Living next to the main railway going north from Oslo, Norway, I have some firsthand experience when it comes to environmental noise. There are more than 60,000 train passes outside my window in a year. To many people’s surprise I have little problem with it. The main reasons are that I live in a modern building with triple glass windows and the majority of the trains passing are modern low noise train passing slowly. However, some nights there will be a fast passing diesel locomotive or a squeaky goods train that can wake up the whole neighbourhood.

Noise pollution tends to increase as population grows and rapid urbanisation takes place. Enlargement of traffic systems, international airports and rail networks brings noise pollution to more and more people. It is estimated that about half of the European Union citizens are living in zones which do not ensure acoustical comfort to its residents. As surveys have shown that noise pollution is regarded as one of the most disturbing forms of pollution, more research has been conducted to identify the negative effects of noise. The results indicate that insomnia, high blood pressure, heart diseases and hearing impairment can all be caused by noise.

In this article I will present what is understood by environmental noise, the main regulations dealing with it, and alternative approaches to quantify noise pollution through computer modelling and in-situ measurements.

What is environmental noise?

Noise can be defined as any unwanted sound. Noise from road, rail and air traffic, industries, construction and public work is considered as the main sources of environmental noise.

All noise except noise at the industrial workplace is considered as environmental noise.

Calculation of colour coded noise maps

When governments and local authorities need to assess environmental noise over a larger area such as a whole city, it is not possible to do physical measurements and calculation of noise maps is the preferred method. Computerised calculations based on digital maps with simulation of noise sources and noise barriers provides the data to create a map which is coloured according to the noise levels in the different areas.

Noise maps can not predict what the noise level will be at any particular point at any instant in time, but where the noise sources are well-defined, such as road or rail traffic, or aircraft, it is possible to say with some confidence what the long-term average noise level will be.

The European Union has a Directive on Environmental noise (Directive 2002/49) which obliges the member states to draw up “strategic noise maps” for major roads, railways, airports and agglomerations, using harmonised noise indicators Lden (day-evening-night equivalent level) and Lnight (night equivalent level). They are also required to inform and consult the public about noise exposure and its effects, and they must address local noise issues by drawing up action plans to reduce noise. All member states should have submitted noise maps for all agglomerations with more than 250 000 inhabitants by 2007. A summary report from the European Commission based on those noise maps shall be prepared for July 2009.

Noise mapping has an advantage when it comes to predicting future noise levels as planned noise sources and noise barriers can be added in the calculations. Hypothetical situations can therefore be assessed. However, reliable calculations require that the input data such as topography, characterisation of buildings, source description are of high accuracy. This requires much work and advanced acoustical skills. Further on the calculation should be calibrated. To this end representative noise measurements should be carried out and compared to the calculated results. Noise measurements add to the credibility and reliability of calculated noise maps.

When is long term measurements required?

As soon as you are experiencing regular complaints on a noise source, a calculated noise map of predicted noise is of little help. Single events are not picked up by a noise map, and it is impossible to predict noise from installations or activities with irregular activity patterns. In this case only noise measurements can document the noise, and regulations defining absolute maximum levels generally require continuous noise monitoring to be in place. In fact, long term measurements of noise level can be a very positive community relations step for local authorities and the noise polluter. Often the very act of continually monitoring the noise levels leads to a decrease in the number of complaints, especially when the data is available to community members for review. Noise monitoring can also lead to more precautious acts from the noise polluter. Furthermore, permanent monitoring can pick up long term trends in the noise situation and thereby give important input to noise abatement policy making.

State of the art noise monitoring systems

Modern noise monitoring systems offer a wide range of advanced features and come with software that can analyze and structure long term measurements data efficiently. A complete noise monitoring system consists of one or more noise monitoring terminals, a noise server and work stations for presentation, post processing and reporting.

Noise monitoring terminal

The noise monitoring terminal is the piece of equipment that acquires sound level and other data and transmits it to the server. It comprises the sound level measurement apparatus (microphone, power supply, sound level meter) along with an enclosure, a communication device and possibly a weather station and camera.

The bases for a noise monitoring station are the microphone and the sound level measurement device. In order to be certain that your measurements are of the highest quality, the sound level meter should be approved to satisfy the standard IEC 61672:2003 “Electroacoustics – sound level meters” by a national institution such as PTB (the German national metrology institute). An approved meter is tested to give correct results under varying physical conditions. The standard defines the precision of two categories of meters: Class 1 and Class 2. A Class 1 meter is required for noise measurements in most countries.

