Abstract: An apparatus for attenuating low frequency sound in a high static pressure environment comprises a hollow, tube-shaped structure having two, free-bending spans disposed opposite each other and enclosed in an evacuated thin plastic envelope or bag. Each of the hinged spans has a flexural strength exceeding 200,000 psi and an elastic modulus exceeding 20,000,000 psi. The shape of the spans can be planar or curved, and the ends of the spans are closed. The spans having a planar cross section are supported adjacent their peripheries by T-shaped members, each of which includes an inwardly projecting flange which contacts and supports the spans. The curved-span embodiment employ two suitably shaped plugs which contact and support the two ends of the spans.