Environmental noise monitoring is undertaken to understand and assess the effects of sound on people living and working within the vicinity of sound sources. There are several purposes for monitoring such as to understand the baseline sound climate, to compare levels against defined limits, to investigate noise complaints or perhaps understand trends such as whether levels are increasing or remaining the same over a certain period of time.
Aside from establishing the baseline, monitoring is commonly undertaken around airports, road and rail corridors, large industrial premises and construction sites. On construction sites, it is common to measure and record noise along with other construction effects such as vibration levels, dust and air quality, together with a weather station to record meteorological conditions.
In order to assess the effects of noise on people living and working within the vicinity of construction sites, it is common to measure and record construction noise and vibration levels. However, it is important to recognise that measured noise levels are simply a proxy for and may not always be well correlated with the effects of noise on people. All too often, noise is monitored without necessarily thinking about the links between measured noise levels and effects.
Types of monitoring
Continuous monitoring at a number of discrete locations around the site can be used to readily identify patterns in the sound climate. These may be considered over different time periods (daily, weekly) to allow the identification of unusual peaks or gradual increases in sound levels over time. Sudden increases may correspond to and assist in the investigation of complaints. Conversely, if sound levels remain steady or maintain a continuous pattern, this can assist in negating or defending against complaints. This type of monitoring can be done by periodic visits to site for manual download; this is often the case when no mains power is available and regular visits are required to change batteries. However, modern systems incorporating remote data download (via 3G modem or similar) are becoming increasingly popular. These systems can either provide an automated regular data download, or may link to web-based systems providing almost real-time data availability. However, such systems are usually only viable for long term installations where there is a mains power supply available. Considerations, such as security of equipment, maintenance and calibration, and availability of a suitably strong mobile phone signal for the modem to function satisfactorily also need to be considered when specifying and installing such a monitoring system.
Several suppliers now offer multi-channel systems capable of measuring several parameters simultaneously, for example, vibration levels in three axes and noise levels can be logged simultaneously in a single system, or weather data may be logged alongside noise levels.
Continuous unattended monitoring cannot usually be used to identify specific noise sources. The inclusion of audio recording functions (e.g. recording a ‘snap-shot’ of audio when a pre-defined trigger level is exceeded), can provide some further information, although this is still often not sufficient to provide conclusive evidence of the source of sound. Consideration should also be given to privacy and it may not be appropriate to record audio. Attended monitoring will therefore always be necessary to some extent. Attended monitoring is particularly important for the validation of source sound levels which are used in noise predictions.
Where to monitor
There is often a compromise to be had regarding monitoring location, in terms of site access, security and power supply. Monitoring is often undertaken at the site boundary or on a lamp post and the measurements can in most cases be correlated with those at the receptor and, where necessary, a correction factor applied. It may also not be possible to measure sound from the source of concern reliably at the receptor due to other sources, such as local roads. In such circumstances, a proxy location may be selected closer to the source, and used to calculate the resulting levels at a receptor.
It is generally not possible, or necessary, to install continuous monitors at every receptor, but attended measurements can be used to supplement the continuous data. Where the results of both are considered together, it should be possible to identify trends at a greater number of locations and estimate sound levels over longer time periods than the actual duration of attended monitoring. This approach works well where a small number of dominant sound sources (such as a major road) dominate the sound climate over a wider area.
Key to the delivery of any large scale monitoring programme is careful planning, and consideration of the detailed aims of the project. Modern sound level meters allow for the collection of large quantities of data, but in order to prevent overloading the analysis with too much data, a pragmatic approach needs to be taken to collect all the necessary data without simply logging every available parameter.
The use of standard site survey sheets for the collection of all other site data, and the use of databases and GIS systems for storage, processing and display of data have been discussed and found to be beneficial to ensure firstly that instrumentation is set up correctly and consistent approaches are taken to monitoring across all sites; secondly for managing the large datasets and providing a useful tool to assist with quality control checking of data. This includes:
- Checking of noise data on site by staff at start and end of measurements, including site calibration checking of instrumentation and ‘reasonableness’ check of measured noise levels
- Detailed QA checking of data analysis processes and spreadsheets
- Review and approval procedures for all final outputs, reports, etc.
Where more than one meter is used then all equipment must be time synchronised. This will ensure that all data are accurately recorded and data from sites located close to each other measured at the same time can be considered in combination. In some circumstances it can also be used to identify extraneous sound sources not relating to the site.
Multiple visits to long-term measurement sites are recommended to allow detailed notes of the soundscape to be taken at different times of day. The use of sampled audio recordings to allow for future subjective and/or quantitative analysis of the sound climate for periods of unattended monitoring is also discussed.
Peer review and specialist input is required to ensure that the measurement and analysis methods are robust and to technically review data and reports prior to final review by the project manager and approval by the project director.
Monitoring should only be undertaken by those competent to do so. Anyone undertaking monitoring should be a member of the Institute of Acoustics. Membership of the Association of Noise Consultants provides additional confidence in the competency as all member companies undergo a rigorous interview process and have available a wide range of professional development training resources.
Health and safety requirements
All staff should have health and safety training in monitoring, which may include training in working at height, together with any site specific training. A specific risk assessment and method statement should be produced and all staff briefed on this. For health and Safety reasons and based on experience of previous work each team should comprise of at least two staff. One will be largely responsible for engagement with residents, the other for instrumentation set up.
Creating the right impression
When monitoring at a property we should not underestimate the importance of creating the right impression and to reassure the homeowner. Project success relies to a large extent on persuading people to let us leave noise monitoring kit in their garden or hanging on a window for extended periods. Where possible there should be a letter drop to the places we want to measure, but we will need to follow this up with phone calls and door knocking. Even when we get a positive response from the letter drop it’s really important we create a good impression. Hence we need a combination of technical acoustic skills and skills in engaging with the public and obtaining agreement from residents for the noise monitoring.
Interpreting the monitoring data
While large data sets are useful in understanding the range of sound levels in an area, the disadvantage of these is that they introduce additional uncertainty by extending the range of measured values. Analysis of the data should be completed with an awareness of the differences and uncertainty introduced by the averaging methods, as well as the range of the data set.
The analysis of the baseline data for a large infrastructure scheme did not indicate that use of any one of the mean, mode or arithmetic averages is more representative, however, the following key conclusions were drawn:
- Logarithmic average values (i.e. longer duration LAeq levels, and using logarithmic averaging across days) tends to result in levels biased towards the higher measurements, and hence may result in unduly high trigger levels or thresholds for identifying significant effects.
- Use of mode values can give a good indication of the ‘typical’ baseline levels, but are open to significant variation, particularly when there is a wide range of measured baseline levels, or significant variation in level with time over the calculation period.