[Code of Federal Regulations]
[Title 49, Volume 5]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR325.53]

[Page 13-14]
 
                        TITLE 49--TRANSPORTATION
 
                      DEPARTMENT OF TRANSPORTATION
 
PART 325_COMPLIANCE WITH INTERSTATE MOTOR CARRIER NOISE EMISSION 
STANDARDS--Table of Contents
 
        Subpart E_Measurement of Noise Emissions; Stationary Test
 
Sec.  325.53  Site characteristics; stationary test.

    (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.

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    (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 Sec.  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 Sec.  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 Sec.  325.75(b) of this part shall be applied to the 
measurement.

[40 FR 42437, Sept. 12, 1975, as amended at 41 FR 10227, Mar. 10, 1976; 
54 FR 50385, Dec. 6, 1989]