Playing It By Ear

Are you one of the relatively few people that has the ear rumbling “superpower?” The tensor tympani, a muscle located in the middle ear, is responsible for that ability. This muscle normally dampens loud sounds like chewing and thunder by reducing vibrations of the eardrum. But for those that have voluntary control of the tensor tympani, it is possible to induce a dull rumbling sound at will.

What is a technophile to do with this information? Build an ear rumbling gesture recognition system to control digital devices, of course. At least, that is what a group at the Karlsruhe Institute of Technology did in developing EarRumble. The method provides a hands- and eyes-free input method by performing gestures via voluntary contraction of the tensor tympani muscle.

EarRumble can be incorporated into earphones to allow for discreet, low-effort gestures. Four different types of gestures — a single and double rumble, holding a rumble, and repetitive rumbles were tested in the study.

The prototype hardware consists of a gutted pair of commercial earphones, in which a Bosch BME280 pressure sensor was installed in order to capture in-ear barometry measurements. Foam earplugs provide an airtight seal of the ear canal such that small pressure differences can be captured. A speaker is also included in the design to allow the earphones to perform their normal function of playing audio.

The software pipeline begins with a gesture detection step, with a 360 millisecond sliding window. Pressure sensor readings are analyzed to compute the sum of absolute difference (SAD) within each window. When a SAD above twenty Pascal is found, the window is flagged as containing gesture activity. Detected gestures were then classified with a gradient boosting classifier (XGBoost).

A total of eighteen individuals were recruited to assess the accuracy of EarRumble. Participants were asked to perform the four gestures, four everyday activities, and were also asked to do absolutely nothing. An overall classification accuracy of 93% was achieved. The device was received very well by users who described it as both easy to use and “magical.”

Unfortunately, not everyone can ear rumble. It is not certain what percentage of the population is capable, but in a small-scale online survey, the team found that 43.2% of respondents reported having this ability. The requirement for an air-tight seal of the ear canal caused issues with discomfort in some cases, and the temperature and humidity changes that occur with prolonged sealing of the ear canal could possibly support the entry of bacteria into the middle ear.

Based on feedback from participants, EarRumble appears to be an interesting new method for gesture recognition — but as one of the many that cannot ear rumble, I guess I will never know.