Microphones for permanent outdoor installations must be designed to withstand variations in temperature and humidity, and also resist heavy rain and corrosive climate. They should have a self-check functionality using an electrostatic actuator or a charge injection method. This way the microphones can be automatically verified so that the interval for manual inspection of the terminals can be prolonged. Still, the microphones are subject to a yearly calibration in an accredited laboratory to ensure lasting accuracy. Regular laboratory calibration is also required if measurements taken shall be valid for court cases. Additionally a weather station might be required to do proper analysis of the measurements; especially important is wind direction and wind speed. With advanced software for noise analysis all measurements done during high wind speed causing a high noise level measured by the microphone can be easily extracted from the measurement results.

The noise monitoring terminal should be robustly designed with battery backup, tamper proof enclosure and proper temperature regulation to secure stable long term logging of noise data. It should also have a self-monitoring function so that the status of the terminal (such as external power and battery status, door openings etc) is logged and can be verified by the operator.

The communication revolution with broadband data traffic even wirelessly has opened for a new world of noise monitoring. Some years ago the most advanced systems would only allow transmission of small amounts of data. Now, transmission capacity has increased and communication prices have decreased. This means that higher resolution results can be transmitted and even sound recordings and video images may be streamed close to real time. State of the art noise monitoring terminals support all modern communication platforms such as ADSL, WIFI and 3G mobile networks. An acoustic consultant can therefore have access to noise measurements from a number of monitoring stations in her office and measurements can be made available real time on the Internet.

Event detection – what happened?

For those responsible of noise polluting activities, it is often of high interest to get an immediate warning when sound levels are too high. For example will the sound technician on an outdoor concert be able to reduce the volume directly if he gets a warning from a noise monitoring terminal that noise level at neighbours’ premises exceeds the local limitations. Such a warning can be sent as email or SMS with information about location, duration and of course the measured sound level.

In order to securely identify what actually caused the noise, advanced systems have triggers that can start recording of audio, video or both. These can then be streamed remotely near to real time for immediate identification of the source, or they can be replayed later during post-processing of the data. Video recording can be very efficient to later prove what caused the noise, be it a person, a vehicle or other.

Realtime access to noise levels through the Internet?

In a noise measurement system with multiple terminals, the noise data are streamed to a database located in a central server. The data are being analysed and from the database, your results can be presented in any format you like. Do you wish to give a large number of people access to the measurement results, time profile graphs can be made accessible for anyone through an Internet webpage. Or do you wish to have a custom made report summarising all events the last 24 hours? Modern software packages can provide such reports in known formats such as Microsoft Excel or PDF.

For the specialist, advanced post processing software makes further analyses possible: Automatic selection of specific noise patterns and recalculation of average levels of different time lengths can be done. Date logged during certain weather conditions can be discarded. If sound files are recorded an FFT analysis for pure tone detection can be performed, this is a typical application for noise monitoring from wind power farms.

An advantage of a central database is that data can be easily accessed by different groups that need access to data, such groups may include local authorities, acoustic consultants, the company or organisation that controls the noise source and even the wide public.

Configuration of the noise monitoring system

There are a number of national and international standards that recommend how noise from different sources shall be characterised through measurements. Traffic noise, railway noise, airport noise and wind power plants all have their own standards (see examples in table 1). The standards usually describe in detail how the measurements shall be carried out and under what meteorological conditions. Table 2 summarises the most used environmental noise parameters.

Modern noise monitoring systems are capable of logging or calculating all of these parameters in parallel with a time resolution down to a second. However, it is always recommended to reduce the number of parameters to a minimum and to work with as low time resolution as possible. This way one reduces the data traffic and the amount of data to work with.

