Abbreviation for Equivalent Input Noise. EIN is a specification we most commonly encounter when looking at microphones and preamps. Because the output of most microphones is so low the amount of self-noise they produce can be important. Apply a lot of gain and any little bit of noise becomes pronounced. Further, any self-noise of the preamp also becomes pronounced under the high amounts of gain required. There are established theoretical noise floor limits for electronic equipment. All devices operating at a temperature above absolute zero produce their own noise. Even a simple resistor, or any source of resistance in a circuit will produce noise. In fact, a 200 ohm resistor on its own produces 0.26 microvolts of noise. Referenced to standard line level signals this is equal to -129.6 dBu of noise (for more on dBu, see our Summit on dBu versus dBV). When a microphone is connected to a preamp you can think of the microphone as a ‘source resistance.’ 200 ohms is often considered typical, though mics do vary quite a bit, however the EIN specification is supposed to be measured with a 200 ohm source impedance (for the sake of comparing apples to apples). So you start with .0.26 microvolts of noise, and then add whatever noise the preamp has and you get the real working noise of the system (the system being the mic and the preamp). Preamp manufacturers know they are more or less starting at this theoretical noise limit (-129.6 dBu) so the value they quote is Equivalent Input Noise in their specs. EIN basically takes this ‘source noise’ into account. Therefore the theoretical lowest EIN spec you could encounter with preamps is -129.6 dBu, which would mean the preamp itself produces no noise at all. If the preamp produces the same amount of noise as the source resistance this value will go up by 3 dB to -126.6 dBu (you may also see dBm). Most mic preamps fall somewhere within this range, however, like most things in specs it is fairly easy for the manufacturer to tinker with the methods to produce better results. Consequently it is not unheard of to see values in the -130 to -135 range. This is usually accomplished by measuring with a lower source impedance, or even a direct short across the input. Resistors of lower impedance will produce less noise, but also offer an unrealistically low source impedance to the preamp, which means the measurements don’t have as much ‘real world’ relevance. Occasionally you will see EIN rated in dBV. Be careful there because the dBV standard gives a result that is 2.2 dB better just because it is referenced to a different voltage to begin with (again, see the dBu versus dBV summit). Clearly this information is pretty technical and not for everyone. For those who don’t want to digest it all you can sleep at night knowing that most modern preamps you encounter are of such high quality that they are within a tolerable range in terms of their self-noise. In short, don’t lose too much sleep over this unless you are recording very low volume sounds.