Volts (symbol V – not v) measure the electrical pressure, or potential difference available to force current through a circuit. The name derives from the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first chemical battery.
Voltages can have a fixed polarity (Direct Current volts), or can change more or less cyclically (Alternating Current volts). Batteries are examples of DC potential sources, mains power with its steady sine wave alternation and the less predictable fluctuations of analogue audio are examples of AC ones.
DC voltages have a single value since they do not vary.
AC voltages can have a peak value – or more accurately a peak-to-peak (p-p) one – which describes the maximum range. However for most purposes including power lines and moderately continuous audio signals the Root Mean Square (RMS) value is used. In colloquial terms this is the average voltage; mathematically it is 0.707 times the potential between zero and the positive peak. AC voltages that are mentioned without any other qualification are normally RMS.
A sense of scale
For audio, noise sources typically have levels in the range of µV (microvolts – millionths of a volt).
Analogue audio that registers usefully on a PPM is in the range of mV (millivolts – thousandths of a volt)
If an alignment level of 0 dBu = -18 dBFS is used …
…0 dBFS corresponds to 6.15 V RMS or 17.4 V p-p
…-10 dBFS (PPM6) corresponds to 1.9 V RMS or 5.5 V p-p
Voltages below about 50 V (dependent upon the region) are usually rated as “Low” and need not be securely insulated for safety purposes. Voltages above that are considered dangerous and require care.
Domestic power is approximately 115 V AC in the US and 230 V AC in most other countries.
Industrial power uses 3 lines (phases) which cycle through 230 V in sequence. The voltage between each of these, and each to ground (or a neutral line, if used) will be in the range of 300 – 400 V due to the changing phase relationship.
Any voltage can cause a spark but in order to break down a significant air insulation barrier quite high voltages are needed, normally >1000 V. Lightning is in the range of MV (megavolts – millions of volts).