Case studies

Noise monitoring is necessary for following regulations setting limits of noise emission, for preventing complaints and handling complaints. Typical noise sources that are subject to noise monitoring include:

  • Communications and transports: airports, harbours, railways, road traffic
  • Leisure: outdoor concerts, sports arenas, race tracks, shooting fields
  • Construction and demolition sites
  • Industrial activities
  • Wind power farms
  • Military activity: shooting ranges, training fields
Standard
ISO 1996 Description, Measurement and Assessment of Environmental Noise
ISO 3891 Procedure for describing aircraft noise heard on the ground
ISO/DIS 20906 Acoustics – Unattended monitoring of aircraft sound in the vicinity of airports (standard in preparation)
ISO 3095 Railway applications – Acoustics – Measurement of noise emitted by railbound vehicles
IEC 61400−11 Amendment 1 – Wind turbine generator systems − Part 11: Acoustic noise measurement techniques
Standard
L AEq,T A-weighted-equivalent continuous noise level Calculates a constant level of noise with the same energy content as the varying acoustic noise signal being measured during time T
L AE Sound Exposure Level (SEL) The L AE value contains the same amount of acoustic energy over a normalised one second period as the actual noise event under consideration
L AFMax L ASMax L AIMax Maximum A-weighted noise level measured with Fast, Slow or Impulse time weighting. They are the highest level of environmental noise occurring during the measurement time
L AFMin L ASMin L AIMin Minimum A-weighted noise level measured with Fast, Slow or Impulse time weighting. They are the lowest level of environmental noise occurring during the measurement time
L AFN,T Percentile levels The level of A-weighted noise exceeded for N% of the measurement time
L den A-weighted noise level for day-evening-night Composed of the long-term A-weighted average sound level for day/evening/night respectively plus 0/5/10 dB

Here follows a presentation of three ongoing noise monitoring cases:

Harbour monitoring

Harbours where unloading and loading of containers takes place, are producing high noise levels. Grenland Harbour in Norway experienced regular complaints from neighbours and the local authorities required installation of noise monitoring terminal to make sure that the local noise legislation was followed. There are strict regulations on when port activity such as container loading and unloading can take place.

Noise levels and weather data are stored in a local database. An acoustic consultant analyses the data and compares them with the time of neighbours’ complaints. He has direct access to the database via the Internet connection in his office. Since the audio signal is also recorded, the real source of the noise can be identified and high level measurements that are not related to harbour activities can be sorted out.

The noise monitoring station can give an objective picture of the noise situation in the harbour area at the specific times when neighbours have complained about noise. The data from the noise monitoring station also indicate the trend of the noise situation. The neighbours themselves have access through web to the noise reports that are made by the independent consultant. Every month graphs are made that show maximum and equivalent noise level for day, evening and night.

Surveilling noise level from vehicles on a race track

Permanent installation of noise monitors at race tracks is becoming increasingly important as more circuits are required to measure and control their noise emission to the surrounding environment.

Drive-by measurements can give the circuit a real time display of the noise produced by vehicles passing the test point which in many cases will identify those which will cause complaints to be raised. Systems can also be configured to automatically measure and record exceedences of preset noise limits providing circuit operators with an accurate measurement of the noise of individual vehicles along with a video clip of the episode.

To assure that the noise level from vehicles does not exceed legal noise level limits Scandinavian Raceway at Anderstorp, Sweden have installed a Norsonic Noise Monitoring system. It provides a continuous surveillance of the noise from the passing cars or motor cycles and at the same time stores the measurements for later consultation. The actual noise level is displayed on several computers connected to a local area network through cable and wirelessly. The level versus time data is presented online so that the race leader in that office can see immediately what noise level a passing vehicle is producing. If the level exceeds a certain threshold, he will stop the vehicle and ban it from the race.

Monitoring noise and vibration resulting from construction work

At Rockefeller University in New York the construction of a new building next to a laboratory raised concerns among the researchers; they had to know the level of noise and vibrations that their laboratories were exposed to. The solution was to install sound and vibration metres in various laboratories and hook them up on the local area network. A central database collects all the data, and event analysing software sends an SMS directly to the responsible researcher when too high levels are registered. The researcher can at any time check real-time and historical data of noise and vibration level in his laboratory.

Conclusion

Even though noise monitoring has been done for several decades already, we will see more and more applications for noise monitoring in the coming years. New technologies and reduced costs is one reason, but more important is the increased awareness about the negative consequences of noise pollution. The European Noise Directive harmonises the approach throughout Europe, and the outcome of the current noise mapping initiative will be action planned on local levels that will enforce noise level limits on noisy activities. This will require noise monitoring systems both to assist those responsible for noise emissions and for controlling noise levels in residential areas.

Published: 10th Mar 2009 in AWE International