Brüel & Kjær Microphones; Five Decades of Excellence
by Niels Bøgholm, Transducer Product Manager
19 Apr 2010
What you see when you buy a microphone is just a small metallic device, but don’t let that fool you, there are a lot of processes, special care and dedicated knowledge behind each single unit.
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Technologie
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Before the Beginning
It may come as a surprise to many to learn that until around half a century ago Brüel & Kjær did not even manufacture microphones. Already in 1943, Per V. Brüel had made a drawing of the first Brüel & Kjær transducer (which was actually an accelerometer, later to be Type 4303). At that time, it was felt that design and production of microphones was too complicated for a small company like Brüel & Kjær. Instead Brüel & Kjær became a reseller of microphones from the Danish company Ortofon.
In 1950, Ortofon introduced a 36mm diameter condenser microphone which was used by Brüel & Kjær with some success, but in the mid fifties it was felt that we should develop our own measuring microphones.
From the very beginning it was clear that a polarized condenser microphone would be the best choice, this microphone principle had been invented by E. C. Wente back in 1916, for its time it was a true breakthrough with respect to frequency range and recording quality.
Practical limitations such as the need for 200 V polarization voltage and a high input impedance amplifier limited a widespread use of condenser microphone for many years.
The Beginning
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The first true Brüel & Kjær developed and manufactured measuring microphones were Type 4131 and Type 4132. They were called 1-inch microphones, despite an actual diameter of only 0.936 inches (23.7 mm). They were developed by a bright young engineer, Mr Gunnar Rasmussen, in close co-operation with the company founders. Mr Viggo Kjær defined one of the firm design goals and that was that the new microphone should comply with American Standards Association: Z 24.4-1949 (Laboratory Standard Primary Microphones).
Types 4131 and 4132 were considerably simpler and much more elegant in mechanical construction, and also had other benefits compared with the state-of-the-art microphones at that time, namely the W. E. 640 AA (from which it inherited the strange 1-inch size).
But soon it was realized that a half-inch microphone would be the better for most purposes and Type 4133 (free-field) and Type 4134 (pressure-field) were introduced. From the very beginning it was understood that accurate calibration methods were an essential factor in order to secure measurement integrity and high product quality. Microphone Calibration Apparatus Type 4141 (reciprocity calibration) and an Electrostatic Actuator UA 0023 were launched together with the two new microphones. Ever since then Brüel & Kjær has been seriously committed to calibration activities and international standardization work. By the way, the UA 0023 has proven to be a true survivor. Today, more than 50 years after its introduction, it is still available in our product portfolio. This can be seen as proof both of its remarkable quality, and that our calibration tools and free-field corrections for the same type of microphone could remain nearly unchanged over more than five decades.
Another interesting and important factor was the attention to detail and design that is, and always has been, a hallmark of Brüel & Kjær. Previously, preamplifiers had been bulky, often with a diameter much larger than the microphone and, thereby, disturbing the sound-field severely. The 1-inch preamplifier for Types 4131 and 4132 was designed not only to fit the cartridge, but also to cause minimum disturbance of the sound-field. This understanding of the importance of attention to detail can still be seen in all Brüel & Kjær offerings, for example instruments like hand-hald analyzers Types 2250 and 2270.
The Beginning of a Legend
These new Brüel & Kjær microphones had an accuracy and stability that was in a class of its own and there is no doubt that this head start was the foundation for our leading position in the area of measuring microphones; a position which has been strengthened both by on-going refinement of the product portfolio and introduction of new unique members of the Brüel & Kjær microphone family.
