1. . 
Pertaining to sound. Generally, acoustic describes sound or vibrational events, regardless of frequency. The term sonic is limited to frequencies and tools operated in the frequency range of 1 to 25 kilohertz.
2. . 
In geophysics, acoustic refers specifically to P-waves in the absence of S-waves (i.e., in fluids, which do not support S-waves, or in cases in which S-waves in solids are ignored).