#ScienceOfSound Sunday: DPA Microphones How can a DPA microphone still have perfect data after being exposed to severe environmental conditions like temperature, humidity, dust and age? Our answer is: choosing the right materials, components and technology from the very beginning. In previous articles, we described how our microphones behaved when recording a space shuttle launch only 175 m from the rocket engine and exposed to flames and chemicals and how the microphones were taken to the Arctic to record ambient sounds at -45°C. We have taken our microphones into the rain to record rain drops and we have demonstrated the omni-directional microphones behaviour after being submerged in water (Vichy, since you ask!). Materials It is essential to the stability of a microphone that the materials in the cartridge work well together, i.e. expand and contract together while exposed to heat or cold. We have chosen a nickel foil for our diaphragm material in our pressure microphones. For the microphone housing we have chosen German silver, an alloy with a very high content of nickel. These two materials are so close to one another, that they respond extremely similarly towards temperature. This way we are able to manufacture microphone cartridges with a stable sensitivity regardless of the temperature. Furthermore nickel is extremely resistant to humidity. When it comes to pressure gradient microphones, it gets more difficult, because this microphone principle needs a soft and very compliant diaphragm. The tight and hard metal foil diaphragms will not perform to their optimum when used in directional microphones. We have chosen a plastic material called PVDF with a vaporised layer of aluminum on. This material is one of the few plastic materials that is non-hydroscopic, i.e. it does not absorb water, and is truly resistant to the most aggressive kinds of humidity. The distance between the diaphragm and the back plate in our pressure gradient microphones is about three times as large as in our pressure microphones. Therefore the pressure gradient microphone concept is less sensitive to temperature expansions and contractions. The Pre-aging process When materials are stressed, bent or worked on in any way, tensions will start to occur. Only time can release the tensions again. Microphone stability is of course dependent on the stability of the materials, so it is essential that the tensions in the housing and the diaphragm are released. Heating up the material will speed up the aging-process and therefore release the tension build-up inside it. Brüel & Kjær has named the process pre-aging and it has been used on measuring microphones for many years. Tests are made during the whole process, where the sensitivity of the microphone capsules is measured while being heated up to 200° C. When the sensitivity is stable, the microphone capsules are put into a humidity room with 90% humidity and 40°C. The studio microphone capsules have now undergone more than 150 handlings and more than 300 for the 100 capsules for the 4040 Hybrid Microphones. Electronic components and circuits But DPA condenser microphones are also electronics and low noise amplifiers. To get a low noise amplifier stable under all kinds of conditions takes hard work and a steady hand during the design process. Mainly it is the feedback loops in the low noise amplifier which are problematic, and, when not dimensioned accurately, the amplifier will start to oscillate. Also the components themselves in the feedback loops need to be of highest quality, if you want your amplifier to be stable under all thermal conditions. Running the amplifiers with low amplification will help this process, but then the output of the cartridge needs to have a decent level. We can obtain a high sensitivity on our capsules by having a high polarisation voltage on the back plate. This allows us to use unity gain amplifiers, running them as impedance converters and ending up with extremely stable microphones. Environmental tests When a new design has seen the light of day at DPA Microphones, it has to undergo numerous environmental tests to get approved as a new product. Some of the key points of DPAs standard environmental tests are listed below:
Posted on: Sun, 27 Apr 2014 13:19:54 +0000
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