I was employed by Bruel & Kjser in 1950. At that time there were only 30 employees and everything was produced in house - metal working, sheet-metal forming, electronic assembly and finishing. The product range was not focused on acoustical products, but included a wide variety of voltmeters, precision attenuators and radio transmission equipment. One of the products was a measurement microphone based on a Rochelle salt sensing element. The element was coated with wax but was still affected by humidity. The effects of other factors like temperature and barometric pressure were to a large degree unknown.

A 36-mm condenser microphone for sound pressure measurements was introduced in 1950. The microphone was designed and developed by Dr. Schlegel from the Danish company Ortofon with a brass housing and aluminium diaphragm. The microphone was manufactured by Ortofon. They would produce batches often units and the production was shared between B&K and another Danish company Radiometer. The new microphone was certainly an improvement over the Rochelle salt microphone as it could be calibrated with an electrostatic actuator. This method was also developed by Dr. Schlegel from Ortofon. The most common measurement microphones of the day are shown in Figure 1.

In 19551 was sent to USA to implement service and sales at Brush Electronics in Cleveland, Ohio. This was my first introduction to real measurement microphones in the form of the Western Electric 640AA and the ANSI 224.4-1949 standard (see Figure 1). I knew of the W.E. 640AA from Beranek's Acoustic Measurements and from a doctoral thesis by A. Kjerbye Nielsen "Microphone Measurements." In 1947 he invented a practical reciprocity calibration technique. Later this method was further developed by Dr. P. Rubak for more precise calibration. This was the basis of the IEC 327 standard.

I had the opportunity to travel extensively in the US, meeting many acousticians, and to be exposed to many applications from rocket testing to listening for beetles moving inside oranges. On my return to Denmark, B&K was having problems with the quality of the 36-mm microphones. The aluminium diaphragms were easily corroded and tended to develop short-circuiting whiskers between the diaphragm and backplate. I was asked by Viggo Kjær to work on a 1-inch microphone compatible with the ASA 224.4 standard. I developed a new design, where the diaphragm could be screwed onto the front of the microphone body and allowed diaphragm tension to be adjusted from the front. The W.E. 640AA and the ECL MR-103 were adjusted by moving the internal assembly.

We produced nickel diaphragms by an electroplating process using uncut lacquer disks for phonograph recording as base material. The diaphragms of the W.E. 640AA, MR-103 and MKOOOl were clamped, which caused problems with stability. I tried different methods. One was soldering which was unstable, because it peeled off the threaded diaphragm ring. Another was vacuum deposition of a thin film of metal, which worked, but the production equipment turned out to be too expensive for the company. I ended up using a crimping technique which has worked well for many years. With the addition of today's laser welding techniques, it is possible to choose the most relevant process for a specific microphone type, considering stability and long term corrosion effects.

Free-field calibration required an anechoic room of reasonable size to calibrate a 1-inch microphone at frequencies above 5 kHz. We did not have a suitable room available at B&K. After a lot of testing in different rooms I was finally allowed to get a room in the basement of the factory with outside dimensions of 2 × 2 m and anechoic space of 1.4 × 1.4 m. This was too small for a free-field reciprocity calibration of 1-inch microphones. It was therefore necessary to develop a 1/2-inch microphone to confirm the measurements on the 1-inch design and go even further with a 1/4-inch microphone in order to be able to scale down. The 1/4-inch microphone enabled me to test a new sound level meter configuration on a 1/4-inch scale model made of wood.

The small scale models of the microphones were not well received by the management. I got a salary cut as a reward and was not allowed to take out any patents. My early experiments were redeemed when the 1/2-inch design ended up being the most used and copied microphone for general acoustic measurements.

Condenser microphones are high impedance devices and require an impedance converter to drive the connecting cable and instrumentation. Semi-conductors of the 1950s were not suitable for preamplifiers. The W.E. 640AA used a fairly large vacuum tube preamplifier. It looked like a Coca-Cola bottle, where the microphone was the cap. To gain full advantage of a small microphone, a preamplifier of the same diameter as the microphone is desired for free-field measurements. I designed a 1-inch and 1/2-inch microphone preamplifier based on sub-miniature tubes as shown in Figure 2.

The original B&K 4111 microphone was 36-mm in diameter and used an EF40 tube that caused self-noise problems. The new preamplifiers were a considerable improvement (see Figure 3). Modern preamplifiers are free of microphonics due to the use of ceramic substrates and low noise FET input stages. They have a 20-40 ÏÙ input impedance enabling linear response to below 2 Hz. The old MKOOOl microphones were calibrated to within ±1 dB using an electrostatic actuator. Careful calibrations improved the accuracy until we could show <0.1 dB variation between standards laboratories.

During microphone development work I needed a precise and fast method for determining small changes in microphone sensitivity in order to determine expected long term stability and thermal effects as well as effects of ageing. Reciprocity calibration is very time consuming and normally involves three microphones. So I had to use a more direct absolute calibration method. Traditional methods like comparison of a test microphone to a calibrated standard microphone could not be used because that is what I was trying to develop. Pistonphones available at that time were not very accurate. Optical measurements of piston displacement and motion between the piston actuator and optical read-out could not be implemented. A free floating, dual-piston mechanism actuated by a cam disc overcame these problems (see Figure 4a). Developed sound pressure could be based on a precise cam disc, precise piston diameters and microphone coupler volume. Barometer accuracy for atmospheric pressure correction has been the weak point for many years. Precision laboratory barometers using mercury are easily obtained for the laboratory but not very portable. The development of precision grade barometers has enabled us to develop the modern precision pistonphone. I developed the pistonphone calibrator shown in Figure 4b. The long-term uncertainty is less than 0.1 dB.

The protective grids of the new microphones were designed to allow them to remain on microphones in actual use. This was not the case for the W.E. 640AA, which was typically used without its protective grid. The new grid for the 1/2-inch microphone was designed to extend the high frequency range to 20 kHz.

Dr. Per V. Briiel actively supported my free-field calibration procedures and we published an article on the frequency response of microphones in the B&K Technical Review, 1959. With high quality microphones available it was possible to continue developing sound level meters, artificial ears and artificial mouths as well as numerous applications. Some of these were extremely interesting such as the surface mounted microphones for the Concorde, outdoor microphones with built-in actuator calibration, telephone test equipment, etc. During this time I also designed accelerometers, force transducers and whole-body hand-arm vibration transducers and I invented the Delta shear configuration for accelerometers. Carl Wahrman-Jensen, my colleague in vibration transducer development, made many valuable contributions to calibration techniques.

I left the transducer development department in 1973 and took over a separate department for the development of new measurement techniques and instrumentation. This lead to the production of phase-matched microphones, intensity probes with well defined spacers, hydrophones, railway monitoring systems, sound intensity applications, and sound power measurements. In cooperation with Ole Roth, we developed the first true real-time intensity analysis system including gating techniques.

I was laid off from Briiel & Kjær in 1993 after the take over by A.G.I.V. and started the company G.R.A.S. Sound & Vibration. The company produces a complete line of measurement microphones and accessories.

FAQs. Today's measurement microphones are stable and rugged, but still they are and should be treated as high precision delicate instruments. Just to give an example of the dimensions involved - for a standard 1/2-inch microphone measuring a sound pressure level of 40 dB (corresponding to the level in a quiet living room), the diaphragm will move approximately 10~n m. In order to appreciate the magnitude of this tiny movement, imagine that the microphone diameter was the same as the diameter of the earth (12,700 km). The diaphragm would move only 10 mm.

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