67 Pa. Code § 157.2
The following words and terms, when used in this chapter, have the following meanings, unless the context clearly indicates otherwise:
Ground cover-Any of various low, dense-growing plants, such as ivy, myrtle, low weeds or brush.
Hard test site-A test site having the ground surface covered with concrete, asphalt, packed dirt, gravel or similar reflective material for more than 1/2 the distance between the microphone target point and the microphone location point.
Microphone line-An unmarked reference line running parallel to the vehicle path and passing through the microphone.
Microphone point-The unmarked location on the center of the lane of travel that is closest to the microphone.
Relatively flat-A noise measurement 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 is relatively flat.
Soft test site-A 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.
Traffic railing-A longitudinal highway traffic barrier system installed along the side or median of a highway. A traffic railing shall have at least 35% 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. Posts or other discrete supports shall be ignored when ascertaining open free space.
Vehicle reference point-The location on the vehicle used to determine when the vehicle is at any of the points on the vehicle path. The primary vehicle reference point shall be the front of the vehicle. For vehicles with a gross vehicle rating of 6,000 pounds or more, where the distance from the front of the vehicle to the exhaust outlet is more than 16 feet, the secondary vehicle reference point shall be the exhaust outlet.
67 Pa. Code § 157.2
This section cited in 67 Pa. Code § 157.41 (relating to site characteristics).