Laboratory study of sidewall noise transmission and treatment for a light aircraft fuselage

The noise transmission through a fuselage from a twin-engine propeller aircraft was measured using a pneumatically driven horn sound source to simulate a propeller noise distribution. Sidewall noise treatments included a double-wall, production-type treatment and several combinations of fiberglass and lead-vinyl. The noise reduction through the fuselage sidewall without treatment is shown to agree in magnitude and trend with the reduction of propeller noise through an aircraft fuselage in flight, but differs substantially at low frequency from the transmission loss measured with a diffuse noise source. The effects of the horn source position and angle of incidence are shown. The treatments were evaluated using their insertion loss, defined as the reduction of the interior sound level that occurred when the treatment was installed. The fiberglass is shown to weigh less and to have insertion loss values that are often higher than those of the double-wall and lead-vinyl treatments. Variations of temperature in the test chamber are shown to result in changes of the noise spectrum within the fuselage. The changes observed may explain part of the variability of treatment insertion loss measured in the flight of a propeller aircraft.