11/01/15

MICROPHONE STABILITY

This article explains how DPA microphones still can have perfect data after being exposed to severe environmental conditions like extreme temperature, humidity, dust, and age? The answer is: choosing the right materials, components and technology from the very beginning.
Keywords: Stability, materials, pre-aging, stability of electronic components, environmental tests.
Achieved knowledge: You learn what essential parameters that have an influence on the microphone stability.

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 DPA's standard environmental tests are listed below:
 
Environmental test of microphones
Influence of magnetic field 100 A7m
Temperature -25ºC for three days; 70ºC for four days
Humidity 40ºC and 90% humidity RH for 10 days
Vibration 10 Hz, 58 Hz and 500 Hz (0.3 mm/2g) for x2 hours
Bump 3000 bumps with 25 g in two directions









Among the more bizarre tests was two d:dicate™ 4006 Omnidirectional Microphones put in the toolbox on a fishing boat and exposed to shocks, vibrations, oil and salt water. After one year in the toolbox, they still kept their specifications. Furthermore, the studio microphones were long-term tested in salt mist, to test their resistance towards corrosion. The influence of dust on the diaphragms was also tested by sprinkling talcum powder directly on the diaphragms of the pressure microphones. The difference in frequency response could not be measured. This effect is due to the tight diaphragms of these microphones, where additional weight has less influence on the performance. The soft diaphragms of the pressure gradient microphones are more sensitive to additional weight (dust), so a net is placed directly behind the protection grid and the back port to keep dust away from the diaphragm.

Now, by telling you all this, we are not encouraging you to toss and throw your DPA microphones around and treat them like something you have taken out of your toolbox! We consider microphones as delicate instruments, which you should handle with care. Furthermore it is extremely important to remember not to touch the diaphragms of the d:dicate™ 4006 Omnidirectional Microphones with anything. But, if you take care of your DPA microphones, they will become your most faithful recording tools and probably outlive any of your other recording equipment.

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