Basic study on active acoustic shielding (Improving noise-reducing performance in low-frequency range)

Active acoustic shielding (AAS) is a system that can attenuate a sound passing through an open window. An AAS is constructed from a number of AAS cells set in an array having an approximately co-located microphone and speaker system. The concept of AAS was previously demonstrated and some simple simulations and experiments were performed. Moreover, an AAS window with four AAS cells was proved to be effective for not only a single stable noise source but also multiple noise sources and moving noise sources. However, the AAS window could only attenuate noise with frequencies from 500Hz to 2 kHz, and could not attenuate noise below 500 Hz, which is dominant in construction sites and daily life. Therefore, in this paper, a new AAS window consisting of two different AAS units in one window is proposed. One of the units attenuates low-frequency sound and the other attenuates high-frequency sound. These units are controlled independently. The new AAS window was fabricated and experimentally evaluated in an anechoic room. It was found that the new AAS window reduced noise by 5 to 15 dB in the frequency range from 300 Hz to 2 kHz over a wide area in the room.