The performance of acoustic hoods at low frequencies

An investigation has been made into the performance of acoustic hoods at low frequencies, in the range where difficulties are often encountered in obtaining adequate attenuation. The problem has been analysed for the case of one dimension and, with the aid of small boxes and panels, the solution is shown to be of practical value. The importance of having a sealed enclosure with stiff walls is illustrated, and several methods of introducing wall rigidity have been investigated. When consideration need only be given to higher frequencies, limp wall techniques may be used to advantage, providing adequate mechanical damping resides in the system. Methods of introducing wall damping have been investigated and discussed, and some further factors influencing the low frequency performance are included. It is concluded that further research into devising highly damped stiff structures would be advantageous.