Far-field Noise Levels Research Articles

Effect of centerbody scattering on propeller noise

This paper describes how the effect of acoustic scattering from the hub or centerbody of a propeller will affect the far-field noise levels. A simple correction to Gutin's formula for steady loading noise is given. This is a maximum for the lower harmonics but has a negligible effect on the higher frequency components that are important subjectively. The case of a blade vortex interaction is also considered, and centerbody scattering is shown to have a significant effect on the acoustic far field.

Helicopter rotor speed effects on farfield acoustic levels

The design of a helicopter is based on an understanding of many parameters and their interactions. For example, in the design stage of a helicopter, the weight, engine, and rotor speed must be considered along with the rotor geometry when considering helicopter operations. However, the relationship between the noise radiated from the helicopter and these parameters is not well understood, with only a limited set of model and full-scale field test data to study. In general, these data have shown that reduced rotor speeds result in reduced farfield noise levels. This paper will review the status of a recent helicopter noise research project designed to provide flight experimental data to be used for further understanding of helicopter rotor speed effects on farfield acoustic levels. Preliminary results will be presented relative to tests conducted with a McDonnell Douglas Helicopter Company model 500E helicopter operating with the rotor speed as the control variable over the range from 103% N2 to 75% N2 and the forward speed maintained at a constant value of 80 knots.

The reduction of blast noise with aqueous foam

Experiments were performed to investigate the potential of water-based foams to reduce the farfield noise levels produced by demolitions activity. Measurements of the noise reductions in flat-weighted sound exposure level (FSEL), C-weighted sound exposure level (CSEL), and peak level were made for a variety of charge masses, foam depths, and foam densities (250:1 and 30:1 expansion ratio foams). Scaling laws were developed to relate the foam depth, foam density, and charge mass to noise reductions. These laws provide consistent results for reductions in the peak level, FSEL and CSEL up to a dimensionless foam depth of 2.5. A two part model for the mechanisms of sound level reductions by foam is suggested.

Community noise from air cushioned landing craft

The effect of noise on neighboring communities resulting from operating the U.S. Navy's proposed Air Cushioned Landing Craft (LCAC) at an initial training site is predicted and evaluated in this preliminary impact assessment. Farfield noise levels of two prototype vehicles, each with a maximum gross weight of 160 tons, were measured under various stationary and moving modes at the Naval Coastal Systems Center in Panama City, Florida. These data and appropriate vehicle operating characteristics were entered into NOISEMAP, a noise contour generating computer program which has heretofore only been used in aircraft noise analyses. Four vehicles undertaking a total of 440 annual missions were found to generate a 65-dB DNL contour with an area of only 14 square mile. Nonetheless, single event maximums over 65 dB predicted near residential locations were identified as potential areas of adverse impact on the community. [Work supported by the U.S. Navy.]

Community noise from air‐cushioned landing craft

The effect of noise on neighboring communities resulting from operating the U.S. Navy's proposed Air Cushioned Landing Craft (LCAC) at an initial training site was predicted and evaluated in this preliminary impact assessment. Farfield noise levels of two prototype vehicles, each with a maximum gross weight of 160 tons, were measured under various stationary and moving modes at the Naval Coastal Systems Center in Panama City, FL. These data and appropriate vehicle operating characteristics were entered into NOISEMAP, a noise contour generating computer program which had heretofore only been used in aircraft noise analyses. Four vehicles undertaking a total of 440 annual missions were found to generate a 65‐dB DNL contour with an area of only 1/4 square mile. Nonetheless, single event maximums over 65 dB predicted near residential locations were identified as potential areas of adverse impact on the community. [Work supported by the U.S. Navy.]

Noise Characteristics of Combustion Augmented Jets at Midsubsonic Speeds

Noise generation by a subsonic flow discharging from a combustion chamber is examined with regard to the relative importance of combustion as a source of noise in such a flow system. Measurements of pressure fluctuations inside the combustor are compared with far-field noise measurements by direct cross-correlations. The cross-correlations and derived cross-spectral densities verify that much of the noise originates inside the combustor. A fluid mechanic perturbation model is used to predict exit plane velocity fluctuations due to internal pressure fluctuations. This unsteadiness at the exit plane is assumed to behave as an acoustic monopole which radiates to the far field. Far-field noise levels estimated on this basis are in good agreement with measured values. The over-all noise level from the combustor/jet is found to be 10-20 db higher than for an equivalent clean, cold jet at the same exit velocity, in the range of 500-700 fps.

Flyover Noise Prediction from Static Data

The use of static ground test data to describe the acoustic characteristics of an aircraft flyover can result in a significant savings both in time and money, providing good correlation with actual flight test measurements is assured. This paper addresses itself to the elements associated with the calculation of EPNL values from static ground test data. Farfield noise level measurements were made over a range of engine thrust settings from idle to takeoff power of a JT3D-3B engine mounted on a test stand. Two key engine operation parameters were used to correlate the SPL values: low-pressure compressor rotor speed, which controls fan noise; and net thrust, which controls jet noise. The SPL spectrum, used to simulate a flyover, is generated by interpolating the measured data at the appropriate in-flight values of rotor speed and net thrust. The extrapolation to inflight condition also considers airplane parameters such as altitude, climb gradient, and number of engines, as well as Doppler shift effects, spherical divergence, atmospheric and extra ground attenuation, and ground reflection. Comparisons are made between EPNL values predicted from static ground test data and values obtained from flight tests. A tape will be played which demonstrates the difference between flyovers of a production B-707 airplane and a B-707 airplane equipped with quiet nacelles.