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Measurement Procedures <br />This guide contains two measurement procedures. The general protocols remain the same, but your choice of <br />procedure depends on the capabilities of your sound level meter (SLM). NTP -1 should be used if your SLM is <br />capable of calculating monitoring results and NTP -2 should be used if your SLM only displays instantaneous <br />readings. <br />Your sound level meter and microphone must comply with the specifications for ANSI S1.4 -1983 Type 0, 1, 2, <br />or S. You must also have a calibrator of a known frequency and sound level. Calibrators should be compared <br />to a lab standard periodically. <br />Measurements should not be made when noise from wind or precipitation results in a difference between the <br />background sound level and noise source being measured that is less than 10 dBA. In practice, this means that <br />wind speeds must be below 11 mph, and rainy weather conditions should be avoided. Temperature and <br />humidity should be within equipment specifications. <br />Properly choosing a monitoring location is an important consideration. Measurements should be made in an <br />area of normal outdoor human activity, nearest to the noise source. The monitoring location may not <br />necessarily be at the property line, such as if the property of the complainant is large and evidence of outdoor <br />activity is limited to a backyard patio. <br />Another important part of site selection is the consideration of errors caused by reflecting objects. Figure 9 <br />shows the effect on noise measurements of a reflective object such as a wall. <br />Errors caused by reflecting objects <br />uu�i! &pnlae <br />S ■ WYI &pi <br />Apprommalely 3 r1BA Error <br />Reeled-he hwI ze <br />Airpro io aiely 1 Z. [WA Error <br />c i <br />Figure 9 <br />A Guide to Noise Control in <br />Minnesota • October 2008 Minnesota Pollution Control Agency <br />8 <br />Sound Level Mete <br />R4 fleetive Siu face. <br />