(a)(1) The motor vehicle to be tested shall be parked on the test site. A microphone target point shall be established on the ground surface of the site on the centerline of the lane in which the motor vehicle is parked at a point that is within 3 feet (.9 m) of the longitudinal position of the vehicle's exhaust system outlet(s). A microphone location point shall be established on the ground surface not less than 31 feet (9.5 m) and not more than 83 feet (25.3 m) from the microphone target point. Within the test site is a triangular measurement area. A plan view diagram of a standard test site, having an open site within a 50-foot (15.2 m) radius of both the microphone target point and the microphone location point, is shown in Figure 2.
(2) Measurements may be made at a test site having smaller or greater dimensions in accordance with the rules in subpart F of this part.
(b) The test site must be an open site, essentially free of large sound-reflecting objects. However, the following objects may be within the test site, including the triangular measurement area:
(1) Small cylindrical objects such as fire hydrants or telephone or utility poles.
(2) Rural mailboxes.
(3) Traffic railings of any type of construction except solid concrete barriers (see § 325.5(c)(4)).
(4) One or more curbs having a height of 1 foot (.3 m) or less.
(c) The following objects may be within the test site if they are outside of the triangular measurement area of the site:
(1) Any vertical surface, regardless of size (such as a billboard), having a lower edge more than 15 feet (4.6 m) above the ground.
(2) Any uniformly smooth surface slanting away from the vehicle with a slope that is less than 45 degrees above the horizontal.
(3) Any surface slanting away from the vehicle that is 45 degrees or more and not more than 90 degrees above the horizontal, if all points on the surface are more than 15 feet (4.6 m) above the surface of the ground in the test site.
(d) The surface of the ground within the measurement area must be relatively flat. (See § 325.5(c)(5)). The site shall be a “hard” site. However, if the site is determined to be “soft,” the correction factor specified in § 325.75(b) of this part shall be applied to the measurement.