THE INVISIBLE MICROPHONE 

The Sennheiser MD 31 is one of the most extraordinary microphone designs of the 1950s.
Created for professional stage and broadcast use, it was designed to solve a very particular problem: how do you place a microphone directly in front of a performer without hiding the performer behind a microphone?

Sennheiser's solution was radical. The actual dynamic transducer was moved all the way down to floor level, while only a thin acoustic tube and a remarkably small pickup head remained in front of the artist. From the audience — and particularly through the television cameras of the period — the microphone almost disappeared.
The concept first appeared with the Sennheiser MD 3 around 1949. The later MD 31 retained its remarkable construction while introducing an improved, more sensitive dynamic transducer and a specified frequency range of 50 Hz to 12 kHz.

The design quickly found its place in the highest levels of European broadcasting and live performance. It was used by the NWDR and later NDR, and microphones of this remarkable family appeared in major concert halls, prestigious live-performance houses, broadcast studios and high-end venues of the period. Édith Piaf was particularly fond of Sennheiser's “invisible microphone” concept, which allowed her to perform without a conventional microphone obscuring her face and stage presence. Historical photographs and material also connect these early Sennheiser systems with Quincy Jones, Jerry Lee Lewis and other major artists and celebrities of the era.

The MD 31 was therefore not simply an experimental curiosity. It belonged to the professional world it had been designed for: major artists, broadcasters and prestigious live stages where both sound and presentation mattered.
More than seventy years later, the MD 31 remains an astonishing piece of microphone engineering: not a conventional microphone placed on a stand, but a complete acoustic system designed around making the microphone itself nearly invisible.

REMARKABLY PRESERVED

The condition of this MD 31 system is extraordinary.
Despite its age, the system appears retaining an exceptional degree of originality and preservation throughout. The surfaces, finishes, mechanical elements and delicate acoustic components have survived in a condition that is remarkable for professional equipment from this period.
This is not simply a well-kept vintage microphone. It is a fully preserved historical system, complete with its original packaging and papers, presenting today remarkably close to how it would have appeared during its working life.

Considering the age, complexity and intended professional use of the MD 31, finding an example preserved to this degree is exceptional.

The photographs form part of the description and document the remarkable condition of the actual system offered here.

TECHNICAL CONDITION

FULLY FUNCTIONAL — TESTED
The technical preservation is equally remarkable.
The MD 31 has been carefully tested and is fully functional, with the microphone still performing as intended and reproducing the appropriate specified frequency range.
The dynamic transducer remains healthy, and the complete acoustic system — including its unusually long acoustic path — continues to operate correctly more than seventy years after its manufacture.
This is therefore not simply a historically complete display or collector's piece. It remains a working professional microphone system, preserving both the physical construction and the functional performance for which the MD 31 was originally designed.
For an instrument of this age, rarity and mechanical complexity, its combination of historical completeness, exceptional cosmetic preservation and correct technical performance is truly remarkable.

BUILD & DESIGN
The MD 31 achieves its unusual appearance by separating the point where sound enters the microphone from the location of the transducer.
Only the slender tube and small acoustic head rise to the height of the performer. The substantially larger dynamic transducer remains hidden inside the floor unit.
This arrangement gives the MD 31 its unmistakable silhouette and explains virtually every element of its construction.

THE MODULAR CONSTRUCTION
The MD 31 consists of several interconnected mechanical and acoustic sections, allowing the unusually tall assembly to be transported, assembled and adjusted while retaining its extremely narrow profile.
The floor unit, tube sections and interchangeable upper acoustic elements together form a remarkably versatile microphone system.

THE FLOOR UNIT
The large lower housing is not simply a weighted stand. It contains the actual dynamic moving-coil transducer.
It also provides the stability required by the tall, slender construction and protects the microphone's internal components.
What appears at first to be the base of a microphone stand is therefore the microphone's main electromechanical housing.

THE ACOUSTIC TUBE
Rising from the floor unit is the characteristic slender metal tube system, extending toward the acoustic head at the top of the microphone.
The tube forms the acoustic connection between the sound inlet and the dynamic transducer housed below. Sound entering at the top travels through this path before reaching the moving-coil system.

THE TELESCOPIC SECTION
The telescopic section allows the microphone to be adjusted continuously to suit different performers and applications.
With this element, the acoustic head can be positioned at virtually any height from approximately 110 cm to 210 cm, giving the MD 31 an unusually wide adjustment range while preserving its exceptionally slender appearance.

