Conventional methods of suppressing acoustic noise using passive sound absorbers generally do not work well at low frequencies. This is because at these low frequencies the acoustic wavelengths become large compared to the thickness of a typical acoustic absorber. A sound wave of frequency 100 Hz, for example, will have a wavelength of about 3.4 meters in air under normal conditions. It is also difficult to stop low frequency sound being transmitted from one space to another unless the intervening barrier is very heavy. For these reasons, a number of practically important acoustic noise problems are dominated by low frequency contributions. These problems are sometimes difficult to solve using passive methods since the solutions are expensive in terms of weight and bulk.