Here are some examples of calculations involving decibels. Decibels are ratio measures that depend on standard reference values against which a variable amplitude is compared.
Example 1: We are connecting a device with a nominal output level of −10 dBV to a device with a specified input level of +4 dBu. When a signal is at the nominal amplitude, the device meter reads 0 VU. What will the input VU meter read when fed from the output?
Because the input expects an input of +4 dBu, the meter will read −7.8–4 dB, or −11.8 VU.
Example 2: How much peak-to-peak voltage is necessary to allow a +4 dBu RMS sinusoidal signal to pass through a circuit undistorted?
From Figure 1-5, peak-to-peak measures are 2.828 times RMS measures for sine waves:
The power supply needs to provide at least 4.1 volts peak-to-peak to accommodate a +4 dBu signal.
For a signal of +24 dBu RMS (a good amount of headroom), we would need a peak-to-peak voltage of:
This voltage is beyond the limits of op-amps that run on standard +/−15V power supplies.