A novel time-of-flight estimation method of acoustic signals for temperature and velocity measurement of gas medium

Acoustic thermometry and velocimetry are widely recognized as promising methods for in situ measurement of gas temperature and velocity due to their non-invasive, low-cost, and high-resolution advantages. Accurate estimation of acoustic time-of-flight (TOF) is the key to improving the accuracy of acoustic thermometers and velocimeters. In this paper, a new TOF estimation method is developed based on the digital lock-in filtering (DLF) technique. The signal source is an audible sound with linearly modulated frequencies, and the relationship between the time and frequency domains of the signal is established using digital lock-in and low-pass filtering techniques. The linearity of the time–frequency series and the accuracy of the TOF are improved by using a least-squares-based linear fitting method. Experimental studies of gas temperature and velocity measurements with acoustic methods in static, dynamic, and different signal-to-noise ratio (SNR) environments have been carried out. The DLF method performs well in terms of accuracy and robustness to extract the modulated signals of known carriers in a strong noise environment. Furthermore, the very nature of the method predisposes the time-domain data of the acoustic source signal not to be repeatedly processed at each TOF calculation, which saves system resources for processing the source signal.