The Evolution behind a Revolutionary Surface Microphone
01 Sep 2004
Ever since one of the company founders, Per V. Brüel, designed his first measuring microphone way back in 1945, Brüel & Kjær has pioneered innovative solutions for microphones and preamplifiers. The revolutionary Surface Microphone was designed and manufactured using this proud heritage of the well-proven Brüel & Kjær measurement microphones. But this is combined with new design principles (patent pending) taking microphone development into a whole new generation.
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Ground Breaking Technology
After years of close cooperation with one of the world’s leading aircraft manufacturers, Airbus (based in Toulouse, France), Brüel & Kjær has proudly launched the groundbreaking Surface Microphone Type4948. A prepolarized pressure microphone, it is not only flat and small (height: 2.5mmwith built in DeltaTron® preamplifier capable of driving long cables and easily integrated in multichannel test systems), but also robust, accurate, stable and supports TEDS.
The Challenge
It all started three years ago when Airbus contacted Brüel & Kjær with a request for a new aerodynamic microphone for use during flight tests. It was not an option to use traditional flush-mounted microphones, which required drilled holes, so from the very beginning it was clear that we had to produce something very different that would require technology and solutions unlike those we had used in the past.
Part of the challenge was the level of miniaturization required. We started the project knowing that we were about to embark on developing a completely new way of making microphones and integrating electronics and mechanics known only from MEMS (Micro-Electro-Mechanical Systems) and similar advanced technologies.
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But these technologies on their own were not enough to produce the Surface Microphone. During development we encountered several problems caused by miniaturisation, problems that don’t normally occur for traditionally sized microphones. Because of the physical limitations, the only way of finding the best solution was through the use of numerical modelling techniques such as BEM (Boundary Element Modelling) and FEM(Finite Element Modelling). Our knowledge of these techniques was crucial to the success of the project.
Guiding us towards a better solution, Airbus involved and engaged themselves through out the development process – from concept and early specifications to rigorous performance testing and approval. In total, three generations of prototypes were developed and tested at Airbus before completion. Verification was also far more exacting than for normal measuring microphones, and included demanding flight tests performed at Airbus. The verification test results proved that the surface microphone could withstand all the requirements demanded of sensors mounted outside an aircraft during flight, including de-icing detergent, turbulence, hailstorms, rain, high g levels, a temperature range from –50°C to100°C, cruising altitudes of 30000 feet, and mach 0.8.
The frequency range is from 5 Hz to 20 kHz, the operating range covers a span from 55 to160 dB SPL, and the long-term stability is excellent at 1 dB per 1000 years.
The microphone is available in 3 different versions:
- Type 4948 has 1.2 m coaxial cable with10 – 32 UNF coaxial connector
- Type 4948A features both TEDS and CIC(Charge Injection Calibration, a Brüel & Kjær patented method for in situ verification of microphone and cabling). 4948A features a4-pole micro-connector and an 8 cm cable
- Type 4948 B has two 1.2 m coaxial cables, one of which is used for CIC
Applications
With the new surface microphone acoustic measurements are possible in areas that have previously been limited, such as:
- Measurement of sound pressure directly on the surface of an aircraft, car, lorry, wing of a wind turbine, etc.
- Acoustical fatigue testing – in the aerospace industry, acoustical fatigue testing is one possible application. Until now, acoustical fatigue testing has often been performed in large ground-based test cells. Now data can be gathered during real flight conditions, and the risk of acoustic fatigue can be realistically judged
- Transfer path analysis
- Measurement of sound pressure (noise) in limited spaces, and where flush mounting of conventional microphone/preamplifier combinations would be impossible or cause undesired side effects
Brüel & Kjær would like to take this opportunity to thank all the people at Airbus who have been involved with this project for the excellent and fruitful cooperation.
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