A system and method for treating biomass so as to convert the biomass into a useful form is provided. In some embodiments, the system and method allow treating a biomass mixed in a fluid medium in an acoustic resonator chamber. The chamber may be used to mix the biomass with other chemical agents or catalysts. The chamber is also coupled to one or more acoustical drivers to provide an acoustical (e.g., ultrasonic) field in the chamber, which can also be driven to cause acoustic cavitation in some embodiments. The acoustic resonator chamber may also be placed under static pressure to enhance a mechanical, acoustical, or chemical effect during processing. Various examples of co-processing or pre-processing stages are also provided, including acid, base, AFEX, ammonia and other stages to enhance a desired effect.