In 2014, the Federal Aviation Administration amended the Airport Improvement Program Handbook to clarify guidance for the funding of noise mitigation projects requiring eligible structures not only to be located within the day-night average sound level 65-decibel (dB) noise contour, but also experience existing interior noise levels that are 45 dB or greater with the windows closed.  This restated guidance places greater emphasis on the accuracy of the measurement of the existing noise reduction of a structure.

Historically, two methods have been used to measure the noise reduction of structures:  aircraft overflight and loudspeaker.  The most common approach to testing structures has been to adapt the American Society for Testing and Materials (ASTM) E966-10 test procedure that incorporates an outdoor loudspeaker to simulate an aircraft noise source, together with noise level measurements outdoors and indoors.  This method is repeatable, but tests have shown that it results in lower measured noise level reduction than the aircraft overflight test by several decibels.  The ASTM E966-10 guidance contains adjustment factors to account for reflection from the façade surface when using a loudspeaker as the noise source.  However, the appropriateness of these adjustments is in question as a result of the noted discrepancy between measurements using aircraft and loudspeakers for the tests.

The objectives of this study were to determine and understand the contribution of the factors influencing the sound level at and near a building façade, identify appropriate adjustment factors for measurements of noise reduction in airport sound insulation programs, and validate these factors through application to existing data and by conducting field measurements. 

A time-domain simulation model, which was validated through a series of field measurements, was developed to aid in this study.  Updated adjustment factors were developed from measurements and modelling to account for façade reflection and were validated by application to previously published noise reduction data and additional field measurements.

This study yielded the conclusions that the adjustment factor to account for façade reflection in the measurement of noise reduction using a loudspeaker and flush-mounted microphone should be the theoretical vale of 6dB, and not 5dB as quoted in ASTM E966-10.  Additionally, the adjustment factor to account for façade reflection in the measurement of noise reduction using a loudspeaker and near-façade microphone should be 3.5 dB, and not 2 dB as quoted in ASTM E966-10.

DOT/FAA/TC-18/15
Authors: Sharp, Ben H.; Cox, J. Eric; Zheng, Z.C.