Index of content:
Volume 48, Issue 3C, September 1970
48(1970); http://dx.doi.org/10.1121/1.1912207View Description Hide Description
Because of the rapidly increasing noise pollution in the vicinity of large commercial airports, the government established an interagency panel in October 1965 to develop recommendations concerning the implementation of a coordinated noise‐abatement program. A part of NASA's contribution to this program resulted in the awarding of contracts to The Boeing Company and The McDonnell‐Douglas Corporation in May 1967, directed toward reducing the noise under the flight path of the current large four‐engine fan‐jet commercial transports. This paper is a brief review of the goals and scope of these contracts to study and develop nacelle modifications to minimize fan compressornoise radiation.
48(1970); http://dx.doi.org/10.1121/1.1912208View Description Hide Description
A summary is given of a research program to develop acoustic‐lining technology for engine ducts. The environment to which linings are exposed in the inlet and duct of a typical turbofan engine installation and the noise spectrum to be attenuated are described. Concepts of acoustic linings and their mechanisms for attenuating the propagation of sound in ducts are discussed, with particular application to turbomachinery. The procedure used for screening and selecting acoustical facing materials for linings is reviewed. A summary is given of the experimental methods employed in this procedure—measurement of the specific flow resistance, the specific acoustic impedance, and of the acoustic absorption coefficient—and a synopsis of the results is made. Suitable materials were evaluated as part of duct linings in a laboratory flow‐duct apparatus, simulating an engine‐duct environment in which linings were subjected to sound levels up to 155 dB over‐all sound‐pressure level (SPL) and airflows up to Mach number 0.4. The most important acoustic and geometric characteristics that influence the acoustic attenuation and the results obtained are summarized. The attenuation of linings in a laboratory flow duct and in a full‐scale engine are compared. Finally, a description and some results of a theoretical study developed for predicting the attenuation of linings is given and compared with the experimental measurements.
48(1970); http://dx.doi.org/10.1121/1.1912209View Description Hide Description
This paper is a review of the structural aspects of current turbofan noise‐suppression designs presented in conjunction with a summary of the environmental conditions, which determine the design criteria of duct‐lining acoustical materials. The subject is confined to structural and environmental studies of acoustical materials and to the analyses, tests, and design criteria necessary to provide a certifiable noise‐suppression system for commercial aircraft. The paper parallels, but updates, similar paper presented by the author to the October 1968 Conference on Progress of NASA Research Relating to Noise Alleviation of Large Subsonic Jet Aircraft [R. A. Mangiarotty, A. H. Marsh, and E. Feder, NASA SP‐189 (1968)]. Studies related to the structure and environment of duct‐lining acoustical materials proposed for use in the noise‐suppression systems of turbofan aircraft may be more clearly understood when preceded by: (1) an indication of where the systems are used within a power plant nacelle, (2) a complete definition of the environmental conditions under which the systems are expected to function, (3) an explanation of the basic acoustical function of the materials, and (4) a detailed description of typical duct‐lining configurations.
48(1970); http://dx.doi.org/10.1121/1.1912210View Description Hide Description
Fan noise radiated from the engine inlet and fan discharge of current fan jet airplanes during landing makes the largest contribution to perceived noise. This noise can be reduced by lining the engine inlet and duct walls with sound‐absorbingmaterials. Because of the scarcity of analytical lining design methods which include duct airflow effects, an experimental program was conducted to investigate the influence of acoustic and geometric duct‐lining parameters on fan‐noise attenuation. The significant parameters were found to be the acoustic impedance, treatment length, depth, core‐cell dimensions, channel width, number of walls lined, number of layers in the linings, air velocity, and direction of flow with reference to sound propagation. The influence of these parameters is discussed. For simple single‐layer linings a design technology has been developed by means of empirical equations. Experimental results show that this technology is valid, for duct sizes from 4 to 12 in., lining depths of to 1 in., and duct airflow Mach numbers from −0.4 to 0.4.
48(1970); http://dx.doi.org/10.1121/1.1912211View Description Hide Description
Design studies of various acoustically treated inlet and fan‐exhaust ducts were conducted for the JT3D turbofan engine installation on DC‐8 airplanes. These design studies resulted in the selection of two configurations of treated engine inlets and one configuration of treated fan‐exhaust ducts for full‐scale fabrication and testing. On the basis of ground runup test results, one configuration was chosen for subsequent flyover noise and cruise‐performance flight testing. This paper discusses some of the design concepts that were studied, presents some of the results of the full‐scale ground runup tests, and also presents predictions of the change in flyover perceived noise level owing to installation of treated nacelles on DC‐8–50/61 airplanes.