|Fig. 1: Several frequency shifts recorded. (a) Represents a no motion. (b and c) Represents toward and away motions. (d) Represents a large shift in frequency which requires further analysis and a second scan.  (Courtesy of ACM)|
The comparison of user interface on cell phones since the very first cell phone to today's latest smart phone shows remarkable changes and upgrades. Large clunky buttons are a thing of the past, and touchscreen displays are the standard. Even with all of the innovation taking place one can't help but ask, where do we go from here?
One of the latest innovations in the cellphone-user experience is one being sought after by Microsoft called Soundwave. 
Soundwave is a new concept being researched exclusively by Microsoft that uses the Doppler effect to sense gestures. The sensing is achieved by first implementing an inaudible tone via the cellphone speaker in the users environment. When a user moves their hand in gesture, the tone experiences a frequency shift which is measured. The frequency shifts are gathered and analyzed. Any frequency shift measured that is within the threshold of a certain gesture is finally what determines which function the cell phone will utilize. Some of the fundamental measurable properties that determine the gesture are velocity, direction, proximity, size of target, and time variation. An algorithm based on these quantities, in accordance with the frequency shift, determines the gesture.
Fig. 1 shows an example of how different Doppler shifts are classified as different gestures.  As of now, Soundwave can successfully implement scrolling, see-saw motions, single or double taps, and sustained motion.
The main limitation with Soundwave is the fact that it is based solely on the Doppler effect, which means motion is required. Because of this static poses are not measurable which obviously puts a limit to the available gestures that can be used, and requires much more complicated techniques if static poses wanted to be implemented. In addition, the inaudible tone in the user environment may actually not be completely inaudible for children and animals.  This could be quite an annoying drawback of Soundwave. And lastly, to create a distinct shift in frequency, the gestures require movement of palms or the whole hand rather than just fingers. In initial tests, this factor played a part in decreasing the accuracy of Soundwave for users with small hands or those that tend to use too much of their fingers in their gestures. 
© Donish Khan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. M. Chang, "Next Up in Kinect-Style Motion Sensing: Ultrasound? Popular Mechanics, 25 May 12.
 S. Gupta, D. Morris, S. Patel, D. Tan, "SoundWave: Using the Doppler Effect to Sense Gestures" Association for Computing Machinery, 5 May 12.
 H. F. Olson, Music, Physics and Engineering (Dover, 1967).