An apparatus for use in a borehole, comprising: an acoustic sensor including: a pressure transducer (302; 402; 430; 470) comprising a first flexing member (420; 421; 491) attached to an optical medium (433; 483), the transducer (302...470) configured to convert a low-amplitude, high-frequency acoustic pressure signal within a nominal borehole pressure incident on the transducer (302...470) to an alternating strain on the optical medium (433; 483) along an axis of the medium (433; 483) via movement of the first flexing member (420; 421; 491) while suppressing conversion of low-frequency signals and constant pressure into strain of the optical medium (433; 483), the acoustic pressure signal propagated via a downhole fluid coupled with the transducer (302...470), wherein the pressure transducer (302...470) comprises a first interface (424; 444; 474) between reservoirs (408, 410; 438, 440; 478, 480) containing a reservoir fluid, the first interface (424; 444; 474) comprising the first flexing member (420; 421; 491) and at least one opening (404; 451, 452; 481, 482) providing for communication of the reservoir fluid between the reservoirs (408...480), and wherein high-frequency refers to frequency characteristics corresponding to typical acoustic signals used for measurement applications downhole, and low-frequency refers to frequency characteristics corresponding to acoustic signals below a level representing desired acoustic measurements; and a detector configured to generate acoustic measurement information responsive to received electromagnetic radiation transmitted through the medium (433; 483) along the axis, at least one optical property of the medium (433; 483) being responsive to the alternating strain on the medium (433; 483) such that the electromagnetic radiation received by the detector represents the acoustic signal, characterized in that: the pressure transducer (302...470) comprises a second interface between one of the reservoirs (408...480) and the downhole fluid, the second interface comprising a second flexing member (422; 432; 472) responsive to the high-frequency acoustic pressure signal incident on the transducer (302...470). an acoustic sensor including: a pressure transducer (302; 402; 430; 470) comprising a first flexing member (420; 421; 491) attached to an optical medium (433; 483), the transducer (302...470) configured to convert a low-amplitude, high-frequency acoustic pressure signal within a nominal borehole pressure incident on the transducer (302...470) to an alternating strain on the optical medium (433; 483) along an axis of the medium (433; 483) via movement of the first flexing member (420; 421; 491) while suppressing conversion of low-frequency signals and constant pressure into strain of the optical medium (433; 483), the acoustic pressure signal propagated via a downhole fluid coupled with the transducer (302...470), wherein the pressure transducer (302...470) comprises a first interface (424; 444; 474) between reservoirs (408, 410; 438, 440; 478, 480) containing a reservoir fluid, the first interface (424; 444; 474) comprising the first flexing member (420; 421; 491) and at least one opening (404; 451, 452; 481, 482) providing for communication of the reservoir fluid between the reservoirs (408...480), and wherein high-frequency refers to frequency characteristics corresponding to typical acoustic signals used for measurement applications downhole, and low-frequency refers to frequency characteristics corresponding to acoustic signals below a level representing desired acoustic measurements; and a detector configured to generate acoustic measurement information responsive to received electromagnetic radiation transmitted through the medium (433; 483) along the axis, at least one optical property of the medium (433; 483) being responsive to the alternating strain on the medium (433; 483) such that the electromagnetic radiation received by the detector represents the acoustic signal, characterized in that: the pressure transducer (302...470) comprises a second interface between one of the reservoirs (408...480) and the downhole fluid, the second interface comprising a second flexing member (422; 432; 472) responsive to the high-frequency acoustic pressure signal incident on the transducer (302...470).