Recently, my colleague Mike Bifulco wrote a blog post about using Near-Field Communication (NFC) technology to request payment using NFC tags and Stripe Payment Links. In a similar spirit, I came across the concept of “Ultrasonic payments”, a technology based on transferring data through inaudible sounds, and decided to experiment and look into how to implement such a payment method with Stripe.
Here’s the result below. My iPad is in airplane mode, so not connected to the internet, and transmits a payment link to my phone via ultrasound.
If you would like to try this out, check out the live demo or have a look at the repository on GitHub.
Disclaimer: This is an experiment. The ultimate goal would have been to implement a solution similar to tap-to-pay using ultrasounds, however, even though this is possible and has been done by companies such as LISNR, this is not something that can be easily prototyped. Instead, I decided to explore by sending Payment Links, a no-code payment solution that allows you to request payment from customers with a simple link. I still wanted to share what I tried and learned, as this technology can still be applied to different applications than payments.
Soundwaves
Before focusing on payments, it’s important to understand how devices transform analog sound into digital data so, without going into too much detail, here’s a brief explanation of how it works.
Sound is created by vibrations that produce shifts in the flow of molecules. These molecules bump into each other, creating a chain reaction called a sound wave. When this wave reaches a microphone, the movement of air molecules creates pressure against the thin membrane found in the device, creating electrical signals that are then converted to digital values using an Analog to Digital converter (ADC).
Source: “How do microphones work?”
Humans can usually hear sounds from a frequency range of 20Hz-20kHz. However, it doesn’t mean that sound does not exist outside of these frequencies, it only means we cannot hear it. As a result, you can transmit data at frequencies outside of what the human ear can detect, through inaudible sounds. This is a technique called ultrasonic data transmission.
So, how can we use this to transmit a payment?
Ultrasonic data transmission in JavaScript
This prototype is going to rely on quiet.js, a JavaScript library to transmit data using sound.
The same way that usual tap-to-pay systems require both the terminal and personal device to be NFC-enabled, sending data through sound requires the devices involved to have a microphone and speaker.
For the purpose of this experiment, the merchant’s device should have a speaker as it will be transmitting the payment link, and the customer’s device should have a microphone to receive it.
This scenario would allow merchants to use an iPad in-store to sell their products, without having to acquire a terminal device.
Transmitting data
Using quiet.js to transmit text via sound can be done by creating a transmitter instance, specifying some parameters including the frequency, gain and frame length, and finally calling the .transmit() method with the payload.
Let’s look into the code needed to do this.
Initial setup
To start, the library needs to be imported. Quiet.js includes a blob of libquiet that it relies on, and JavaScript bindings, so in my prototype, I used script tags to import quiet.js and
