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That capsule is very special. The reason Milab made it rectangular was that all normal capsules, being circular like a tambourine, will have a resonant peak that is very often in the midrange. Milab found that by making a rectangular capsule, you broke up that resonance and moved it higher up in frequency where it was less dangerous. It was moved out of the critical midband area. So that capsule is in a class by itself.

Kavi Alexander
Grammy award winning engineer and owner of Water Lily Acoustics
The Milab 2700 full condenser capsule, uses two halves which are mounted into one piece to a "Rectangular large dual membrane capsule". The principle of such a dual, back to back, membrane condenser was first described in 1935 by Braunmuhl and Weber (article in Hochfrequenztechnik und Elektroakustik). Each side of the capsule has a cardioid pattern. By adding or subtracting the two signals all patterns from omni to figure eight are obtainable.

Typical 1 inch circular capsule next to the Milab 2700Almost all manufactured microphone capsules in the world are circular. Milab Microphones has always aimed to make small microphones with a big sound and one of the benefits of the rectangular capsule is the possibility to fit it inside small microphone bodies.The capsule is more than 1 inch in the length direction, but the width is much smaller.

The 2700 rectangular capsule was developed during the fifties and the sixties and was used in several microphone models - many considered as classics today. In 2006 the capsule was modernized which meant improved frequency response and lower self noise. The new version of the capsule goes, internally, by the name 2900 and was first used in the DC-196.

The membrane is a polyester film covered by a thin layer of aluminium, giving good electrical properties as well as low weight. The weight, membrane stiffness and air cavity stiffness are all properties that influence the high frequency response as well as the transient response.

The main movements of the membrane are of course in and out from the backplane, regardless of the shape, but some secondary effects may be noticeable.

Acustical differences
The acoustic properties of a rectangular shape are somewhat different from the circular shape. If the microphone is placed vertically, so that the diaphragm's longer axis is upright, it will exhibit a wider directional pattern in the horizontal plane than in the vertical. In the horizontal plane the capsule will act as small diameter microphone with better frequency properties than a large circular membrane capsule. It reacts in much the same way as a line-array loudspeaker does. This can be of great benefit when you're trying to minimise spill or reflections from specific directions.

Difference between the frontal and side (90) frequency response - 2700 capsule
Figure 1. The 2700 capsule
Difference between the frontal and side (90) frequency response - typical 1 inch capsule
Figure 2. Typical 1 inch capsule

The curves above illustrate discrepancies between the two microphones' frontal and side frequency response. They were measured at 90 in the horizontal plane.

Mechanical differences
Rectangular shapes have different mechanical resonance phenomena than circular. In the 2700 the length and width are calculated so that peaks in the length direction never coincide with peaks in the width direction.

Summary of benefits when compared to conventional capsules
  • The off-axis response is smoother and more consistent. This is one of the reasons why the simulated capsules in the SRND 360 work so well. The following quotes from magazine reviews of Milab microphones with rectangular capsules confirm this statement: "Off-axis response on the cardioid setting is extremely smooth and consistent, with minimal colouration." (Resolution Magazine on the DC-196) - "One if its most impressive traits is the distinct lack of tonal colouration as sound sources move off-axis to the directional patterns." (Resolution Magazine on the DM-1001).

  • The dimensions of the rectangular capsule allow us to build some of the smallest large membrane microphones on the market. The DC-196, DC-96C and DC-96B are significantly smaller than conventional LDC microphones and are often mistaken for small diaphragm mics because of their size.

  • The rectangular shape eliminates the resonant peak in the midrange that is a negative consequence of large circular diaphragm capsules.

  • The horizontal and vertical polar patterns are not identical (the vertical is narrower), which makes the microphones even more versatile. When micing an acoustic guitar, for instance, placing the microphone vertically in front of the guitar will pick up more of the strings and placing it horizontally will pick up more of the body and less of the strings.

  • Best of both worlds: The rectangular capsule combines the superior resolution and off-axis response of a small membrane condenser with the sensitivity and low self noise of a large membrane condenser.
Learn more:
Acoustics and Vibration Animations - by Dan Russell, Ph.D.
Animations of Acoustic Waves - Institute of Sound and Vibration Research at the University of Southampton

References:
Backus, John, The Acoustical Foundations of Music, 2nd Ed, W. W. Norton, New York, 1977, ISBN 0393090965
Berg, Richard E. and Stork, David G., The Physics of Sound, 2nd Ed., Prentice Hall, 1995, ISBN: 0131830473
 
 
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