A medical device for non-invasively outputting a reduced noise signal responsive to acoustic vibrations indicative of one or more physiological parameters of a medical patient, comprising: a first acoustic sensing element configured to be acoustically coupled to the body of a patient, the first acoustic sensing element being configured to output a first signal comprising a physiological signal component and a noise component; a second acoustic sensing element being configured to output a second signal comprising at least a noise component; and a noise attenuator (740; 1740) configured to produce a reduced noise signal in response to the first and second signals, the reduced noise signal comprising a physiological signal component and a noise component, the ratio of the physiological signal component of the reduced noise signal to the noise component of the reduced noise signal being greater than the ratio of the physiological signal component of the first signal to the noise component of the first signal, wherein the first acoustic sensing element and the second acoustic sensing element are provided on or in a medical sensor unit, characterised in that the medical sensor unit comprises a frame shaped to provide a first acoustic cavity, the first acoustic sensing element being mechanically coupled to the frame and comprising at least one piezoelectric film stretching in tension across the first acoustic cavity, wherein the medical sensor unit further comprises an acoustic coupler supported by the frame and positioned to apply pressure to the first acoustic sensing element to bias the first acoustic sensing element at a predetermined tension into said first acoustic cavity, wherein the acoustic coupler comprises a first protrusion or bump configured to press against the body of the patient when the medical sensor unit is fastened into place on the patient, wherein the acoustic coupler further comprises a second protrusion designed to abut against the first acoustic sensing element to apply the pressure to the first acoustic sensing element, and wherein the second acoustic sensing element is in proximity to the first acoustic sensing element, and wherein an amount of acoustic coupling between the first acoustic sensing element and the patient is greater than an amount of acoustic coupling between the second acoustic sensing element and the patient. a first acoustic sensing element configured to be acoustically coupled to the body of a patient, the first acoustic sensing element being configured to output a first signal comprising a physiological signal component and a noise component; a second acoustic sensing element being configured to output a second signal comprising at least a noise component; and a noise attenuator (740; 1740) configured to produce a reduced noise signal in response to the first and second signals, the reduced noise signal comprising a physiological signal component and a noise component, the ratio of the physiological signal component of the reduced noise signal to the noise component of the reduced noise signal being greater than the ratio of the physiological signal component of the first signal to the noise component of the first signal, wherein the first acoustic sensing element and the second acoustic sensing element are provided on or in a medical sensor unit, characterised in that the medical sensor unit comprises a frame shaped to provide a first acoustic cavity, the first acoustic sensing element being mechanically coupled to the frame and comprising at least one piezoelectric film stretching in tension across the first acoustic cavity, wherein the medical sensor unit further comprises an acoustic coupler supported by the frame and positioned to apply pressure to the first acoustic sensing element to bias the first acoustic sensing element at a predetermined tension into said first acoustic cavity, wherein the acoustic coupler comprises a first protrusion or bump configured to press against the body of the patient when the medical sensor unit is fastened into place on the patient, wherein the acoustic coupler further comprises a second protrusion designed to abut against the first acoustic sensing element to apply the pressure to the first acoustic sensing element, and wherein the second acoustic sensing element is in proximity to the first acoustic sensing element, and wherein an amount of acoustic coupling between the first acoustic sensing element and the patient is greater than an amount of acoustic coupling between the second acoustic sensing element and the patient.