  The promising application of femtosecond laser filamentation in atmospheric
remote sensing brings imperative demand for diagnosing the spatiotemporal
dynamics of filamentation. Acoustic emission (AE) during filamentation opens a
door to give the insight into the dynamic evolution of filaments in air. In
particular, the frequency features of the acoustic emission provide relevant
information on the conversion of laser energy to acoustic energy. Here, the
acoustic emission of femtosecond laser filament manipulated by energy and the
focal lengths was measured quantitatively by a broadband microphone, and the
acoustic parameters were compared and analyzed. Our results showed that the
acoustic power presents a squared dependence on the laser energy and the
bandwidth of the acoustic spectrum showed a significant positive correlation
with laser energy deposition. It was found that the spectrum of the acoustic
pulse emitted from the middle of the filament has a larger bandwidth compared
to those emitted from the ends of the filament and the spectrum of the acoustic
pulse is also an indicator of the filament intensity distribution. These
findings are helpful for studying the plasma filament properties and complex
dynamic processes through acoustic parameters and allow the optimization of
remote applications.
