  We present direct measurements of the gas acoustic dynamics following
interaction of spatial single- and multi-mode 50 fs, 800 nm pulses in air at 10
Hz and 1 kHz repetition rates. Results are in excellent agreement with
hydrodynamic simulations. Under no conditions for single filaments do we find
on-axis enhancement of gas density; this occurs only with multi-filaments. We
also investigate the propagation of probe beams in the gas density profile
induced by a single extended filament. We find that light trapping in the
expanding annular acoustic wave can create the impression of on-axis guiding in
a limited temporal window.
