|What is special on this circuit and the microphone array principle?|
After modification of the electrets capsule, the internal FET in it is now connected as a source follower, with a gain of 0dB. The balanced output rises the gain to +6dB.
Panasonic states for the WM61A omni capsule a sensitivity of -35dBv/Pa and a load resistor of 2k2.
The total sensitivity reduction measured between an unmodified electrets capsule with UN balanced output, and a modified capsule with balanced output, was 4 dB.
R2 therefore introduced 10dB negative feedback to the capsules internal FET of the WM61A ! This simple modification reduces distortion at high sound levels, AND improves the max. SPL by 10dB.
When the unit is placed on a large hard surface, the layout of the L2-floor-array boosts sensitivity further by 7dB. Resulting in a practical sensitivity of -34-10+6+7 = -31 dBv/Pa.
The max output level of the capsule is now +1dBv. Max. audio input therefore is calculated at 94+31+1=126dBspl.
The electrets capsules Signal-to-Noise-Ratio is unchanged by the modification, as the output noise of the FET is reduced together with the audio output. Without a special silent room the measuring of microphone noise levels is difficult. Tests showed very low noise level in silent living room conditions.
Compared to a microphone in free air, a L2-floor-array placed on a large hard surface improves the signal to electronics noise ratio significantly. For instance: the boundaries of a L2-stereo-floor-array (see photos) cause a 7 dB higher microphone output. The electronics noise level of the microphone is unchanged. Resulting in 7 dB better S/N (!).
Stereo image and frequency respons.
The boundaries of these PZM arrays with omni capsules make the system UNI directional above 500Hz and therefor give very good stereo images (photo 1).
The capsules however are OMNI, and low frequency roll-off does NOT exist, as is the case with cardioids capsules.
To compensate for the "low frequency shelf" introduced by the limited size of the PZM baffles, a boost of EQlow in the mixer is enough (adjust to 15:00 Hrs). Resulting in a flat and wide frequency response from 15kHz down to 20Hz.
Further benefits of this L2-floor-array stereo directional system are (see photo's and read this Crown publication):
- Reflected sound (reverb) is amplified LESS than direct sound. Resulting in better direct-to-reflected-sound-ratio. In other words: sound sources seem to be closer by positioned (larger "reach").
- Placed on de stage floor (photo 2) and made from acrilic sheet, this stereo system is much less conspicuous then a high microphone stand with stereo mikes on it.
- Quickly to install. Just connect the cables and put it down.