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Open cavities generating cavity tones |
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The goal of this research is to investigate the acoustic resonances of 2d and 3D open cavities by a fast and efficient method in the time domain. This method models the time response in any point as the convolution of the source waveform with the impulse response of the cavity, which, in turn, is obtained as a sequence of attenuated and delayed impulses coming, the first from the real, and the subsequent from the mirror imaged sources (Image Source Model). This method, which main advantages with respect to others that work in the frequency domain are that the results for all frequencies can be calculated at once, the time domain data is recovered directly, and the computational cost does not increase with frequency, can easily provide the frequency response at each point of the cavity by Fourier transform. From these frequency responses, the relevant resonances of the cavity can simply be obtained. Illustration and experimental validation of the proposed method is presented by its application to a rectangular open cavity.
A 3D open cavity for the measurement of resonances in the Anechoic Room
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| Experimental (left) and numerical (right) mode (2,2,0) for the 3D open cavity | |
The cavity noise can be reduced by changing its geometry, by passive control strategies, or by active control techniques.
After optimizing the design of the cavity, the cavity walls will be lined with absorbent materials. This is a classic strategy in order to reduce the noise. In the case where there is no air-flow, fibres and porous materials are used. However, when there is an air-flow, porous metals and metallic foams or microperforated panel (MPP) are employed instead.
Frequently both passive and active noise control strategies are often combined, since passive insulation is more efficient for high frequencies, whereas active control is better suited for lower frequencies. Active control essentially tries to eliminate sound or vibration components by adding the exact opposite sound or vibration.
Polytechnic University of Madrid (ETSI Aeronautics), National Institute of Aerospatiale Technologies (INTA), AIRBUS Spain
