{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T10:15:04Z","timestamp":1788171304055,"version":"build-2803163510"},"reference-count":21,"publisher":"MDPI AG","issue":"17","license":[{"start":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T00:00:00Z","timestamp":1788134400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>This paper presents a smartphone-based sonar-sensing system for contactless monitoring of respiration and gesture-based user activity, using a single commercial phone with no hardware modification. A 20 kHz audio signal is transmitted from the phone\u2019s speaker and reflections are captured by its microphone; motion-induced Doppler shifts are recovered using an envelope detection method that removes the need for a reference copy of the transmitted signal and improves robustness to transmitter frequency drift. Actuator experiments at a target distance around 50 cm demonstrated motion sensitivity at two conditions, 6 mm at 0.15 Hz and 2 mm at 0.6 Hz, with a measured signal-to-noise ratio of approximately 21.7 dB. Respiration rate detected by the system was compared against a pulse oximeter across two subjects, with an error within 0.1 Hz of the reference. Gesture-based interactions (tapping, scrolling, typing, talking) were analyzed using short-time Fourier transform features as a first step toward device authentication. Distance and direction estimation are outside the scope of this work and are identified as directions for future development.<\/jats:p>","DOI":"10.3390\/s26175512","type":"journal-article","created":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T09:21:19Z","timestamp":1788168079000},"page":"5512","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Smartphone Acoustic Sensing for Contactless Respiration Monitoring and Gesture-Based Authentication"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0009-0007-8896-7564","authenticated-orcid":false,"given":"Raj","family":"Nandini","sequence":"first","affiliation":[{"name":"Department of Electrical Engineering and Computer Science, Syracuse University, Syracuse, NY 13244, USA"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"software","vocabulary":"credit"},{"role":"validation","vocabulary":"credit"},{"role":"formal-analysis","vocabulary":"credit"},{"role":"investigation","vocabulary":"credit"},{"role":"writing-original-draft","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0073-6707","authenticated-orcid":false,"given":"Ashish","family":"Mishra","sequence":"additional","affiliation":[{"name":"Independent Researcher, Irvine, CA 92618, USA"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"formal-analysis","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"},{"role":"supervision","vocabulary":"credit"}]}],"member":"1968","published-online":{"date-parts":[[2026,8,31]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"979","DOI":"10.1109\/TCSVT.2014.2302516","article-title":"Fully Unsupervised Learning of Camera Link Models for Tracking Humans Across Nonoverlapping Cameras","volume":"24","author":"Chu","year":"2014","journal-title":"IEEE Trans. Circuits Syst. Video Technol."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Nowara, E.M., Marks, T.K., Mansour, H., and Veeraraghavan, A. (2018, January 18\u201322). SparsePPG: Towards Driver Monitoring Using Camera-Based Vital Signs Estimation in Near-Infrared. Proceedings of the 2018 IEEE\/CVF Conference on Computer Vision and Pattern Recognition Workshops (CVPRW), Salt Lake City, UT, USA.","DOI":"10.1109\/CVPRW.2018.00174"},{"key":"ref_3","unstructured":"Yu, M.-C., Liou, J.-L., Kuo, S.-W., Lee, M.-S., and Hung, Y.-P. (September, January 28). Noncontact respiratory measurement of volume change using depth camera. Proceedings of the 2012 Annual International Conference of the IEEE Engineering in Medicine and Biology Society, San Diego, CA, USA."},{"key":"ref_4","unstructured":"Chekmenev, S., Rara, H., and Farag, A.A. (2005, January 19\u201321). Non-contact, wavelet-based measurement of vital signs using thermal imaging. Proceedings of the First International Conference on Graphics, Vision, and Image Processing (GVIP), Cairo, Egypt."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"195","DOI":"10.1049\/el.2019.3380","article-title":"Human identification based on radar micro-Doppler signatures separation","volume":"56","author":"Qiao","year":"2020","journal-title":"Electron. Lett."},{"key":"ref_6","first-page":"8557","article-title":"Estimation of Doppler Velocities from Sub-Nyquist Ultra-Wideband Radar Measurements","volume":"16","author":"Sakamoto","year":"2016","journal-title":"IEEE Sens. J."