Noise monitoring entails long-term sound monitoring without the need for human interaction. There are two main types of sound monitoring: workplace monitoring and environmental monitoring, each depending on the location of the sound source. Environmental noise monitoring is one of the most common kinds of environmental monitoring and is most often carried out using a monitoring system.
In 2022, the Physikalisch-Technische Bundesanstalt (PTB) has granted the world’s first Class 1 Sound Level Meter with MEMS microphone approval to SV 307A Noise Monitor. It is an important turning point in the history of MEMS microphones in environmental sound monitoring.
In 2014, after SV 104 noise dosimeter had been type-approved by Austrian BEV (Bundesamt für Eich- und Vermessungswesen) as the first Class 2 Sound Level Meter with MEMS microphone, the workplace noise monitors market was revolutionized.
The monitoring system, as described in ISO 1996-2, comprises a Noise Monitor and data collection center, as well as all hardware and software utilized for environmental noise monitoring.
The term Noise Monitor also called a “Noise Monitoring Terminal” (NMT) refers to instrumentation used for automated continuous sound 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).
Class 1 noise monitors are the same as class 1 sound level meters when it comes to meeting IEC 61672 performance criteria. Because there is no specific standardization for Noise Monitors, the two standards used to build noise monitors are IEC 61672-1 about the instrument’s ability to measure sound levels and ISO 1996-2 for monitoring applications.
The following are the essential noise monitor performance criteria defined by IEC 61672-1: linear operating range, directional response, frequency response, 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.
The objective of a noise monitor is to provide data regarding the level of noise in a location so that it may be compared to the established noise limits.
A noise monitor can be used to evaluate the quality of various types of noise. Noise monitors comply with ISO 1996-2 standards and are intended to measure the following main environmental noise sources: road traffic, rail traffic, air traffic, and industrial plants.
Environmental noise monitoring equipment shall meet requirements 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).
Noise dosimeters, sound level meters, or noise monitors are used for noise measurements in different areas. The noise levels sampling (short) or monitoring (long) happens depending on the length of time measurements are taken. To monitor noise levels in the environment, the noise monitor should be installed at the measurement site following ISO procedures. The importance of the selection of the measurement site is emphasized in ISO 1996-2: 2017 which states that sites for measuring microphones shall be chosen to minimize the effect of residual sound from relevant sound sources.
Following the ISO 1996-2, environmental monitoring is done with Noise Monitors placed at a height of 4m in a way to minimize the influence of residual sound from non-relevant sound sources.
Noise monitoring is typically conducted outdoors for a long period. The unattended monitoring means that the noise monitor is continuously recording noise without requiring human attention.
Online noise monitoring is becoming increasingly important as we strive to create more sustainable cities and protect the health and well-being of people living in urban areas. The noise data is sent to a data collection center with the use of remote communication. Websites like SvanNET provide access to noise monitoring online.
Following the ISO 9612, the microphone is attached to the worker’s shoulders with a small sound level meter called a noise dosimeter, which is placed around 10 cm from the ear. This kind of noise exposure monitoring is known as noise dosimetry.
Noise monitoring should occur whenever there is a risk of exceeding these noise levels limits. As a result of studies on noise and health connections, as well as policy-making procedures in various countries, governments have established national limit values and regulations for environmental noise.
Depending on local regulations, the dB limits permitted for environmental noise may differ. Typically, for daytime, the permitted dB limit is 65 dBA, and for nighttime noise levels, it is 55 dBA.
Noise quality monitoring is the process of continuously monitoring the noise levels in an environment to ensure that they remain within acceptable limits. Depending on the application, noise quality monitoring may be conducted in a variety of ways. Sound level meters and noise dosimeters are often used for noise quality monitoring in workplaces. The noise monitors are used for environmental measurements.
The appearance of new-generation MEMS microphones in 2019 allowed them to be utilized in environmental monitoring. Since 2013, MEMS microphones have been used in noise dosimetry. Until recently, noise dosimeters with MEMS microphones had a linear measurement range of 55 dBA RMS ÷ 140.1 dBA Peak, which was insufficient for environmental monitoring.
MEMS microphones reduce the cost of the monitoring system without sacrificing performance. The use of MEMS microphones has a very similar effect to that of classic condenser microphones in terms of monitoring systems. As a result, the use of MEMS microphones ensures that parameters such as linear operating range, frequency response, and temperature operating range are conformed to IEC 61672-1.
The appearance of MEMS microphones has shattered the price barrier, on average halving the price of noise monitoring terminals. In addition to NMT cost savings, repair service pricing dropped as well. MEMS microphones are immune to radio frequency interference (RFI) and electromagnetic interference (EMI), as well as environmental resilience. Long-term acoustic monitoring applications in the harsh sub-zero winters and hot, humid summers necessitate this resiliency to changing environmental conditions, which is especially crucial.
MEMS (Micro Electrical Mechanical System) microphones consist of three main parts: MEMS, ASIC, and package. The MEMS microphone 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 MEMS sensor is a silicon capacitor made of two electrically isolated surfaces. One surface, called the backplate, is fixed and covered by an electrode. The other surface, called the diaphragm, is movable and has many holes, that is, acoustic holes.
The other is movable and is called the membrane or diaphragm. 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).
The ASIC delivers an analog or digital output, depending on the microphone type. For analog MEMS microphones, the electrical output signal from the ASIC is sent to an external pre-amplifier, which is also in charge of converting the output to a signal that can be used as input of an acoustic chain.
For digital MEMS microphones, the ASIC output is sent to an internal analog-to-digital converter (ADC) to provide a digital signal, either as a pulse density modulated PDM format (1-bit high sample rate data stream) or I2S format (same as PDM microphone but including a decimation filter and a serial port to produce a standard audio sample rate).
The microphone class and sound level meter class are often confused with each other. Although the microphone is a removable part (to allow direct insertion of electric test signals) the standard for IEC 61672-1 standard does not specify requirements for a microphone separately. The IEC 61672-1 class performance requirements are applied to a sound level meter with a microphone as a whole.
Henceforth, a noise monitor will be considered to satisfy IEC 61672-1 as the entire device, and as a sound level meter with a microphone.
The performance of noise monitors based on MEMS and classic condenser microphones is quite comparable.
As a result, the use of MEMS microphones in noise monitors ensures that parameters such as linear operating range, frequency response, directional response, and temperature operating range are conformed to IEC 61672-1.
The future of sound monitoring is rapidly changing, as new technologies are emerging that allow for more accurate and efficient ways to measure and analyze noise levels. For example, recent developments in sensor technology have enabled the creation of sound monitors that can not only detect environmental noise, but also track data on a real-time basis and provide detailed analysis of noise trends over time. Additionally, machine learning algorithms are being used to develop more sophisticated noise prediction models that can help identify potential sources of sound pollution and recommend mitigation strategies. Ultimately, these and other advances in sound monitoring technology will help create a more sustainable and livable world for all.
Because of the design low cost and very good performance NMT systems based on MEMS microphones are the only right choice for multipoint noise monitoring in the future.