Acoustic sensors receive signals which are emitted by ultrasonic emitters with frequencies above the audible range of the human ear, at approximately 20 kHz, and determine their location via time-of-flight. A si mple emitterreceiver pair delivers the distance obtained from the simple beat timing. For estimation of a position in a three-dimensional space at least one emitter and three receivers are required. An incre ase in resolution can be achieved by three emitters and three receivers tune d at different frequencies. Similar to infrared, the signals are distributed and mostly stay in a room, which is why the technology is commonly used to achieve room level o r subroom level accuracy. Its low propaga­tion speed of about 340 m/s returns results with a l ow time resolution. In addi tion, precision is affected by environmental variables like me dia density, reflections of the signal wave, temperature, and humidity. The limited accu racy and working volume limits the use of an acoustic tracking system to applicat ions where low resolution is required. However, with improvements of the tracking algorithm and within a short tracking range, acoustic trackers are used to realize nonelectro­magnetic tracking, achieving submillimeter accuracy, and h ave been found u seful in cardiology studies where minimum electrical signal is expected