Abstract: A central vacuum cleaner having an air flow exhaust pipe is provided with a muffler comprising an elongated muffler pipe having a reduced diameter inlet and a reduced diameter outlet and a noise reducing foam liner disposed in the muffler pipe between the inlet and outlet. The foam liner includes a smooth inner surface that is generally flush with inner surfaces of the inlet and outlet. The foam liner comprises a combustion modified polyether polyurethane foam tube inserted in the muffler pipe between the inlet and outlet.

Abstract: A central vacuum unit having an acoustic damping system is provided. The central vacuum unit includes a canister having a sidewall forming a hollow interior and a lid closing an end of the sidewall, a vacuum motor within the canister which emits noise during operation, at least one cooling air inlet for admitting cooling air into the hollow interior, and at least one cooling air outlet in the sidewall for exhausting the cooling air from the hollow interior. Additionally, the sidewall has an exhaust port and the motor has an exhaust pipe extending through the exhaust port. The acoustic damping system includes an acoustic damping tunnel, an acoustic damping canopy, and an exhaust port seal. The acoustic damping tunnel is within the hollow interior and forms a pathway between the hollow interior and the cooling air outlet. The pathway is lined with a sound absorbing material so that the tunnel reduces noise emitted from the hollow interior through the cooling outlet.

Abstract: Sound intensity is determined by simultaneously measuring instantaneous acoustic pressure and instantaneous particle velocity at a single point. A condenser microphone having a vibrating diaphragm generates an output voltage proportional to the instantaneous acoustic pressure. A laser Doppler vibrometer measures the velocity of the vibrating diaphragm and a Doppler frequency tracker converts the velocity measurement into another output voltage which is proportional to the instantaneous acoustic particle velocity. The sound intensity is then calculated from the known components.