In the audio world there are actually four different signal levels that are relevant. The two most discussed in regards to input are line level input and microphone input. First, lets take a look at all four:
- microphone level - weaker audio level than line, such as that from a microphone
- instrument level - weaker audio level than line, such as that from instrument pickups
- line level - used to interconnect audio components
- speaker level - an amplified audio signal for headphones and speakers
Microphone and instrument level audio is weaker than line level. The signal strength can refer to voltage and impedance to produce output power.
Line Level
The line level jacks on consumer electronics are typically unbalanced accepting a TRS Phono Plug. Unlike the weak signal from a microphone, a line level signal is at the highest signal level before final amplification and output to speakers. Consumer line level is rated around -10dBV while there is a different level for professional studio equipment. The most common nominal level for professional equipment is +4 dBu. A line level describes a line's nominal signal level as a ratio, expressed in decibels, against a standard reference voltage.
You never want to send a line level signal to the microphone input of an audio device.
Microphone Level
After the microphone level input goes into an audio device it goes through a preamplifier to boost the signal up to line level. Instrument level input is also very weak, however, may have more variance than that from a microphone. Both will be increased by a preamp prior to mixing or further amplification.
When a microphone picks up an acoustic sound wave, it generates a very small voltage waveform which is measured in microvolts.