Outdoor Noise
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From 1963 we offered microphones suited for permanent outdoor measurement, for example, in airport noise monitoring systems. This is an area which is still being developed and consolidated, recently with the acquisition of Australian-based Lochard by Brüel & Kjær and the launch of Noise Sentinel. This product differs from the classic hardware
/software combination in that all you buy are measurement results. We own, run and maintain all the equipment. But even in this case, spearheading the whole solution is a trustworthy, high quality transducer. Exceeding the Limits
The range of (acoustic) sound if often defined as an upper frequency of 20 kHz and a maximum sound pressure level (SPL) of 140 dB. Quite early on in teh study of acoustics it became clear that applications such as scale testing, aerodynamic measurements and measurements of impulsive noise required microphones exceeding the limits of audible sound. The solution came in form of microphones with 1/4-inch and 1/8-inch diameters. In fact, 1/8-inch Type 4138, introduced in 1967, is still available in our present product portfolio.
It's (not only) in the Air
Sound waves are most often thought as propogating in air. But they also propogate in water, and so underwater acoustics is an area of increasing importance, and Brüel & Kjær has been quietly there for a long time. In 1972 we introduced our first hydrophone, Type 8100, leveraging knowledge from the acoustic world with knowledge from the accelerometer world (piezo-elements).
Today, for underwater acoustics, Brüel & Kjær provides a complete program covering everything from hydrophones, preamplifiers, underwater cables, special underwater applications and, of course, hydrophone calibration systems.
Yes!
"Yes" might very well have been the reaction when we were officially requested to develop a replacement for the Western Electric W.E. 640A Laboratory Standard Microphone. The outcome was Type 4160. This microphone has a high precision front cavity particularly suited for reciprocity pressure calibration in couplers. Ten years later, a similar 1/2-inch microphone, Type 4180, was launched, and from that day primary precision microphone calibration is synonymous with Brüel & Kjær.
Less than Nothing
The threshold of hearing is defined as a dynamic pressure of 20µPa or 0dB (SPL). But we wanted to measure sounds below zero-level, so, in 1984, we introduced a system for “very-low-level sound measurements” consisting of Type 4179 and its associated preamplifier, Type 2660. This combination of a 1-inch cartridge optimized for extremely low noise in combination with the special pre-amplifier has an inherent noise specification of –5.5 dBA, a value that remains unbeaten to this very day!
Look Ma, No Hands!
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The need for 200V polarization voltages for measurement micophones is often a cause of frustration in portable equipment, and the presence of such a high voltage can be challenging in humid environments. The solution is to incorporate pre-polarized microphones, also called electret microphones, into the measurement chain. There had been prototype electrat microphones around since 1962, but due to lack of stability, they were not suited for precision microphones.
But not to be outdone, in the mid seventies Brüel & Kjær set up a special task force, headed by the legendary Erling Frederiksen, to conduct numerous experiments with electrets, and this eventually lead to the launch, in 1979, of two pre-polarized microphones, Types 4155 and 4175. These back-electret type microphones had a metal diaphragm, thereby combining the benefits of pre-polarized microphones (no need for external polarization voltages) with the proven stability of a metal diaphragm microphone.
The principles and know-how gained in this project was further utilized and refined in the Falcon series of microphones introduced in 1993. This series is still the back-bone of our microphone portfolio, exemplified by Types 4189, 4190, and 4191. The Falcon series of microphones uses a stainless alloy diaphragm, thus extending the microphone lifetime considerably. Until the introduction of the Falcon series, we had used Nickel as the diaphragm material in most of our microphones (with known issues in highly humid environments).
Smarter Solutions
| Today TEDS transducers are common, 20 years ago they were but a dream. |
The availability of pre-polarized condenser microphones was a breakthrough for portable instruments and opened the way for the use of Constant Current Line Drive (DeltaTron) techniques, where the expensive 7-lead microphone cable is replaced by a cheaper coaxial cable that carries the signal and supply voltage for the preamplifier on the same conductors.
Much later the term “smart” transducer was used to describe TEDS transducers (Transducer Electronic Data Sheet) that followed IEEE 1451.4. Needless to say, the initial work in the TEDS standardization committee was driven by, and strongly supported by, Brüel & Kjær, represented by amongst others Mr Torben R. Licht. Today, TEDS transducers are common, 20 years ago they were but a dream. Sound Intensity
Noise source identification in noisy environments was long considered impossible. The advent of sound intensity techniques solved that challenge once and for all, but in order to achieve reliable results in practice, microphones with extremely precise phase-matching were needed.
