Click here to purchase the entire book in PDF format. dBV Nowadays, the 600$\Omega$ specification doesn't apply anymore. The input impedance of a tape deck you pick up off the shelf tomorrow could be anything - but it's likely to be pretty high, somewhere around 10 k$\Omega$. When the impedance is high, the dissipated power is low, because power is inversely proportional to the resistance. Therefore, there may be times when your power measurement is quite low, even though your voltage is pretty high. In this case, it makes more sense to measure the voltage rather than the power. Now we need a new reference, one in volts rather than watts. Well, there's actually two references... The first one is 1 Vrms. When you use this reference, your measurement is in dBV. So, you measure the voltage output of your piece of gear - let's say a mixer, for example, and compare that measurement with the 1 Vrms reference, using the following equation. $$\textrm{Voltage (in dBV)} = 20 \log \left ( \frac{\textrm{Voltage 1} }{ 1 \textrm{ Vrms}} \right )$$ (3.36) Where Voltage1 is measured in Vrms. Now this time it's a 20 instead of a 10 because we're measuring pressure and not power. Also note that the dBV does not imply a measurement across a specific impedance.