Development Of Low-Cost Noise Monitoring Terminals (NMT) Based On MEMS Microphones

The article shows and discusses an example of NMTs with MEMS microphone meeting class 1 by IEC 61672-1.

The rapid development of MEMS microphones (Micro-Electro-Mechanical Systems) in the last decade years made it possible to use them in noise measurement instrumentation meeting the IEC 61672-1 specification. Fifteen years ago, the available MEMS microphones offered only a 60 dB dynamic range, whereas modern MEMS microphones offer 100 dB dynamics. Such a wide dynamic range of MEMS microphones, along with their improved repeatability and long-term stability, enabled the development of low-cost noise monitoring terminals for noise monitoring. In particular one of such NMTs (Svantek SV 307A) offers the linear measurement range of 30 dBA Leq÷ 128 dBA Peak which proves to be optimal for urban noise monitoring applications.

A low MEMS microphones cost enables the development of innovative designs for low-cost noise monitoring terminals with features such as a multi-microphones arrangement for a dynamic system check.

Noise Monitoring Terminals With Mems Microphones

Because there is no specific standardization for Noise Monitoring Terminals, the two standards used to build noise monitoring terminals are IEC 61672-1 about the instrument’s ability to measure sound levels and ISO 1996-2 for noise monitoring application.

The following are the essential NMT characteristics defined by IEC 61672-1 that were mentioned in this article: linear operating range, frequency response, directional characteristics, and temperature operating range. More requirements are associated with the measurement application, including long-term stability, environmental robustness, powering, and communication. ISO 1996-2 has additional criteria, such as GPS, frequency analysis, and monitoring of weather conditions (wind, rain, temperature, humidity), which are not addressed in this article.

What is an NMT?

The term “Noise Monitoring Terminal” (NMT) refers to instrumentation used for automated continuous noise monitoring which monitors the A-weighted sound pressure levels, their spectra, and all relevant meteorological quantities such as wind speed, wind direction, rain, humidity, atmospheric stability (ref. ISO 1996-2:2017).

Development of low-cost Noise Monitoring Terminals

Noise monitoring terminals that meet ISO and IEC specifications are rather expensive devices. The condenser microphones, which must meet additional requirements for calibration checks with the electrostatic actuator, are one of the reasons for their high cost. The concern that must be also addressed when selecting a microphone is how well it will endure over time. Then there’s long-term stability and environmental durability, which are important factors in microphone selection at the top of pricing ranges.

The appearance of MEMS microphones shattered the price barrier in that, on average, a MEMS microphone costs less than 5 Euros. In addition to NMT cost savings, repair service pricing dropped as well.

The use of MEMS microphones has expanded due to their versatile design, greater immunity to radio frequency interference (RFI) and electromagnetic interference (EMI), low cost, and environmental resiliency. This resiliency to varying environmental conditions is particularly important for long-term acoustic monitoring applications in the harsh sub-zero winters and hot and humid summers.

What Is A MEMS Microphone?

MEMS (Micro Electrical Mechanical System) microphones consist of three main parts: SENSOR (microphone), ASIC, and package. The SENSOR and the ASIC are packaged together in a cavity that is surrounded by a substrate and a lid.

A sound inlet (acoustic port) is present either in the substrate or in the lid, and, most of the time, positioned directly in the MEMS cavity.

The SENSOR shown in Figure 1 is a miniaturized polarized condenser microphone with a typical polarity of 50V. One surface, called the backplate, is fixed and covered by an electrode. The other surface, being the diaphragm, is movable and has many holes, that is, acoustic holes.

A sound wave passing through the acoustic holes of the backplate will set the diaphragm in motion, creating a change of capacitance between the two corresponding surfaces. This is converted into an electrical signal by the Application-Specific Integrated Circuit (ASIC).

There are two types of MEMS microphones: analog and digital. In the analog type, an ASIC contains an impedance converter (preamplifier) and a charge pump for generating a polarization voltage. The digital microphone’s ASIC additionally includes a sigma-delta A/D converter with PDM output. The PDM format is a standard input for most Codecs available on the market (a pulse density modulated PDM format is an a1-bit high sample rate data stream).

In this article, we will discuss the NMT using a MEMS microphone with the analog output.

Electrical testing of NMT with MEMS according to IEC 61672-1

The IEC 61672-1 requires providing an electrical equivalent of the microphone for electrical testing. In the case of MEMS microphones, it is a challenging, but possible task.

Figure 1. Example of MEMS microphone construction.

Figure 2. Transducer and ASIC of an Analog MEMS Microphone

Figure 3. Typical analog MEMS microphone block diagram

Study: Comparison Of Key Specifications Of Two Nmts: Mems-Based And Condenser Microphone-Based

In this section, we compare the features of a MEMS-based NTM to those of an NMT based on a condenser microphone. Both NMTs were authorized by the Physikalisch-Technische Bundesanstalt (PTB) for conformance with IEC 61672-1 in 2022. The performance of two NMTs was compared:

Svantek SV 200A based on a MK 255S pre-polarized free-field ½” condenser microphone with a nominal sensitivity of 50 mV/Pa
Svantek SV 307A based on ST 30A MEMS microphone (½” housing) with a nominal sensitivity of 36 mV/Pa

As shown in Figure 4, the SV 200A NMT uses four MEMS microphones mounted on a side of the housing to detect noise directivity.

