Ice, Ice, Safety

Summer may not be quite over just yet, but it is starting to sputter here in the Northern Hemisphere. For many, that means putting the beach chairs and coolers in storage in preparation for the coming winter. In their place, ice shanties, skates, and other wintertime toys will soon start to reemerge. And with the arrival of that season once again comes the need for reminders about ice safety.

Lots of fun and sport can be had on the surface of a frozen lake, but there is also plenty of danger if that sheet of ice is not thick enough. Thousands of people die after unexpectedly plunging into an icy body of water every year. To prevent these tragedies, the thickness of ice is often measured by chiseling a hole in it and using a tape measure. This method is prone to error, however.

More accurate measurement techniques, like ground-penetrating radar, are also available. However, the instruments cost tens of thousands of dollars and require specialized training for use, so they are rarely used for recreation. Engineers at the Dalian University of Technology have come up with a solution that could offer the accuracy of technologies like radar with the ease and low cost of a tape measure. They have developed a method that turns a smartphone into an ice thickness sensor that leverages acoustics.

The system, called IceSound, uses the phone’s built-in speaker to emit a specially designed chirp signal spanning from 3 kHz to 22 kHz. When the sound waves travel through the ice, they bounce off two key boundaries — the air-ice interface at the surface and the ice-water interface at the bottom. The smartphone’s microphone then picks up these echoes. By calculating the time delay between the reflections, the app can determine the ice’s thickness using a formula based on the speed of sound in ice.

However, this is more difficult in practice than in theory. Sound travels through ice much faster than through air (about 3,250 m/s at 0 degrees C) making it harder to distinguish between the closely spaced echoes. Thick ice also weakens the bottom reflection, making it difficult to detect. And since sound speed varies with temperature, ranging from 3,250 m/s near freezing to around 3,800 m/s at -30 degrees C, simply assuming a constant value could lead to big errors. Furthermore, the phone’s orientation matters. If the speaker and microphone are not pointed directly at the ice surface, the results can be skewed.

To overcome these obstacles, IceSound layers in some clever engineering. Signal processing techniques like wavelet denoising, high-pass filtering, and combining multiple measurements help tease out the faint bottom echo from background noise. For thicker ice with distinct echoes, the system relies on time-domain correlation to extract the delay; for thinner ice, where echoes overlap, frequency-domain analysis proves more effective. To deal with temperature variations, the app integrates data from public weather APIs, which it uses to model how sound speed changes with depth. The smartphone’s accelerometer and magnetometer also provide orientation data so that the app can compensate for tilt.

In field trials, IceSound achieved a mean absolute error of just 0.3 centimeters when measuring ice in the critical 10-15 cm range. More importantly, when assessing whether the ice was safe for human activity, the system’s error rate dropped to just 0.13%.

By turning a device nearly everyone already carries into a powerful ice safety tool, IceSound could make winter sports and recreation much safer. Instead of lugging around an auger, chisel, or radar rig, future skaters and ice fishers may only need to pull out their phone, run an app, and know within seconds if the surface beneath their feet is safe.