History of the B&K Microphone

Brüel & Kjær has more than 70 years of proven commitment to continuous product improvement and groundbreaking new innovations in measurement transducers. This article explores the history of microphone development at Brüel & Kjær and provides an insight into the research and development invested in microphone products. An overview of the production of microphones is also given.

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Table of content

Historical Background to Microphone Development
Microphone Innovations
Development of Brüel & Kjær Microphones
Production of Microphones at Brüel & Kjær

The First Microphones

Brüel & Kjær began producing microphones in 1945. By the late 1950s they were established as a leading supplier of measurement microphones, due largely to the inspirational enthusiasm of Dr. Per V. Brüel in the field of microphone and general transducer development.

In parallel, Brüel & Kjær also invented, designed and developed complete acoustic measurement systems, analyzers and recorders. The accuracy achieved from our measurement microphones formed an important part in the success of these products.

From humble beginnings, Brüel & Kjær gained an increasingly good reputation amongst microphone users, both in the field of acoustics and in academic research, providing a high standard of service, well-built products and collaborative research and development programmes, ensuring a continuous improvement in the accuracy and performance of new instruments.

Today, this approach continues to deliver innovative measurement instrumentation, including a comprehensive range of measurement microphones in sizes from 1/8-inch to 1-inch. Together these microphones cover all aspects of measurement microphone usage.

Fig.1.1 Range of condenser measurement microphones

By the early 1970s, Brüel & Kjær ’s strong presence in the measurement microphone field had become firmly established with the development of high sensitivity 1/2-inch microphones.

Calibration equipment, such as reciprocity equipment for laboratory calibration and the pioneering hand-held pistonphone, was released. This convenient way of calibration effectively improved the accuracy of everyday microphone usage by allowing users to check measurement accuracy in the field.

In 1973 Brüel & Kjær solidified the position as a leading microphone supplier, meeting a request from Western Electric to supply 1-inch microphones to replace their successful but ageing WE 640AA microphone.

The Brüel & Kjær solution took the form of the classic Type 4160 microphone.

Fig.1.2Sound Intensity probe using a phase-matched pair of microphones

Further innovations ensued, notably in improvements to electret processes during the 1970s, resulting in the production of stable prepolarized microphones which became standard for use with class 1 sound level meters.

The 1980s brought further developments, in particular in the field of sound intensity measurement. The mid 1980s also saw the development of specialized types of sound intensity probe microphone. This microphone made use of a revolutionary and now patented tube system which gives a flat frequency response and allows for measurements in places where access for standard measurement microphones is difficult.

Improvements have continued into the 1990s with the introduction of highly accurate yet robust microphones (the Falcon Range^TM), and outdoor microphones and terminals for permanent outdoor noise monitoring. The outdoor microphones have since proven very effective in harsh environments.

Microphone Innovations

1956 Type 4131

World’s first volume‐produced measurement microphones, Types 4131 (free‐ field) and 4132 (pressure‐field), developed, amongst others, by Dr. Per V. Brüel

1967 Type 4138

World’s first 1/8-inch measuring microphone. Due to on‐going product improvements, this type is still available

1975 Type 4160

Brüel & Kjær is requested to produce a replacement for the Western Electric WE 640 AA Reference Microphone. As a result, Type 4160 and later the ½" Type 4180 were introduced (still the world de facto acoustical standards)

1980 Type 4155

Brüel & Kjær launches the world’s first high‐stability, measurement grade, electret microphone, which is now discontinued.

1984 Type 4179

Using advanced modelling and clever design, this now discontinued microphone had a noise floor of –2.5 dB(A), that was still unbeaten after nearly 30 years!

1987 TYPE 4182

Brüel & Kjær introduces probe microphone for measurement in extremely confined spaces and up to more than 600°C

1993 Type 4188

Falcon series is introduced. Featuring stainless steel, press‐fitted diaphragms, these microphones result in a step change in microphone technology

1994 Type 4189

Falcon series at peak performance. Type 4189 is probably the world’s most popular ½-inch free‐field microphone

2000 Type 4297

The world’s only one‐unit Sound Intensity Calibrator enables calibration without dismantling the probe

2003 Type 4948

Surface Microphone – a Brüel & Kjær first: an 'all titanium' sensor originally developed for aerospace applications

2004 Type 4949

Surface Microphones now also find their way into the automotive industry, where they break new frontiers in wind‐tunnel testing

2005 Type 4952

World’s first outdoor microphone where all parts exposed to the weather are made from polymer materials

2006 Type 4955

Continuing the 'all titanium' concept, this TEDS microphone has 1.1 V/Pa sensitivity and a typical noise floor of 5.5 dB

2009 Type 4961

Multi‐field Microphone – Type 4961 was the world’s first ¼" measurement microphone that guaranteed accurate and error‐free measurements in both free and diffuse fields and at any angle, now discontinued

2012 Type 1706

World’s first microphone preamplifier that can handle temperatures up to +125°C/+257°F

2015 Calibration in the Cloud

Access calibration data anytime. Calibration data is stored in the cloud for every transducer serviced at a Brüel & Kjær calibration laboratory. Furthermore, correction files for each individual microphone are accessible via the Web

Development of Brüel & Kjær Microphones

Development of microphone products is performed by a dedicated team of engineers. This is something which is not easy to provide given the relatively small and specialist market for precision microphone products. However, at Brüel & Kjær this investment is seen as essential because it allows the design parameters and physical properties of microphones to be based on a solid foundation of knowledge, skills and experience.

Research and development work also encompasses a number of areas that reflect the different aspects of microphone design and construction, in particular, where highly accurate measurements need to be performed. These include:

acoustic measurements, such as sound pressure and free-field reciprocity calibration
mechanical engineering, for example, when controlling small mechanical tolerances
electrical engineering, such as frequency analysis and capacitance measurements
environmental testing, such as measurement of resistance to humidity and temperature tests.

Development skills and knowledge are also applied in research into the optimum choice of materials and to devise effective forms of testing microphones before they go into full production.

These tests include resistance to shock, vibration, temperature, humidity and in the case of preamplifiers, resistance to electromagnetic fields is also tested. Bump tests in which the microphone is subjected to repeated knocks simulate everyday use, while shock tests reproduce the possible effect of impacts received in transport (typically up to the equivalent of 1000 m/s²).

Fig.1.3 Design of a classic condenser microphone

Finally, microphones are also tested for resistance to corrosion, as proven by the most recent range of condenser microphones which have been found to be very robust in harsh measurement environments.

Fig.1.4 Microphone undergoing shock tests

Production of Microphones at Brüel & Kjær

Microphones are precision instruments and while the design of a conventional measurement microphone may appear to be quite simple, its production must be very precisely controlled to meet specified tolerances. Such tolerances impose great demands on the materials and construction methods used, yet the products created must be extremely reliable and robust.

At Brüel & Kjær the emphasis is therefore on quality rather than mass-production.

Two components which receive a lot of attention during production are the microphone diaphragm and backplate. During production the surfaces of these components are made extremely smooth as a very high electrical field strength must exist across the diaphragm to backplate gap.

Another critical area of production is the distance between the microphone diaphragm and backplate which must be constructed to very small mechanical tolerances. Typically this is set to 20 _µ with a tolerance of 0.5 _µ. The required distance is monitored and then implemented precisely, once the correct tolerances have been adjusted.

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