FMSCA Regulation 325.5
(a) Statutory definitions. All terms defined in the Noise Control Act of 1972 (Pub. L. 92 574, 86 Stat. 1234) are used as they are defined in that Act.
(b) Definitions in standards. All terms defined in Sec. 202.10 of the Interstate Motor Carrier Noise Emission Standards, 40 CFR 202.10, are used as they are defined in that section.
(c) Additional definitions. (1) Hard test site means any test site having the ground surface covered with concrete, asphalt, packed dirt, gravel, or similar reflective material for more than ½ the distance between the microphone target point and the microphone location point.
(c)(2) Soft test site means any test site having the ground surface covered with grass, other ground cover, or similar absorptive material for 1/2 or more of the distance between the microphone target point and the microphone location point.
(c)(3) Ground cover means any of various low, dense-growing plants, such as ivy, myrtle, low weeds, or brush.
(c)(4) Traffic railing means any longitudinal highway traffic barrier system installed along the side or median of a highway. For the purpose of this part, a traffic railing must have at least 35 percent of its vertical height, from the ground surface to the top of the railing, open to free space in order to qualify as an acceptable object within a noise measurement test site. Further, for the purposes of this part, posts or other discrete supports shall be ignored when ascertaining open free space.
(c)(5) Relatively flat when used to describe a noise measurement site means a site which does not contain significant concave curvatures or slope reversals that may result in the focusing of sound waves toward the microphone location point.
