*3.1.2 Microphone locations*

The standard recommends the use of a reference microphone, which allows for calibration of "before" and "after" measured levels and helps to consider variations in the characteristics of the noise source [17, 39].

When the reference microphone is used, it is placed in most cases according to the ISO standard, i.e., at a point on a vertical plane including the barrier, and at a height, at least, 1.5 m above the barrier edge. When the barrier is located less than 15 m from the near road lane, the microphone may be placed at 15 m from the center of the road lane, and at a height such that the line-of-sight angle between the microphone and barrier top, as measured from the center of the near road lane, is at least 10° (**Figure 4**) [17].

Microphone location at receiver positions depends on the study objectives since the location of the microphones (distance from the barrier, height above the ground) are determinants to establish diffraction effects. In some studies, a single microphone is located at a height of 1.5 m above the ground. The most common situation is, however, to place microphones at different distances and heights [17, 39–41] for a better understanding of the performance of the shadow zone (**Figure 5**).
