Probably the only Sennheiser mike with any alpha numerical nomenclature, the Ambeo VR Mic
is a A-format
first-order Ambisonic microphone with four KE 14 polarized electret capsules powered by Phantom 48V with a reduced 3,5 mA comsumption, when SPS200 is designed with static condenser capsules also powered by Phantom 48V.
With a background noise of +18 dBA for a 130 dB dynamic range and a 20-20 kHz frequency response, the acoustic characteristics of the capsules should be relatively correct for ordinary sound fields, but the noise will be clearly audible in very quiet environments (out of question to record with this microphone the tiny quivering of the ice floe at the North Pole, and still less the tickling of the sacred scarab beetle on the sand in the Egyptian desert).
The Ambeo four capsules, in a tetrahedral arrangement, are enclosed in a metallic windshield (where the SPS200 leaves them bare). It is rather heavy (more than 400 g) and is larger than the SPS200 (21.5 cm long, 25 mm diameter body and 49 mm diameter head). Bottom is a threaded 12-pin DIN connector. A relatively flexible 3-meter Mogami multicore cable with Lumberg connectors, manufactured by Cinela for DC Audiovisuel, and a 12-pin connector to 4 XLR-3M split cable, allows the microphone to be connected to a 4-mike-input recorder (Sonosax SX-R4, Cantar Mini). A lyre shockmount designed by Sennheiser allows it to be used indoors.
Cinela has specially adapted a Pianissimo windscreen for outdoor use of the Ambeo VR Mic, with a Lumberg threaded 12-pin DIN connector directly integrated into the swivel.
Spotting of the capsules is of paramount importance, so an engraving on microphone body indicates front (front
) and top (up
) of the mic (on the same side as the logo and the name Sennheiser Ambeo VR Mic
). In fact, the signals of the four capsules (1=front-left-top, 2=front-right-bottom, 3=rear-left-bottom and 4=rear-right-top) must then be combined in a defined order to provide B-format
Vertical is the standard use, head up, front-facing engraving (Upright
mode in the Sennheiser plugin, normal in the Soundfied plugin). It can be used vertically, head down, front-facing engraving (Upside Down
mode in Sennheiser, Invert
in Soundfield) or horizontally, head forward, up-facing engraving (Endfire
mode in Sennheiser, End Fire+Invert
in Soundfield), or horizontally, head forward, down-facing engraving (End Fire
mode in Soundfield only, the Sennheiser plugin does not accept this position).
It is very important to communicate to the person who will exploit the Ambisonic files the position of the microphone when recording!
Seldom used in movie and television fiction, Ambisonic is the result of researches, in the 70s, by two researchers from the British National Research Development Corporation at Oxford University. They subsequently founded Soundfield, recently bought by the microphone manufacturer RØde, for the commercialization of Ambisonic microphones. With the expiry of the Ambisonic patents, very few other manufacturers have designed microphones using this technology. Sennheiser being the latest to surf on the wave of virtual reality.
Ambisonic, based on the decomposition of a sound field in spherical harmonics, aims at a three-dimensional reproduction of the sound space at a given point. B-format
corresponds to this decomposition limited to first-order. Four channels are necessary: W (omnidirectional pressure) and XYZ (bidirectional gradients according to the three axes).
9 channels would be needed for the decomposition of a sound field in second-order, and 25 for that in third-order! The higher the order, the more spatially accurate the result. First-order Ambisonic is a simple and minimal compromise that results in an image, which can be considered by some as somewhat fuzzy
, of the sound field captured.
The devices of higher order are called HOA (High Order Ambisonic). They are very rare, complex and costly. The reproduction of such signals is still a vast field of investigation, but there are many potential applications (hyper-directivity or classical multichannel diffusion by plugin). The CNAM (Conservatoire National des Arts et Métiers
, Paris) in association with Cinela, works on a HOA project using a sphere including 50 capsules, made of 200 Mems sensors, according to a Lebedev geometry.
simply uses the signal of the four capsules, more or less cardioid, in a tetrahedral arrangement. It is not directly processable. It must be encoded to B-format
using a plugin (mainly VST or AAX) processing the four capsules signals to the four WXYZ components that can be recorded in a polyphonic WAV file in standard format .amb or in more specific Apple format AmbiX .caf.
In practical terms, Sennheiser builds B-format
with W (omnidirectional) equal to the sum of the four capsules, the X equals the sum of the two front capsules minus the two rear capsules, the Y equals the sum of the two left capsules minus the two right and finally, the Z equals the sum of the two top capsules minus the two bottom capsules.
In movies, as in television, the Ambisonic WXYZ components need to be distributed on broadcast systems, stereo, 5.1 or 7.1, or Dolby Atmos.
Sennheiser provides only a VST plug-in (no AAX for Pro Tools for Windows, only for MacOS X) 32 or 64-bit, encoding the A-format
of the four capsules of the Ambeo to the B-format
WXYZ. It is then necessary to use another one to transform this B-format
into broadcast format. Sennheiser advises against using a plugin other than its own to switch from A to B, arguing a specific treatment to the characteristics of the capsules.
Soundfield offers a 32-bit or 64-bit VST plug-in that lets you go from either A-format
, or B-format
, to go to B-format
or different multichannel formats, from simple stereo to 7.1 through 5.0, 5.1, 6.0, 6.1 and 7.0.
Many other software developers provide Ambisonic plugin
A complete kit including the Sennheiser Ambeo VR Mic, Cinela Pianissimo windshield, lyre shockmount, multicore cable and split cable, all in a suitable carrying case, is already available on rental from DC Audiovisuel.