A reduced pressure treatment system for collecting bodily fluids from a tissue site using reduced pressure, comprising: a source of reduced pressure; a dressing adapted to be disposed proximate and in fluid communication with the tissue site; a canister having sidewalls closed by a lower wall and an upper wall for collecting body fluids from the tissue site, an inlet adapted to be in fluid communication with the dressing, and an outlet disposed in the upper wall over the sidewalls proximate the upper wall and adapted to be in fluid communication with the source of reduced pressure, wherein the bodily fluids have a surface rising as the canister fills from an empty state to a full state while collecting the bodily fluids to define an airspace above the surface within the canister; a first acoustic transducer acoustically coupled to one of the sidewalls of the canister, characterized in that the first acoustic transducer is configured to insonify the airspace of the canister at predetermined acoustic frequencies along a first axis of insonification, and adapted to generate echo output signals in response to acoustic waves reflected within the airspace at the predetermined frequencies, said first acoustic transducer adapted to insonify at a plurality of frequencies between a first frequency and a second frequency; and by a processor in electrical communication with the first acoustic transducer, and adapted to provide input signals to the first acoustic transducer for insonifying the airspace at predetermined acoustic frequencies between a first frequency and a second frequency and then for receiving the echo output signals from the first acoustic transducer to determine a first harmonic frequency therebetween corresponding to the volume of the airspace. a source of reduced pressure; a dressing adapted to be disposed proximate and in fluid communication with the tissue site; a canister having sidewalls closed by a lower wall and an upper wall for collecting body fluids from the tissue site, an inlet adapted to be in fluid communication with the dressing, and an outlet disposed in the upper wall over the sidewalls proximate the upper wall and adapted to be in fluid communication with the source of reduced pressure, wherein the bodily fluids have a surface rising as the canister fills from an empty state to a full state while collecting the bodily fluids to define an airspace above the surface within the canister; a first acoustic transducer acoustically coupled to one of the sidewalls of the canister, characterized in that the first acoustic transducer is configured to insonify the airspace of the canister at predetermined acoustic frequencies along a first axis of insonification, and adapted to generate echo output signals in response to acoustic waves reflected within the airspace at the predetermined frequencies, said first acoustic transducer adapted to insonify at a plurality of frequencies between a first frequency and a second frequency; and by a processor in electrical communication with the first acoustic transducer, and adapted to provide input signals to the first acoustic transducer for insonifying the airspace at predetermined acoustic frequencies between a first frequency and a second frequency and then for receiving the echo output signals from the first acoustic transducer to determine a first harmonic frequency therebetween corresponding to the volume of the airspace.