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En route noise levels from propfan test assessment airplaneThe en route noise test was designed to characterize propagation of propfan noise from cruise altitudes to the ground. In-flight measurements of propfan source levels and directional patterns were made by a chase plane flying in formation with the propfan test assessment (PTA) airplane. Ground noise measurements were taken during repeated flights over a distributed microphone array. The microphone array on the ground was used to provide ensemble-averaged estimates of mean flyover noise levels, establish confidence limits for those means, and measure propagation-induced noise variability. Even for identical nominal cruise conditions, peak sound levels for individual overflights varied substantially about the average, particularly when overflights were performed on different days. Large day-to-day variations in peak level measurements appeared to be caused by large day-to-day differences in propagation conditions and tended to obscure small variations arising from operating conditions. A parametric evaluation of the sensitivity of this prediction method to weather measurement and source level uncertainties was also performed. In general, predictions showed good agreement with measurements. However, the method was unable to predict short-term variability of ensemble-averaged data within individual overflights. Although variations in absorption appear to be the dominant factor in variations of peak sound levels recorded on the ground, accurate predictions of those levels require that a complete description of operational conditions be taken into account. The comprehensive and integrated methods presented in this paper have adequately predicted ground-measured sound levels. On average, peak sound levels were predicted within 3 dB for each of the three different cruise conditions.
Document ID
19950005928
Document Type
Technical Publication (TP)
Authors
Garber, Donald P. (Lockheed Engineering and Sciences Co. Hampton, VA., United States)
Willshire, William L., Jr. (NASA Langley Research Center Hampton, VA, United States)
Date Acquired
September 6, 2013
Publication Date
September 1, 1994
Subject Category
ACOUSTICS
Report/Patent Number
L-17339
NAS 1.60-3451
NASA-TP-3451
Funding Number(s)
PROJECT: RTOP 535-03-11-02
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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