Designing and using a MEMS-based SLM

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Using MEMS microphones in sound level meters
Bruno Paillard,Ph.D, chief designer, Convergence Instruments
A measurement-class microphone such as a 0.5 in ICP microphone is typically a sizable part of the cost of a sound-level meter (SLM). That high instrument cost in turn constrains applications such as construction site monitoring or environmental studies, where many instruments are desired. Micro-electro-mechanical system (MEMS) microphones offer a high-performance alternative to traditional ICP microphones, at a cost that is at least two orders of magnitude lower. MEMS microphones have great qualities, mostly driven by the demands of the consumer-electronics market. However, they also have idiosyncrasies that need to be accounted for to design a high-quality SLM.

MEMS are electro-mechanical systems that are designed and manufactured using the same materials (usually silicon) and etching techniques used to make micro-circuits (see Figure 1). These techniques allow the creation of micron-scale structures with great precision and repeatability. MEMS microphones are extremely small, yet sensitive (the noise floor is usually better than 30 dBA). Many of them integrate amplification and digital sampling electronics at the chip level, thereby providing a digital signal directly and lowering the cost of the rest of the instrument.

The integration of analog-to-digital circuitry directly at the chip level also eliminates electro-magnetic noise that can be coupled to the analog input line in a traditional design.

MEMS microphones are manufactured using tightly controlled micro-etching processes, and, therefore, their characteristics are extremely consistent from one individual to the other. They are very linear (total harmonic distortion is 0.1% or better at 1 kHz/94 dB SPL) and they also have a very wide dynamic range (typically better than 30 dBA to 120 dBA). Additionally, MEMS microphones exhibit very little sensitivity to temperature changes. Likewise, their membrane is so small and light that they are more than 10 times less sensitive to vibrations than an electrostatic microphone. MEMS microphones are mass-produced for the consumer electronics market and are therefore very inexpensive. Their sensitivity is very stable over time. Usually requiring no recalibration to stay within the limits of the type I specification.

These advantages make MEMS microphones ideal candidates to be used in the design of SLMs. However, they also have some shortcomings that need to be corrected for, or at least understood to design an efficient SLM. 

Because they provide a digital signal at the chip-level, it is not possible to remove the pressure-sensitive capsule from the rest of the circuitry and test the analog chain alone. All relevant SLM standards were written in the 1970s and assume that the SLM design consists of a separate microphone capsule, driving an analog processing chain, or an analog-to-digital converter (ADC) followed by a digital processing chain. They mandate the testing of the SLM using electrical signals in place of the microphone. This is not possible when the analog-to-digital conversion is done in the microphone itself, at the chip level. This means that even though an SLM may have the performance required to comply with a certain standard, it cannot be tested using the methodology prescribed by that standard.

Because of the extremely small size of the silicon structures, even very small dust particles can easily damage them. Very high static and dynamic pressures (typically above 160 dB-SPL) can also cause damage to those small silicon structures.

MEMS microphones generally have a sharp resonance in the 10 kHz to 20 kHz range. That resonance needs to be corrected for so that the frequency response of the SLM falls within the limit lines of the appropriate standards....more....

 

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