Physics Colloquium: Biological Motion Sensors and Vertigo

Samuel Owen, Otolith Labs

Abstract: Spatial awareness is the product of the brain taking in information primarily from the eyes, muscles/tendons, and vestibular system – a set of motion sensors in the inner ear. When there is an information mismatch from these sources the result can be motion sickness or even a false perception of motion (vertigo). If the problem is from the vestibular system, one way to overcome the mismatch is to introduce ‘white noise’ and rely on the brain’s natural tendency to ignore a constant/non-informative signal. This ‘white noise’ can be created through mechanically vibrating the motion sensors by applying a bone conduction transducer. While conceptually simple, there are several engineering challenges to overcome:

1. Vibration is sound by another name. Since the vestibular system is in the inner ear only low frequency vibrations can be used to avoid dangerous levels of audible sound.
2. Transducers designed for lower frequencies are large. In order to make a practical product, the transducer has to be small enough to wear comfortably and inconspicuously.
3. Driving a transducer at a non-resonant frequency wastes energy. In a small transducer, this wasted energy will result in dangerous levels of heat.
4. Small transducers resonate at higher frequencies. When a small transducer is driven at a low frequency it will naturally vibrate at frequency’s harmonics and create audible sound.

A traditional bone conduction transducer is designed with a permanent magnet fixed to the housing and a copper coil mounted to a flexible membrane/spring, allowing the coil to move back and forth as an alternating current is passed through it. The engineering challenges faced were overcome by applying the fundamentals of spring mechanics f ≈√(k/m) in order to lower the resonant frequency. To increase the moving mass, the traditional speaker design was reversed with the permanent magnet mounted to a spring and the coil fixed to the housing. To decrease the spring constant, the spring was then elongated by passing it through the center of the magnet and mounting it to the opposite side. The result was a transducer capable of creating the necessary level of low frequency vibration while being 15x less massive, 30x more efficient, and >1000x less audible than a commercially available bone conduction transducer creating the same vibration.

The patented transducer technology is now being tested in clinical trials for motion sickness, VR sickness, and as the first acute treatment for chronic vertigo.