Microphones with documented quality and long-term stability were already part of the Brüel & Kjær program, so it was a small step to develop Sound Intensity Probe Type 3519, needed for the world’s first portable Sound Intensity Analyser Type 3360, launched in 1982.
Picture of the sound ?
Why only measure sound, why not take a picture of the sound field instead ? That must have been the idea which lead to the first acoustic holography systems introduced in 1986. These applications require multi-channel systems and hence cost-effective microphones, the so called array microphones. A modern beamforming system can have dozens of such microphones.
Flat as a Pancake
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Flat as a pancake, but much smaller, was the world's first surface microphone, Type 4948, launched in 2003. This microphone was developed by request of Airbus and was used intensively for development of, amongst other aircraft, the Airbus A 380. With a total height of only 2.5 mm (0.1”) it is still a unique product in the world of sound and vibration measurement.
Type 4949 is a variant of the surface microphone that has been optimised for automotive applications. This microphone has, due to its exceptional form factor and quality, found many applications that nobody had dreamed of when it was originally developed. The surface microphone introduced another Brüel & Kjær technology; “all Titanium".
The use of titanium as material for constructing measurment microphones has made it possible to introduce other unique products, for example Type 4955, a true 1/2-inch low-noise microphone with unique specifications. Type 4955 may not quite have the performance of Type 4179 (still available), but it is a lot smarter with its true slim-line design featuring a 1/2-inch cartridge and 1/2-inch pre-amplifier, both made in titanium. Titanium brings additional benefits such as no sensitivity to magnetic fields, very-low diaphragm mass, and extreme corrosion resistance.
Sound field, which sound field?
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Sound field, what sound-field? And which microphone should I use?
These are questions many customers have asked themselves, and often the consequences of choosing a wrong microphone type are not so serious. But not always. It is said that the consequence of choosing a wrong microphone becomes more serious as the frequency spectrum of the measured signal moves towards higher frequencies.
In automotive engineering, the use of low-mass isolation materials and smaller engines can result in the noise spectrum moving towards higher frequencies in the future. Another challenge is when multiple or non-stationary sources are present in an unknown environment.
In order to meet these challenges, and solve the dilemma of “right” or “wrong” microphone, we introduced in 2009 a completely new concept called the multi-field microphone. The world's first multi-field microphone, Type 4961, can, due to its small size and clever design, replace conventional 1/2-inch microphones in most applications with negligible error, whatever the nature of the sound field.
Behind the Scenes
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For decades we have known that the volume behind the diaphragm is crucial to the quality of a microphone, especially for its long-term stability and noise performance.
Dust and other particles between the back-plate and the diaphragm may cause leakage and arcing, and this leads to excess noise and, in the case of an electret microphone, eventually a drop in sensitivity.
For this reason, our microphones are assembled in a clean room environment. Indeed, some of the most critical processes are done at a laminar-flow bench inside the clean room where the cleanliness is according to ISO1; 10 or less 100nm particles per cubic metre.
The number of components in a microphone may be few, typically 10, but the number of production processes such as polishing, cleaning, charging, artificial ageing and testing can be many times higher. And at the end of all this comes, of course, a documented individual calibration performed in accordance with the highest international standards.
Many Brüel & Kjær microphones are also supplied with an individual mini CD with specific calibration and correction data thatcan be used by the PULSE family of front-ends for real-time response equalization using our REq-X principle.
Protect Your work
Without the knowledge of the many processes and many years of research involved in producing professional measuring microphones, seen in isolation the price of a high-quality, professional, measuring microphone may seem high. But judging whether the price is high or fair, also consider the value of your work. How much will a faulty measurement mean to the product or service you are offering to your customer?
The cost of test time in a wind tunnel, the cost of a flight test, or the consequence of delays due to non re-producible measurements, together with the added value of our expert support, makes the premium price of a high-quality Brüel & Kjær measuring microphone one of the best investments you can ever make.
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