Click here to purchase the entire book in PDF format. Ballistics Since VU Meters are essentially RMS meters, we have to remember that they do not respond to instantaneous changes in the signal level. The ballistics for VU Meters have a carefully defined rise and decay time - meaning that we know how fast they respond to a sudden attack or a sudden decay in the sound - slowly. These ballistics are defined using a sine tone that is suddenly switched on and off. If there is no signal in the system and a sine tone is suddenly applied to the VU Meter, then the indicator (either a needle or a light) will reach 99% of the actual RMS level of the signal in 300 ms. In technical terms, the indicator will reach 99% of full-scale deflection in 300 ms. Similarly, when the sine tone is turned off and the signal drops to 0 V instantaneously, the VU meter should take 300 ms to drop back 99% of the way (because the meter only sees the lack of signal as a new signal level, therefore it gets 99% of the way there in 300 ms - no matter where it's going). Figure 10.14: A simplified example of the ballistics of a VU meter. Notice that the signal (plotted in green) changes from a 0 $V_{RMS}$ to a 1.228 $V_{RMS}$ sine wave instantaneously (I know, I know... you can't have an instantaneous change to an RMS value - but I warned you that it was a simplified description!) The level displayed by the VU Meter takes 300 ms to get to 99$\%$ of the signal level. Similarly, when the signal is turned off instantaneously, it takes 300 ms for the VU Meter to drop to 0. Notice that the attack and decay curves are reciprocals. Figure 10.15: The same graph as is shown in Figure 10.14 plotted in a decibel scale. Note that the logarithmic decay of the VU Meter appears as a linear drop in the decibel scale, whereas the attack curve is not linear. Also, there is a provision in the definition of a VU Meter's ballistics for something called overshoot. When the signal is suddenly applied to the meter, the indicator jumps up to the level it's trying to display, but it typically goes slightly over that level and then drops back to the correct level. That amount of overshoot is supposed to be no more than 1.5$\%$ of the actual signal level. (If you're picky, you'll notice that there is no overshoot plotted in Figures 10.14 and 10.15.)