TOP-TUBE EXTENSION
An additional top-tube extension can be fitted to the main acoustic tube, extending the microphone approximately another 30 cm.
This simple modular element allows the acoustic head to reach farther outward when required, giving the system additional positioning flexibility without changing the location of the floor unit.

THE ACOUSTIC HEAD
At the top sits the remarkably small acoustic head — the only part that needs to occupy the conventional microphone position in front of the performer.
Its minimal dimensions are central to the MD 31's purpose, keeping the area around the performer's face visually clear.

THE TRANSPARENT ACOUSTIC DISC
One of the system's most distinctive elements is the transparent circular acoustic disc surrounding the small upper head.
The disc adds directionality to the microphone while minimizing its visual presence. Sennheiser used related acoustic-disc principles on other, more conventional microphone designs, but nowhere was the idea integrated as elegantly as here: even the element used to shape the microphone's pickup remains almost visually transparent.

THE LARGE DIRECTIONAL HEAD
For applications where visual transparency was less important, the MD 31 system could be configured with a completely different upper assembly: a large directional microphone head.
Unlike the transparent disc, this is deliberately substantial and immediately visible. It transforms the appearance and acoustic behaviour of the system, allowing the same basic microphone to be adapted for other professional applications.
Together with the different upper acoustic elements, the MD 31 could therefore cover a remarkably broad range of directional behaviour — from a very narrow, highly directional pickup, through cardioid operation, to an almost omnidirectional response.
This modularity is one of the most remarkable aspects of the system. Rather than being a single fixed microphone design, the MD 31 could be physically reconfigured according to the acoustic and visual requirements of the application.

THE MD 31
The MD 31 followed Sennheiser's earlier MD 3 and represents the developed version of the same remarkable concept.
Externally, the two systems are essentially identical. The important development occurred inside.
The MD 31 received an improved dynamic transducer offering greater sensitivity and improved performance while preserving the architecture of the MD 3.
Its specified frequency range extends from approximately 50 Hz to 12 kHz, impressive performance for a dynamic stage microphone of this unconventional construction and period.
The MD 31 captures a fascinating moment in broadcasting and performance history, when engineers were simultaneously trying to improve sound reproduction and reduce the visual presence of the equipment producing it.

TECHNICAL SPECIFICATIONS
Manufacturer: Laboratorium Wennebostel / Sennheiser
Model: MD 31
Country of manufacture: Germany
Type: Dynamic moving-coil microphone
Transducer location: Floor unit
Acoustic system: Remote acoustic inlet with tube transmission to the transducer
Frequency response: Approx. 50 Hz – 12 kHz
Sensitivity: 0.1 mV/µbar
Frequency tolerance: ±3 dB
Application: Professional stage, broadcast and television use
Construction: Floor-standing modular acoustic microphone system
Era: Mid-1950s
Predecessor: Sennheiser / Labor W MD 3

RARITY & HISTORICAL SIGNIFICANCE
The MD 31 belongs to a period in which microphone design was remarkably experimental. Rather than accepting the microphone as a visually unavoidable object, Sennheiser reconsidered the relationship between performer, microphone and audience.
Historical photographs are particularly revealing. In images of performers such as Édith Piaf and Jerry Lee Lewis, the extremely narrow Sennheiser structure is barely noticeable despite standing directly in front of the performer. Historical material also connects microphones of this family with Quincy Jones and other major figures of the period.
Surviving examples are exceptionally scarce. Museum and collector archives document the MD 31, but there is remarkably little evidence of complete examples appearing for sale on the modern online market.
Most importantly, the example offered here is a historically complete MD 31 system, preserved together with its original packaging and original papers.
This degree of completeness is extraordinary for a professional microphone system of this age. The MD 31 was never simply a single microphone, but an elaborate modular system intended to be configured for different professional applications. Having the system survive together in its historical context — including its packaging and documentation — makes this example particularly significant.
Rather than an isolated surviving piece of equipment, this is a remarkably complete historical ensemble showing the MD 31 as Sennheiser originally supplied and conceived it.
A historically complete MD 31 system such as this represents considerably more than a collectible vintage microphone. It is an exceptionally rare surviving artifact from the formative years of Sennheiser and from an era when microphone engineering, broadcasting and stage design were developing together.

THIS LISTING INCLUDES

COMPLETE AND ORIGINAL

Sennheiser MD 31 microphone system

• floor unit containing the dynamic transducer
• acoustic tube system
• top-tube extension
• small acoustic head
• transparent acoustic disc
• large directional head
• packaging
• papers and documentation​