Microphones
...sented by a Th¨¦venin equivalent circuit with a generator in series with an internal impedance. At a minimum, the microphone specifications should supply these two parameters. The generator voltage is typically specified in dB re 1 V per ¦̀bar, or e = 20 log p, where p is the acoustic overpressure in dyne/cm2 or ¦̀bar. Sometimes the pascal is used as the reference pressure, 1 Pa = 10 ¦̀bar. This adds 20 dB, so manufacturers like to use it to imply that their microphones are more sensitive. The sensitivity is specified at some reference frequency, usually 1000 Hz. Equally important is the specification of the internal impedance of the microphone, which varies over an extremely wide range for different microphone types, from less than 1¦¸ for a ribbon microphone to many mega ohms for a capacitor microphone. This is more difficult to find, and sometimes you must assume a typical value for the microphone type involved. A microphone with a low internal impedance, 50¦¸ or less, is normally used with a step-up transformer that provides 40 dB of gain effortlessly (with a 1:100 transformer) and matches well to the high input impedance of many amplifiers. A microphone with a high capacitive internal impedance can¡¯t be used with a cable of any length, so the amplifier must be very near. In the modern electret condenser microphone, the amplifier is an FET in the microphone cartridge itself. All but a few microphones used in professional audio applications have balanced, low impedance outputs. Microphones with unbalanced outputs of either low or high impedance are commonly found in consumer and communications applications. A low impedance output for most microphones is nominally 35 to 600 ohms with the vast majority being around 200 ohms. This means that the microphone is intended for connection to a similarly labelled input, having an actual input impedance which is about 10 times that of the microphone. A microphone converts acoustic energy to electric energy. If the output impedance of the microphone is matched to the input impedance of a device the maximum amount of power will flow into the input circuit. When the microphone's output is matched to that of the connected input, its internal impedance is in series with the equal impedance of the microphone input. This divides the voltage output from the microphone between its own internal impedance and the impedance of the input being fed. In this configuration only half of the voltage available from the microphone will be seen at the microphone input. This is a 6 dB loss of signal. Since there is a minute amount of power (typically around 10-9 Watt) coming from a microphone it is more useful to obtain the maximum voltage level from the microphone. This will occur if the output of the microphone is "unloaded" or "bridged". This results in the maximum voltage from the microphone being available to the input circuits.  In the unloaded configuration the noise from the microphone increases as well. However, the increase in noise, 3 dB, is only half that of the signal because the noise adds randomly whereas the signal adds linearly. Additionally, input circu...