The Dorrough 21B Vertical Analog Loudness Meter with Percent Modulation is a straight line version of Dorrough's standard 40 series loudness monitor. The 21B requires rack mounting and needs an external power supply. It also comes in a handy vertical orientation.
Like the 40B, the 21B measures Relative Loudness to Peak Modulation. The meters are calibrated in percent modulation, referenced at 100%. Access phase correlation, peak hold, sum/diff, and reference mode allowing 0 reference setting to 0.1 dB accuracy, directly or with the optional remote control.
21B Vertical Analog Loudness Meter with Percent Modulation