Absorption Characteristics of Acoustic Resonators Combined with Porous Foam

Acoustic resonators are often used to control low frequency noise in buildings. However, size, weight, and cost constraints limit their use in many aerospace applications. To address these limitations, alternative structures with integrated acoustic resonators have been developed. This technology makes it possible to include a large array of acoustic resonators within the primary structure of the vehicle without increasing the weight or reducing the useable interior volume. However, the use of resonant noise control does not necessarily alleviate the need for porous blankets, which provide broadband noise control and thermal insulation. Consequently, a practical implementation of structurally integrated resonators will likely include an adjacent layer of porous material. The purpose of this study is to assess the acoustic performance of porous material used in conjunction with a structurally integrated resonator array. Specifically, this paper compares diffuse-field absorption measurements and predictions for flat sandwich panels containing integrated resonator arrays, foam treatment in isolation, and multilayer treatments consisting of foam and resonators. Results show that the acoustic resonators significantly increase the absorption in targeted low-frequency bands. Both predictions and measurements also show that the addition of foam treatment increases high-frequency absorption while simultaneously improving the low-frequency performance of the resonators. A physical explanation for the results is provided.