Publications on the topics of propeller source noise, airborne noise transmission, and passenger comfort response to noise and vibration are reviewed. Of the 187 publications referenced, 140 have appeared since 1978. Examples of research accomplishments are presented to illustrate the state of the art. Emphasis is on comparisons of theoretical and measured results, but the description of the theories is left to the references. This review shows that substantial progress has been made in understanding the characteristics of propeller noise, airborne noise, and passenger response, and in the development of prediction methods. Application of the technology to cabin noise control and possible future research directions are discussed.

Review of recent research on interior noise of propeller aircraft

The noise transmission characteristics of test panels and acoustic treatments representative of an aircraft sidewall are experimentally investigated in the NASA Langley Research Center transmission loss apparatus. The test panels were built to represent a segment sidewall in the propeller plane of a twin-engine, turboprop light aircraft. It is shown that an advanced treatment, which uses honeycomb for structural stiffening of skin panels, has better noise transmission loss characteristics than a conventional treatment. An alternative treatment, using the concept of limp mass and vibration isolation, provides more transmission loss than the advanced treatment for the same total surface mass. Effects on transmission loss of a variety of acoustic treatment materials (acoustic blankets, septa, damping tape, and trim panels) are presented. Damping tape does not provide additional benefit when the other treatment provides a high level of damping. Window units representative of aircraft installations are shown to have low transmission loss relative to a completely treated sidewall.

Noise transmission through an acoustically treated and honeycomb stiffened aircraft sidewall

The insertion loss of side-wall add-on acoustic treatments was measured using a light aircraft fuselage. The treatments included: no treatment (i.e., baseline fuselage), a production-type double-wall interior, and various amounts of high-density fiberglass added to the baseline fuselage. The source used to simulate propeller noise was a pneumatic-driver with attached exponential horn, supplied with a broadband signal. Data were acquired at the approximate head location for each of the six possible passenger positions. Insertion loss results for the different configurations were analyzed in space-averaged narrowband levels, one-third-octave band levels, and overall levels, and at specific frequencies representing propeller tone spectra. The propeller tone data include not only the space-averaged insertion loss but also the variation of insertion loss of these particular frequencies across the six microphone positions.

Laboratory study of cabin acoustic treatments installed in an aircraft fuselage

Review of recent research of interior noise of propeller aircraft

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.

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

The noise transmission loss characteristics of the sidewall treatment in the propeller plane of a twin-engine turboprop aircraft are experimentally investigated in the NASA Langley Research Center Transmission Loss Facility. The sound attenuation properties of the individual elements of this treatment are evaluated showing least noise transmission loss in the low frequencies (below 500 Hz) where the excitation levels at the propeller blade passage frequency and the first few harmonics are highest. It is shown that single and double wall resonances play an important role in the noise transmission loss values of the treatment at these low frequencies suggesting that a limp mass with a very low resonance frequency serves better as a trim panel than a trim panel having a high structural stiffness. It is indicated that the window structures might be a potential noise control problem.

F. W. Grosveld

Papers

Noise transmission through sidewall treatments applicable to twin-engine turboprop aircraft