Active control of the exhaust noise radiated by an enclosed generator

The aim of this work was to design and implement a hybrid passive/active system to control the noise radiated by a small generator. Passive control is afforded by enclosing the generator with a close-fitting, leaky, rectangular box. The measured Insertion Loss is higher than 20 dB above 500 Hz. Special attention is paid to technical aspects such as air refreshing and temperature inside the enclosure. Low frequency noise escapes from the enclosure through air intake and gas exhaust openings. A single-input single-output (SISO) feedforward active noise control (ANC) system, implemented in a commercially available device, is used to reduce the radiated exhaust noise below 500 Hz. The reference signal to the SISO ANC system is supplied by an accelerometer located on the air filter case of the generator. The error signal is provided by an electret microphone along the exhaust pipe. The control source consists of a high temperature loudspeaker, positioned in a side-branch configuration to avoid direct contact with the exhaust gas. Some harmonics were attenuated more than 30 dB.