Method and apparatus for controlling acoustic treatment of a sample including a liquid. A processing volume in which the sample is acoustically treated may be controlled, e.g., by positioning a suitable element so as to reduce and/or eliminate a headspace size at a sample/gas interface. An interaction between the acoustic energy and the sample may be controlled, e.g., by using a headspace control element positioned at least partially in the sample that helps to reduce splashing or other sample ejection that would otherwise occur.