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"558","DOI":"10.1109\/JSEN.2011.2134083","article-title":"Respiratory Monitoring and Clutter Rejection Using a CW Doppler Radar with Passive RF Tags","volume":"12","author":"Singh","year":"2012","journal-title":"IEEE Sens. J."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"5384","DOI":"10.1109\/TMTT.2019.2951142","article-title":"Sensitivity and Distortion Analysis of a 125-GHz Interferometry Radar for Submicrometer Motion Sensing Applications","volume":"67","author":"Rodriguez","year":"2019","journal-title":"IEEE Trans. Microw. Theory Tech."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"158284","DOI":"10.1109\/ACCESS.2019.2950347","article-title":"Intermodulation-Based Nonlinear Smart Health Sensing of Human Vital Signs and Location","volume":"7","author":"Mishra","year":"2019","journal-title":"IEEE Access"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"9206","DOI":"10.1109\/JSEN.2019.2926189","article-title":"A Low Power 5.8-GHz ISM-Band Intermodulation Radar System for Target Motion Discrimination","volume":"19","author":"Mishra","year":"2019","journal-title":"IEEE Sens. J."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"501","DOI":"10.1016\/j.envres.2018.11.042","article-title":"Exposure and health risks perception of extremely low frequency and radiofrequency electromagnetic fields and the effect of providing information","volume":"169","author":"Gallastegi","year":"2019","journal-title":"Environ. Res."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1088","DOI":"10.1049\/iet-its.2018.5291","article-title":"Feature-based detection and classification of moving objects using LiDAR sensor","volume":"13","author":"Guo","year":"2019","journal-title":"IET Intell. Transp. Syst."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"789","DOI":"10.1049\/iet-its.2018.5258","article-title":"Trajectory tracking and prediction of pedestrian\u2019s crossing intention using roadside LiDAR","volume":"13","author":"Zhao","year":"2019","journal-title":"IET Intell. Transp. Syst."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"6906","DOI":"10.1049\/joe.2019.0596","article-title":"Investigating vertical motion of small insects in atmospheric boundary layer using millimetre-wavelength radar and Doppler LIDAR","volume":"2019","author":"Wainwright","year":"2019","journal-title":"J. Eng."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"6056","DOI":"10.1109\/JSEN.2017.2734688","article-title":"Food Intake Detection Using Ultrasonic Doppler Sonar","volume":"17","author":"Lee","year":"2017","journal-title":"IEEE Sens. J."},{"key":"ref_16","doi-asserted-by":"crossref","unstructured":"Zedel, L. (2015, January 2\u20136). Modelling Doppler sonar backscatter. Proceedings of the 2015 IEEE\/OES Eleventh Current, Waves and Turbulence Measurement (CWTM), St. Petersburg, FL, USA.","DOI":"10.1109\/CWTM.2015.7098132"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"111897","DOI":"10.1109\/ACCESS.2019.2933987","article-title":"Hand Gesture Recognition Based on Active Ultrasonic Sensing of Smartphone: A Survey","volume":"7","author":"Wang","year":"2019","journal-title":"IEEE Access"},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Liu, C., Jiang, S., Zhao, S., and Guo, Z. (2019). Infrastructure-Free Indoor Pedestrian Tracking with Smartphone Acoustic-Based Enhancement. Sensors, 19.","DOI":"10.3390\/s19112458"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Wang, X., Huang, R., and Mao, S. (August, January 31). SonarBeat: Sonar Phase for Breathing Beat Monitoring with Smartphones. Proceedings of the 2017 26th International Conference on Computer Communication and Networks (ICCCN), Vancouver, BC, Canada.","DOI":"10.1109\/ICCCN.2017.8038412"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"177","DOI":"10.1145\/3287055","article-title":"PCIAS: Precise and Contactless Measurement of Instantaneous Angular Speed Using a Smartphone","volume":"2","author":"Li","year":"2018","journal-title":"Proc. ACM Interact. Mob. Wearable Ubiquitous Technol."},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Tan, J., Nguyen, C.-T., and Wang, X. (2017, January 1\u20134). SilentTalk: Lip Reading through Ultrasonic Sensing on Mobile Phones. Proceedings of the IEEE INFOCOM 2017\u2014IEEE Conference on Computer Communications, Atlanta, GA, USA.","DOI":"10.1109\/INFOCOM.2017.8057099"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/17\/5512\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T09:22:54Z","timestamp":1788168174000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/17\/5512"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,8,31]]},"references-count":21,"journal-issue":{"issue":"17","published-online":{"date-parts":[[2026,9]]}},"alternative-id":["s26175512"],"URL":"https:\/\/doi.org\/10.3390\/s26175512","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,8,31]]}}}