Sennheiser MO 2000 Industrial Optical Analog Microphone

Sennheiser MO 2000 Optical Microphone

This is not a review of a studio toy. It is, however, about audio; industrial audio. I had been hearing about a Sennheiser optical mic. Was this for studio use? Is it analog or digital? How does it work? I reached out to Sennheiser and within a week it was on my door step.

The Sennheiser MO 2000 system consists of a smallish half-rack-space box, a two-way fiber optic cable capped with an omni element. The system is powered by a wall wart power supply. There are both coarse and fine gain controls on the front of the chassis and a simple, lighted Off/On button. The omni mic element of the MO 2000 has a frequency response of 20Hzto 40kHz (+/- .6dB). 

MO 2000 Rear View

The analog balanced output impedance is 600 Ohms, unbalanced 33 Ohms. Analog outputs, both balanced XLR and especially the unbalanced BNC connector, convey that this is some sort of industrial gear. With an SPL of 134 dB, the environment can be at the pain threshold. With a S/N ratio over 50 dB (A weighted), the selfnoise of the microphone will prevent it from being used for critical applications in a recording studio, but that’s not its intended application.

The omni transducer works by shining a lightfrom an LED source generated in the box up one run of the dual fiberoptic cable onto a non-conductive diaphragm made of composite plastic. Sound moves the diaphragm and that movement varies the amount of light that is reflected back down the other fiber optic run. The variations in light intensity are received by a photo diode back at the box, converted into a voltage swing and there’s your audio. 

When would you use a noisier than studio standards microphone system with no metal parts? Maybe as a talk-back mic in a recording studio that pumps pure oxygen in to elevate the mood of the musicians? Inside or near a hydrogen filled dirigible? In corrosive environments that would corrode the metal? In high magnetic fields where the metal may become dangerously projectile; like in a particle accelerator or in an MRI. 

Non-conductive omni microphone element

An email to Sennheiser’s Vanessa Jensen, Sr. Product Specialist, Integrated Systems quickly got to the point. Yes, the MO 2000 system is designed for (but not limited to) deployment in hazardous environments where the absence of electrically conductive components in the transducer avoid the risk of creating sparks or generating heat. The MO 2000 can operate safely in otherwise explosive environments such as for the acoustic monitoring of gas drying plants in natural gas production. In this case, Jensen reports, “The microphone can ‘hear’ slow leaks, which, due to their small size, cause neither a pressure loss nor an alarm message in the other monitoring systems.” So, yes, industrial uses, provided the leaks are noisier than the selfnoise of the MO 2000. The MO 2000 microphone itself is not submergeable in water or presumably in other fluids.

In medical applications, the MO 2000 system can be used in magnetic resonance imaging (MRI), for example to maintain communication with the patient during MRI scans or to achieve active noise cancellation in MRI. The optical microphone does not disturb the imaging process and is itself not influenced by the strong fields inside magnetic resonance imaging equipment. In measuring technology, the microphone is ideal for use in EMI/EMC laboratories, as it does not influence the electric field.

Optical Connector for MO 2000 Microphone

Further, the MO 2000 complies with the requirements regarding intrinsically safe optical radiation according to EN 60079-0:2004 and EN 60079-28:2007. It also meets the requirements of the ATEX Directive on explosion protection, which was documented with the Type Examination Certificate TÜV 07 ATEX 553824 and TÜV 07 ATEX 553825X. This certification allows the MO 2000 to be used in potentially explosive atmospheres of Zone 1 (e.g. gas processing plants). The MO 2000 has the degree of protection IP 54 (5=protection against dust deposits, 4=protection against splashing water from all directions) and is therefore suitable for outdoor applications.

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