Sennheiser MKH 416 The MKH Story - Page 5

The MKH Story Push-pull pulls distortion down Theoretical and practical investigations revealed that the source of the distortions is located in the narrow air gap between the diaphragm and the backplate of the condenser capsule. The viscosity of the air trapped inside this gap causes frictional forces that impede the diaphragm movement. With the diaphragm moving outwards the air motion is easier, due to the wider gap, than with an inwards motion. This difference causes a non-linear reacting force and thus distorts the diaphragm movement. Additional tonal components result, which affect the filigree of the sound. Besides harmonic distortion components (THD) that can enrich the overtone structure and are less conspicuous, annoying disharmonic components may appear due to intermodulation effects. If these components are low, they will not be perceived as distortions but rather like a lack of transparency or tonal resolution - like looking through a not perfectly clean window. The distortion of microphones increases with signal level. This is also a general property of analogue equipment and sound storage mediums. Therefore microphone distortion was previously concealed by the deficiencies of the analogue recording technique. However, digital signal processing exhibits an opposite effect. The linear quantisation of digital signal conversion improves the linearity especially at high signal levels. Thus digital audio can reveal microphone distortions, especially at high sound pressure levels. After the origin of the microphone distortion had been detected, the search for an efficient remedy began. In the end the symmetrical push-pull principle was selected, not only as the most effective, but also the most sophisticated method. The transducer was equipped with an additional plate in front of the diaphragm that was identical to the backplate. Thus two equal air gaps on both sides of the diaphragm were formed. Due to the symmetrical design, the reacting forces in both air gaps sum to an almost constant value if the diaphragm is moved in either direction. So the diaphragm movement is virtually distortion-free. This effect was supported by the acoustic transparency of both plates due to a high degree of perforation. Thus the balanced push-pull transducer was born as the heart of a new professional condenser microphone range. + - a b c Capacitive transducer a Diaphragm at rest position b Positive pressure moves the diaphragm towards the backplate. The narrowed airgap increases the frictional forces and impedes the air motion. c Negative pressure moves the diaphragm outwards. The wider air-gap eases the air motion due to reduced frictional forces. backplate 1 air gap 1 diaphragm backplate 2 air gap 2 Balanced push-pull transducer The diaphragm is centred between two equal backplates creating two equal air-gaps and two equal capacitors. a b c Distortion cancellation due to balanced push-pull design a Diaphragm at rest position b, c Diaphragm excursions in both directions cause equal but opposite alterations of the air gap widths. Thus the sum of the forces reacting on the diaphragm is kept constant. Page 5