Figure 4. Svantek SV 307A (left) and Svantek SV 200A (right)

Linear operating range following IEC 61672-1

The typical outdoor noise measurement is conducted within the range between 30 dBA and 125 dBA, which requires the dynamic range of 100 dB (defined here as the difference between the A-weighted noise floor and the maximum SPL within tolerance).

The first type of MEMS microphones (2008) had a limited dynamic range, which was approximately 60 dB. The second generation of MEMS microphones (2018) has offered a 100 dB dynamic range, allowing them to be used in noise measurements in the environment.

On any level range and at the stated frequency, the deviations of sound levels measured by an NMT need to be within the acceptance of IEC 61672-1. The comparison shows that:

the linear operating range of NMT SV 200A is: 25 dBA ÷ 133 dBA Peak
the linear operating range of NMT SV 307A is 30 dBA Leq ÷128 dBA Peak.

The specifications for environmental noise measurements are met in both cases by the linear operating range.

Frequency response following IEC 61672-1

Noise Monitoring Terminals conforming to IEC 61672-1 should have a specified frequency response for the sound incident on the microphone from one principal direction in an acoustic free field or random directions.

Both terminals comply with the frequency response criteria thanks to compensation filters, which improve frequency characteristics and meet IEC 61672-1 standard, as shown by the below figures.

Figure 5. Frequency response of SV 200A NMT with a condenser microphone.

Figure 6. Frequency response of SV 307A NMT with MEMS microphone.

Directional Response following IEC 61672-1

For any frequency in the range of NMT, the directional-response design goal is an equal response to sounds from all directions of sound incidence. The IEC 61672-1 provides acceptance limits for deviations from the design goals. For class 1 sound level meters, the frequency of the sound signal is specified as up to 12.5 kHz and for class 2 sound level meters up to 8 kHz. The figures below compare the directional response of NMT with a condenser microphone and MEMS microphone, both meeting the IEC 61672-1 specification.

Figure 7. Directional response of SV 307A (left side) and SV 200A (right)

Temperature operating range

IEC 61672-1 defines two tolerance levels for outdoor noise: Class 1 and Class 2. These ranges govern the temperature range of -10°C to +50°C, as well as 0°C to +40°C. They are significant factors when it comes to ambient noise measurements and environmental noise monitoring, respectively.

In real measurements at least the temperature range for NMT should be no less than
(-10°C) to +50°C which is due to the wide fluctuations in temperatures when measuring outside. In practice, temperature operating range from -20°C to +60°C should be provided.

The temperature operating range of:

SV 307A based on MEMS: operating range is specified from (−20 °C) to +60 °C
SV 200A based on condenser microphone: is specified from (−30 °C) to 60 °C

Long-term stability

Long-term stability is a crucial consideration when considering NMT since noise monitoring is an unattended type of measurement. In the case of long-term noise monitoring, the ISO 1996-2 standard refers to the ISO 20906/Amd1:2013 acoustic check for NMT sensitivity verification. The ISO requires the installation of an automated system check that will notify whether the system is functioning properly or is potentially faulty.

The SV 200A uses a classic system check based on an electrostatic actuator. However, the use of an electrostatic actuator in outdoor measurements is troublesome and costly, mainly due to the required high voltage and environmental conditions.

MEMS microphones cannot be tested with electrostatic actuators because of housing, but the small size makes it feasible to design a multi-microphone array inside of an ½” microphone housing.

Using such an array one can make a dynamic system check continuously based on real acoustic signal measured. The concept of the dynamic system check uses a continuous comparison of microphone sensitivity. In addition to the dynamic system check the acoustic signal source can be used for offline microphone testing (e.g. with a 100 dB level).

Powering and communication

When it comes to noise monitoring, one of the most important factors is power and communication. The data must be transmitted to remote servers in the unattended type of measurement. The most popular form of communication is GSM. Both models SV 307A and SV 200A employ 4G modems.

In many cases, the street lights are used as a power source for NMT. In such a situation, there is the possibility of no electricity during the day hours. As a result, NMT should have at least 24 hours of battery life. With the use of MEMS that have extremely low power consumption, it is easier and cheaper to fulfill such requirements.

Microphone Shock Resistance

The damages to classic condenser microphones due to mechanical shock are one of the highest cost sources in noise measurements. Because of their construction, MEMS microphones are extremely robust and can withstand shocks up to 10000 g (100 000 m/s2).

System Integration Capability

Noise monitoring based on MEMS microphones is very easy to integrate with other environmental monitoring systems at a low cost.

Conclusions

As demonstrated in the article, the performance of NMT microphones based on MEMS and classic condenser microphones is quite comparable. As a result, the use of MEMS microphones in NMT ensures that parameters such as linear operating range, frequency response, directional response, and temperature operating range are conformed to IEC 61672-1. Other factors, environmental robustness, power consumption, and data exchange may be considered while drafting Noise Monitoring Terminal requirements.
Because of the design low cost and very good performance NMT systems based on MEMS microphones are the right choice for multipoint noise monitoring in Smart Cities.

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