100 Years Of The Condenser Microphone

We recently marked an important milestone in audio history — the invention of the practical condenser microphone. While names like Edison bring up images of celebrated inventors, the name of Edward Charles (E.C.) Wente, who played a major role in developing the direction of professional audio, is almost completely unknown — even in his own industry.

 

Born in 1889, Wente had a long and fruitful career during his tenure at Western Electric/Bell Labs from 1914 to 1954. One of the first assignments for the 25- year old Western Electric engineer was improving the quality of telephone audio. Wente’s solution came in the form of patent #1,333,744 for a “Telephone Transmitter,” filed on Dec 20, 1916, which we have since come to know as the condenser microphone.

 

 

 

Wente described the core of the microphone capsule (shown in Fig. 1) in his patent: “…The diaphragm (#6) and the plate (#7) constitute the two plates of the transmitter. Sound waves, incident on the diaphragm, cause it to vibrate, thereby causing variations in the effective thickness of the dialectric between the plates and consequently giving rise to variations in the capacity of the condenser… the plate is separated from the diaphragm by a very thin film of air having a thickness on the order of 0.0005 of an inch.”

Here, the “plate” refers to the familiar backplate of modern condenser microphone designs, and the diaphragm in this design was a piece of sheet steel stretched to its elastic limit of approximately 22 microns [0.00086-inch]. A piece of mica separated the plate and the diaphragm, held tightly in place as to create a seal air chamber between the two, so when sound waves push the diaphragm backwards, the compression of the airspace adds stiffness while increasing the elasticity of the diaphragm. And that 0.0005-inch airgap is less than 1/12th the thickness of a human hair, requiring some impressive manufacturing precision to create such a device in that era.

Judging from the schematic, the electronics circuitry in Wente’s microphone is relatively simple, as shown in Fig. 2, with the signal from the “condenser transmitter” capsule (#20) driven by a triode vacuum tube (#22), with a transformer (#25/26) output — all of which are still familiar elements in modern tube mic designs. The preamplifier section was not Wente’s design, but had been created by electronics pioneer Ralph Hartley in 1915, employing using Lee De Forest’s 1908 Audion (triode)—the first vacuum tube.

 

 

This first design was fairly crude, but provided remarkably flat performance compared to the carbon mics used at the time. A year later, Wente described a version in The Physical Review, which featured a large (1.9-inch diameter), 22-micron diaphragm and was capable of reproduction beyond 15 kHz — an astonishing accomplishment in a day when most 78-rpm acoustic recordings topped out around 3k Hz.

Over the years, Wente’s condenser mics continued improving, (such as using aluminum diaphragms, which greatly increased a mic’s sensitivity), and his model 394 condenser microphone capsule fueled the impending revolutions in the electrical recording process and motion picture sound. Western Electric’s condenser lineup expanded with well-known models such as the 7A/8A/9A/10A/47A/53A offering tabletop, floor stand and hanging variations for any application — all featuring Wente’s model 394 “condenser transmitter.”

 

Despite the obvious sonic advantages provided by condenser microphone technology, they were rarely used in early P.A. systems, mostly due to their cost, relative fragility and the bulk and hassle of lugging an external power supply to power the tube electronics.

That would all change with the advent of low-power-draw transistor condenser microphones, a movement that leaped light years ahead in 1966 when Georg Neumann invented the concept of phantom power for his 48-volt KM 83/84/85 condenser mics — now the KM 183/184/185 line. The KM 80 Series mics were a hit, the 48-volt standard was eventually adopted throughout the industry and today, 48V phantom power makes life a little easier for audio engineers everywhere.

 

As a solution for improved (yet easy to employ) microphones that outperformed the carbon granule mics of the day, Wente worked with fellow Western/Electric/Bell engineer Albert Thuras to develop the first commercial dynamic microphone. Their 1928 patent improved on a low-fimoving coil microphone concept that was first patented in 1874 by Ernst Siemens. The Siemens mic never went into production, because at the time, carbon mics were adequate for telephone use and easy to manufacture.

The Thuras/Wente dynamic mic design was eventually released in 1931 as the Western Electric 618A Electrodynamic Transmitter (pictured above). With its thin duralumin diaphragm and new cobalt- steel alloy magnet, the omnidirectional 618A offered high output and a respectable 10 kHz bandwidth. With its ease of operation and tough, compact housing, the 681A remained in use for years among P.A., film and broadcast users. It was also the mic of choice for Franklin Delano Roosevelt’s famous Fireside Chat radio addresses.

 

Wente’s contributions to sound reinforcement went well beyond microphones. There’s no doubt that 1926 marked a busy era for audio technology. Electrical recording — the use of microphones and amplifier-driven cutterheads on record lathes (rather than the recording horns of the acoustical recording process) — was becoming a standard throughout the industry, and the development of talking motion pictures was underway.

Suddenly, the industry needed loudspeakers, with an emphasis on the word “loud.” A year before, General Electric engineers (Chester Rice and Edward Kellogg) unveiled the modern dynamic cone loudspeaker, an important step in the right direction. However, with the low-power amplifiers available at the time, any hope of providing high-output reproduction was pretty much out of the question.

Given that premise, Wente again teamed with Thuras and set out on a radical solution. Their 1926 patent application (#1,707,544) offered what they described as “an acoustical device employing an electrodynamic actuated vibrating element” using a “light piston-type diaphragm which is driven by a light rigid coil.” The diaphragm/voice coil was set within a dense electromagnetic structure fed a signal from an amplifier and was designed to mount onto a horn.

The resulting product was the Western Electric Model 555-w “receiver,” and the modern high frequency compression driver was born. It’s surprising how close the 555-w came to modern designs and how little things have changed since then. The original Wente-Thuras design featured innovations, such as a lightweight 0.002-inch thin aluminum dome diaphragm with a corrugated surround offering strength, stiffness and flexibility; a phase plug; and a threaded horn mount that allowed the driver to be easily fitted on a variety of horns.

Old drawings show the 555-w mounted on a Western Electric Model 12-A, a huge, exponentially tapered horn design that was 67 inches tall, with a 45×45-inch throat opening and an 11- foot overall pathway that provided a frequency response as low as 80 Hz. Although bulky, the driver/horn combination proved highly efficient, and could achieve high sound pressure levels from low-powered amplifiers and could be used alone or combined with cone woofers for extended low-frequency performance.

 

Aside from his work on developing the condenser mic, the dynamic mic, the HF compression driver — those alone should qualify him for the Rock and Roll Hall of Fame — Wente’s innovations were many, including the multicellular horn, a “light valve” for translating audio impulses into variable density patterns for film soundtracks and numerous major contributions in the study of auditory perspective, anechoic room design and acoustical wall materials. In all, he was granted a total of 36 patents and awarded a Scientific and Engineering Academy Award in 1936 for his multicell horn design. Not bad for a guy you never heard of.

This article previously appeared in the Oct. 2016 issue of MMR’s sister publication, FRONT of HOUSE.