begun to double as media players and interest in speech recognition has grown, playback and microphone quality have improved.

Even simple phones that can’t runs apps can use the technology. Text messages with embedded audio files can allow these phones to use the system. “The criterion is: does it have MP3 playback?” says Alsberg. “That’s a lot of devices.”

The first deployment of Zoosh will be in loyalty-program apps created for small merchants by fellow startup Sparkbase. The technology will be in use at “tens of thousands” of small stores and cafes by the fall, says Alsberg.

Laura Chambers, manager for mobile at the payments processor PayPal, wrote in a statement that she and her company were “very excited about Naratte’s Zoosh technology” after seeing it demonstrated. “Zoosh’s approach provides instant scale,” she said, “which is a major hurdle for most mobile payment technologies.” Naratte is keeping quiet about its relationship with PayPal, but even without a partnership, has built a demonstration app (pictured) that allows contactless PayPal transactions, using publicly available application-programming interfaces. Naratte is also in talks with various cell-phone carriers worldwide.

Avivah Litan, an analyst specializing in payments technology for Gartner, an IT research and advisory company based in Stamford, Connecticut, says Naratte’s technology looks promising, but she adds that contactless phone payments have been slow to arrive mostly for business reasons, not technological ones.

“Card companies have offered contactless payments, albeit via plastic cards with special chips, for many years now,” she points out. But there is little evidence that either these or phone-based payment systems will much benefit businesses. “Without a compelling business value—more revenue for equal or lower costs, or significantly higher transaction throughput—we won’t see any more adoption than we have seen to date,” she predicts.

If contactless payments do become more attractive to businesses, though, Naratte may be well placed, says Litan. “[It] would help accelerate adoption,” she explains, “since the market wouldn’t have to wait for consumers to buy NFC-enabled phones.”