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DESIGNING quiet structures is gaining increased attention every day as aircraft manufacturers are continuously aiming to reduce the noise level inside aircraft. To reduce the noise inside an aircraft, an aircraft manufacturer can use two types of noise control techniques: active noise control [1, 2] and passive noise control [3]. Active noise control essentially involves the placement of loudspeakers and microphones throughout the passenger cabin or flight deck in conjunction with a control system. Active noise control is an appealing technique because it can be fitted to existing airframes without major structural modification. Passive noise control, by contrast, seeks to reduce cabin noise by the modification of the airframe structure itself. For example, this could be in the form of acoustic treatments, such as damping layers added to the cabin walls. Designing quiet structures, which is the focus of the present work, is a type …