Showing papers on "Noise reduction published in 1968"

Structural-acoustic response, noise transmission losses and interior noise levels of an aircraft fuselage excited by random pressure fields

Abstract: : A theoretical and empirical study of the structural-acoustic response and sound transmission properties of fuselage structures is described. The external fluctuating pressure environments discussed are boundary layer turbulence, jet noise and reverberant acoustic fields. In order to investigate the complete behavior of the fuselage, equivalent structural models are analyzed whose combined characteristics represent the complex fuselage structure throughout the entire frequency response range of interest. The structure and interior sound field are treated throughout as a coupled dynamic system whose response is describable in terms of the system's normal modes. Prediction methods are developed for structural responses, noise reduction and internal acoustic fields of untreated and acoustically treated fuselage structures. The results of this study have been programmed for computer solution, thus allowing the significant parameters affecting sound transmission to be determined. In addition to the computer programs, empirical design charts are presented for carrying out pre-design estimates of the external fluctuating loads due to boundary layer turbulence and jet noise and overall noise reduction of typical acoustic treatments.

Gain and noise considerations in RF feedback amplifier

Abstract: An equivalent noise model from which open- and closed-loop feedback noise figure expressions are derived is presented. An analytical design procedure for feedback RF amplifiers yielding minimum noise figure is proposed. Experimental results are given.

A realistic assessment of the vertiport/ community noise problem.

Abstract: An increase of background noise has accompanied the steady growth of industrial society. Rapid growth of air travel has focused special attention on aircraft noise, and need for abatement. Anticipated growth of short-haul air transport will bring vertiports into established commercial areas and low-altitude flyovers to populated areas. Noise intrusion may be assessed in perceived noise (PNdb) for residential communities, but a more realistic yardstick for commercial/industrial sections might be speech interference level (SIL). Achievement of the ideal goal—no intrusion of noise above ambient levels—requires reduction of powerplant noise but may be limited by aerodynamic noise from the vehicle itself. Consideration of powerplant noise in terms of "disk loading," progressing from jets at the high end down through fans, propellers, and rotors, leads to selection of rotors and propellers for shorthaul propulsion. Further noise reduction below current state-of-the-art levels will require intensive long-range development programs. Parallel to concentrated effort by industry to provide aircraft acoustically acceptable to the community, the community should plan for these aircraft by judicious selection of vertiport sites and utilization of zoning and building-code functions for development of surrounding neighborhoods.

Results of recent NASA research pertinent to aircraft noise and sonic-boom alleviation.

Abstract: NASA research results pertinent to airport- community noise problems and sonic boom alleviation

Noise of Electrical Machines

Abstract: The control of noise in electrical machines may be effected by two separate and distinct methods. The first is the reduction of the noise at the source and the second the attenuation of this noise before it reaches the ears of the observer. Since noise reduction is costly, the problem is resolved into one of finding the most economic means of obtaining the required noise reduction, without interfering with the efficient functioning of the plant. It is generally considered that a good machine should run smoothly and quietly, so from the earliest days the designers and manufacturers have put a great deal of effort into producing quiet machines and more recently considerable research effort has been expended in explaining the basic phenomena.

Correlation function of teleseismic noise

Abstract: A theoretical calculation is made of the correlation function for the vertical component of Earth motion caused by teleseismic microseismic noise. The calculation pertains to an idealized case in which the Earth is assumed to be spherically symmetric and to transmit seismic waves as a non-dispersive linear system, and in which all noise is assumed to originate at the Earth's surface. Diffraction effects are neglected. Only phases that originate and terminate as longitudinal waves are considered, although the method of analysis is readily applicable to other phases. The correlation function for teleseismic noise is calculated and plotted for the case of isotropic noise having a specific noise spectrum. The theoretical noise reduction is calculated and plotted versus average seismometer spacing for a set of seven-seismometer seismic arrays for the cases of teleseismic noise alone, surface noise alone, and a combination of teleseismic and surface noise.

Noise reduction shape factors in the low frequency range

Abstract: Noise reduction in small enclosed structures in terms of physiological and psychological effects

Noise-Conditioning the Air Conditioner

Abstract: Results of experimental noise reduction work performed with an in-room type air conditioner is presented. Emphasis is placed on what can be achieved in the way of noise reduction by applying basic techniques in a rather straightforward fashion as a "first cut" at the problem.