Variation of attenuation constant and absorption coefficient of fiberglass with altitude

Abstract

ASTM C 423-99a specifies requirements for air temperature and relative humidity during decay rate measurements, but does not address variations in air density even though this parameter decreases with increasing altitude. The effect of changes in air density on absorption of fiberglass was investigated using approximations for characteristic impedance and complex propagation constant [M. E. Delany and E. N. Bazley, NPL AERO REPORT Ac37, March 1969], and design charts for absorbers in diffuse sound fields [F. P. Mechel, J. Acoust. Soc. Am. 83, 1002–1013 (1988)]. At constant temperature and relative humidity, air density decreases by 17% from sea level to 5000 ft altitude, resulting in a 17% decrease in its acoustic impedance and a 12% increase in the attenuation constant of fiberglass. Using measured flow resistivities of four different fiberglass boards and an empirical correction factor to account for diffraction effects at sample edges, NRC values for the same board were found to vary within ±0.04 between sea level and 5000 ft. Transmission measurements, such as those used to qualify aircraft insulation, will be overstated at higher altitudes because transmission loss is proportional to the attenuation constant. Thus correction factors should be developed for data taken substantially above sea level for comparison with data taken near sea level.