Embodiments related to sound sources for marine geophysical surveys. An embodiment provides a sound source, comprising: an outer shell containing a first gas at a first gas pressure; and a compliance chamber in indirect fluid communication with the first gas, the compliance chamber containing a second gas at a second gas pressure, wherein the second gas pressure is lower than the first gas pressure. An embodiment provides a sound source for marine geophysical surveys, comprising: an outer shell; a mass coupled to the outer shell; and an actuator coupled to the outer shell. Additional apparatus and methods are disclosed herein.