Temporal coherence of sound propagated along vertical paths through the atmosphere

Reductions in signal coherence caused by atmospheric turbulence constrain the performance of signal processing methods. This presentation compares theoretical predictions and measurements of acoustic temporal coherence, which describes the similarity of a signal at two times. Combining sound propagation theory with turbulence models yields the theoretical coherence. To be applicable to vertical and slanted propagation, the turbulence models use height-dependent variances and length scales for the fluctuations in temperature, shear-produced velocity, and buoyancy-produced velocity. Meteorological instruments on a 135-m tower measured the required model input data. The coherence measurements used a ground-based source emitting tones between 0.6 and 3.5 kHz and nine microphones on the same tower at heights 39, 80, and 130 m. In most cases, the predicted coherences accurately approximate the measured temporal coherences. In the two early-morning trials, the measured coherence times were much larger than predicted because the atmospheric turbulence was not fully developed. Excluding these cases, the measured coherence times were in the range 0.1–100 s. For each trial, the coherence times decreased slightly more than an order of magnitude from the smallest to largest frequencies. From the shortest to the longest propagation distances, the coherence times decrease slightly less than an order of magnitude.