Measurement Microphone for PM123

While the properties of the sound card can be compensated by a calibration, the properties of the microphone and the microphone amplifier can not. They directly cause wrong results and bad sound.

Required properties

1. Linear frequency response
   Any sound modification introduced by the microphone will be compensated by DRC123. But there is no such thing to compensate for in playback mode. So if your microphone cannot record a certain frequency it will be amplified until your microphone can recognize it - a very bad idea.
2. Omnidirectional characteristics
   Your microphones response should not depend on the direction of the sound. While this sounds complicated it makes things rather cheaper than more expensive.
3. Sensitivity
   Since your speakers are not close to the microphone it should have enough sensitivity to get a reasonable output.

Mircophone types

Electret microphones are the very first choice for this purpose. They are cheap, omnidirectional and have rather high sensitivity.

Analog measurement microphones

Analog microphones like Bayerdynamic MM-1 (also sold as t.bone) are usable and sufficient. However, you need a microphone amplifier with phantom powering to connect it to your sound card. A small mixer is the cheapest way to get them working. But be sure not to have any filtering activated.

Raw electret capsules

Raw electret cartridges without any case are a good choice as well. They are small and do not modify the sound field very much. This causes usually a reasonable good frequency response. You still need a microphone amplifier, this time with bias power.
But you should not chose any electret cartridge. Some of them are really bad. But there are types that are explicitly designed for measurement purposes. Panasonic WM61A is well known to fit the needs. Similar models are also sold as Monacor MCE 2500.

USB measurement microphones

Digital USB microphones like Behringer ECM8000 are not usable. The reason is that their internal ADC has small frequency tolerances. DRC123 needs to suppress ambient noise to a very high degree. This is achieved by using frequency windows with a width of down to 10^-6 (1 ppm) and less. No sound device will fit this need. The result will look like this example. You need the selfsame sound device for recording and playback for the tolerances to cancel.

USB measurement microphones with integrated line out

Unlike the stand alone USB microphones the ones with a built-in sound output device perfectly fit the needs. The hack is that the ADC and the DAC share the selfsame crystal oscillator. So the frequency tolerances cancel in the same way as they do with an ordinary sound card. The difficult task is to get a sound driver for OS/2.

Microphone amplifier

First of all the amplifier must match your microphone type. Especially because electret microphones need powering.

Using the sound cards integrated amplifier

Some sound devices have built in microphone amplifiers. Although it is obvious to use them unfortunately their quality is mostly very poor. Furthermore there are different and incompatible concepts for microphone powering.