  This paper presents the design and implementation of WhisperWand, a
comprehensive voice and motion tracking interface for voice assistants.
Distinct from prior works, WhisperWand is a precise tracking interface that can
co-exist with the voice interface on low sampling rate voice assistants. Taking
handwriting as a specific application, it can also capture natural strokes and
the individualized style of writing while occupying only a single frequency.
The core technique includes an accurate acoustic ranging method called Cross
Frequency Continuous Wave (CFCW) sonar, enabling voice assistants to use
ultrasound as a ranging signal while using the regular microphone system of
voice assistants as a receiver. We also design a new optimization algorithm
that only requires a single frequency for time difference of arrival.
WhisperWand prototype achieves 73 um of median error for 1D ranging and 1.4 mm
of median error in 3D tracking of an acoustic beacon using the microphone array
used in voice assistants. Our implementation of an in-air handwriting interface
achieves 94.1% accuracy with automatic handwriting-to-text software, similar to
writing on paper (96.6%). At the same time, the error rate of voice-based user
authentication only increases from 6.26% to 8.28%.
