A basic characteristic of any audio signal is its amplitude, measured electrically in terms of voltage or acoustically in terms of sound pressure. When assessing the loudness of a signal, the amplitude or pressure is converted to a decibel value. The decibel scale gives a relative number referenced to a certain voltage or pressure. For example, 0 dBV is a popular standard reference for audio levels, and represents one volt. Note that amplitude is expressed as a voltage, while level (or loudness) is expressed using a dB scale.
When working with audio electronics, levels are commonly divided into three ranges: mic level, line level, and speaker level. Mic level is the smallest signal. Microphones and other passive transducers (devices that convert energy from one form, such as sound, to another, such as electricity) produce signals ranging from a few microvolts to a few millivolts.
A typical nominal operating level for a microphone output would be -55 dBV. Line level is hundreds of times greater in voltage terms — typically ranging from several millivolts up to around 1 volt, with a nominal level of 0 dBV. Speaker level is the strongest, ranging from a fraction of a volt (during quiet periods) to several dozen volts depending on the output rating of the amplifier. Of course, sound is very dynamic in nature, so whatever the nominal operating level of your signal is, if you read it with a meter during operation, you are likely to see large fluctuations from moment to moment within that range.
An important function of amplifiers is providing the “gain” needed to raise signals from mic or line level up to speaker level. Gain is another word for amplification, and simply means an increase of the voltage or power. The opposite of gain is attenuation. Both gain and attenuation are commonly measured in decibels.
The dBV scale is not the only one used for audio levels. Another popular reference scale is the dBu, where 0 dBu represents 0.775 volts. The historical predecessor to these two scales is the original dBm scale, where 0 dBm represents one milliwatt, or 0.001 watts. Other scales you might encounter include dBW (referenced to one watt) and dBμV (referenced to one microvolt). These scales are seen mostly in the radio broadcast industry.
Care should be taken not to confuse one scale with another, especially the common dBV and dBu scales. To make things especially aggravating, the term for dBu was previously dBv — with a lower-case “v”; so if you encounter dBv on an old spec sheet, it means dBu, not dBV.