Reflections from Highway Noise Barriers and Use of Absorptive Materials in the United States

For decades, many departments of transportation (DOTs) in the United States have followed widely accepted guidance that single reflections of noise from noise barriers to the opposite side of highways are “generally one to two dBA or less, and therefore not perceptible to the average human ear.” Increases in the average noise level are undoubtedly small, yet the outcry from affected residential communities can be significant and can result in effort and money spent researching the magnitude of sound level increases from reflective noise barriers along highways. In some cases, DOTs have added absorptive materials to barriers after construction. Other DOTs have long-standing practices of using barriers with absorptive surfaces wherever there is noise-sensitive land use on the opposite side of the highway, specifically to avoid any perception of increased highway noise due to the barriers. This paper presents historical information on the study of reflections from noise barriers in the United States, human perception of changes in noise levels, and how reflective noise barriers may change the character of noise from highways as heard in communities opposite the barriers. Although such changes may be small, they may be interpreted as sound level increases and affected residents may feel that they are being treated unfairly by the DOT. Two conclusions are presented: (a) small changes in sound level associated with barrier reflections can be meaningful to the public and to barrier effectiveness and (b) the benefit of implementing absorptive barrier treatments opposite residential areas outweighs the benefit of researching the issue or conducting detailed analyses to justify the use of absorption.