Measurement technics meets recording technology – The unique Decca Tree live recording solution from Gefell

True genius lies in is taking the complex and making it simple – a maxim to which Microtech Gefell has also committed itself.

The innovative Decca Tree solution from Gefell brings the simplest installation to a point with the highest recording demands. High-quality, compact and quickly installed, it opens up new perspectives and fields of application for the demanding user.

The Decca Tree technique was developed at Decca Records to accurately record orchestrals. By using two mics at the sides and one in the center it`s possible to get a greater degree of control over all the facets of the recording, both the source and the room acoustics.

A Decca Tree setup uses three omnidirectional mics arranged in a "T" pattern outlining a triangle. The centre mic is mixed with the two spaced mics to fill the "hole in the middle" in their image and shows the sound source. The placement can be with 3 separate mic stands or with one or more poles. The Decca Tree size  is not fixed (unlike ORTF stereo) and can vary greatly. The distances between the two rear mics can be between 0.6 and 1.2 m. The front mic is set proportionally and can be mounted slightly lower than the outer pair.The sound engineer adjusts these measurements according to subjective view to the size of the ensemble, the dimensions of the room and the music type.

The heart of the Gefell Decca Tree solution is the studio mic M 221, an extremely precise, measurement-grade omni condenser mic, perfect for Decca Tree recordings or recordings where long term stability is crucial.

The Gefell Decca Tree-Set  is supplied complete with a set of three M 221. The tree can be stand mounted or slung (with optional hanging brackets) and has fully adjustable mounting positions. The kit comprises of: 6 x 57 cm arms, 1 x T-piece, 3 x jointing pieces.

Gefell M 221 – The very special mic

Professional audio (a trade magazine for studio professionals, ambitious musical amateurs), for example,  concluded: "The Microtech Gefell M 221 is a perfectionist, created by perfectionists for perfectionists. Its exceptionally natural fine sound makes it the ideal transducer for demanding recordings. Given the sound quality, the outstanding workmanship and the clever ball attachments for mechanical diffuse field equalisation, its price is entirely appropriate."

The  Sound On Sound audio recording technology magazine, based in Cambridge/GB, recognised internationally as the ‚bible‘ of the music recording industry and a firm favourite of top producers, engineers and artists summed up, that the  M 221 is not only "… a remarkably high-quality mic, but a very versatile one too, thanks to some cunning acoustic trickery… There is something very natural about the sound character of omnidirectional mics. They don’t have that slightly lean and phasey quality at the bass end that so many cardioids display, and also, while they inherently pick up a lot of spill from any sound sources nearby (as well as lots of reverberation or room acoustic), that ‚off-axis‘ sound is always tonally neutral and in the right proportion, which makes it sit in a mix very naturally.“

The M 221 unifies universal small diaphragm capsule technology with a metal diaphragm and a modular preamplifier. The use of a certificated measurement capsule combined with a preamplifier with 48 V phantom powering reaches a constant magnitude and phase response and transmission properties which are long-term stable.

Because of the small capsule dimensions this mic is ideally suited for applications where a low interference with the sound field is demanded. It`s usable as a main mic for recordings which should include a certain amount of the room response.

As well the M 221 is suited as a spot mic for recording instruments very close where the low frequencies should be transmitted in detail and original. Moreover it can be used for multichannel main mic arrangements like AB stereophony or decca tree. For these applications the mic is ideal because of its precise and long-time stable transmission properties.

It has an excellent transient response because of the pressure transducer capsule and is really suitable to record instruments with extreme transients. With the both different sized equalization balls (ball attachments for mechanical diffuse field equalisation) a high frequency accentuation can be produced in an acoustical way to increase the presence and to compensate the high frequency roll-off in diffuse sound field.

With a accessory humidity adaptor it  can operate also in unfavorable climate conditions especially referring the temperature and air humidity.

Acoustical Specifications

The pick-up pattern is equivalent to the longitudinal direction of the mic. As sound transducer a condenser capsule with a diameter of 12,5 mm and a very thin metal diaphragm is used. The pressure transducer capsule exhibits an omni-directional polar pattern without proximity effect.

Because of the free field equalization the M 221 has a constant frequency response from 3,5 Hz up to 20 kHz (± 2 dB). The static pressure equalization between the capsule interior and the atmosphere goes through a vent to the preamplifier.

The use of the M 221 with the equalization ball KA 3 effects a frequency response accentuation which begins at approximately 1 kHz and goes constant at 3 dB from approximately 4 kHz up to 12 kHz. With the equalization ball KA 4 the accentuation of the high frequencies starts also at approximately 1 kHz and goes constant at 3 dB between 3 kHz and 12 kHz.

Electrical Specifications

The transformerless circuit design of the electronic impedance converter with symmetrical output facilitates an output capability for sound pressures from15 dB(A) up to 136 dB with a maximum of 0.5 % THD (total harmonic distortion). The mic is connected by a standard 3-pin XLR-male plug with gold-plated pins which realizes the powering by an external 48 V phantom supply according to IEC 61938 with a current consumption of 6 mA.

Mechanical Specifications

The M 221 consists of a mic capsule (MK 221) and a mic preamplifier as independent mechanic parts which are connected to a complete mic through a screw thread. The mic capsule and mic preamplifier have an identical overall diameter of 12.7 mm which enlarges to 21 mm over the length of the mic preamplifier to the connection side of the mic. The shape of this enlargement is designed for a minimum interference with the sound field. Mic capsule and mic preamplifier have together a length of 170.4 mm.

On the connection side of the M 221 there are standard mic holders mountable.

To reach a higher attenuation against solid-born sound and impulses there are elastic holders and suspensions usable. The weight of the M 221 is only 123 g what allows to use the mic with stand mountings as well as to hang up the mic from the ceiling without any problems.

Matched Sets

The M 221 can be supplied in matched stereo pairs on request.  The mics are  also available on request are matched sets of 3 or 5 for Decca Tree or Surround recording use.

The mic capsule – MK 221

The 1/2“ capsule MK 221 is a special capsule originally developed for critical measurement applications. It has a free-field response and is therefore optimized for cor-rect acoustic measurements under free-field con-ditions with 0 degree incidence.The equalization of the static air pressure is bymeans of a capillary to the preamplifier (rear-vented). The mic capsule is suitable for sound level meters of Type 1 acc. IEC 61672

The MK 221 is designed and very carefully constructed to ensure excellent long-time stability of the electroacoustical parameters. All essential components including the diaphragm are made of nickel.

A 2µm,  Nickel  diaphragm, much thinner than normal diaphragms, is used, rather than the more usual gold-sputtered plastic (mylar) membrane, because of an excellent transient response. The diaphragm is bonded by a special galvanic process.

The rear electrode is insulated from the cartridge housing by a quartz glass plate.

The MK 221 can be unscrewed from the  Preamplifier MV 225 to permit either of the the equalization ball KA 3 or KA 4 to be attached.

The ball is slid over the mic body, the capsule refitted, and then the sphere is slid back up until it snaps into position.

Preamplifier MV 225 P48

The 1/2” measuring microphone preamplifier type MV 225 P48 is a high-impedance transducer for 1/2" condenser measuring microphone capsules. It enables direct powering of the capsules by balanced inputs with 48 volts phantom powering usual for studio technique.

Nearly the entire dynamic range of the capsules can be used. Commentary DAT recorder with phantom powering without signal condtioner can be used as data memory for measuring technique applications. Other possible applications are loudspeaker measurements and set up of acoustic equipment.