An experimental study on the effects of an externally-imposed transverse acoustic field in a N2 shear coaxial jets at sub-, near-, and supercritical pressures is presented. Such fields and their interaction with the jets (i.e., breakup, mixing, etc.) are believed to play a critical role during combustion instabilities in liquid rocket engines. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N2 as the working fluid, the chemistry effects on combustion instability are separated from the effects of a transverse acoustic field on coaxial jets. Furthermore, through this choice, ambiguities associated with composition dependence on mixtures critical properties are eliminated. The acoustic field is generated by a piezo-siren and the first resonant frequency is 3kHz. The pressures in the chamber range from 215-716 psia to span subcritical to supercritical pressures. The outer to inner jet velocity ratio varies from 1.2 to 23 and the momentum flux ratio (MR) varies from 0.2 to 23. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. At least 2000 backlit images were taken at 41kHz for each run. The main metric investigated is the length of the dark, or inner jet, core. This length is related to the mixing processes in a coaxial jet. The shorter the core length the faster the mixing occurs. Both the axial and the total, or curved, dark core lengths are studied. For momentum flux ratios 1MR4 the differences in the axial and curved dark core lengths between acoustics on and off are statistically significant, which means acoustics do shorten the core for this range. For subcritical pressures the MR range where the jet is shortened is larger. Preliminary results on the frequency analysis of the dark core lengths and width is also presented.