The index Lden is a day evening night level, which is a logarithmic composite of the Lday, Levening and Lnight levels but with 5 dB(A) being added to the Levening value and 10 dB(A) added to the Lnight value. It would be important to recognise that this is a 24 hour index and that there is no direct comparison of a stated Lden and a perceived noise level at a moment of time i.e. one cannot compare Lden with stated dB(A) levels or charts showing typical/sample perceived noise levels. The Lnight is a measure of the A-weighted, Leq., sound level, over the 8 hour night period of 2300 – 0700 hours and is also known as the night noise indicator and other terms.
The EU’s Environmental Noise Directive requires noise levels to be assessed from road traffic, railways, major airports and industry. It is not a requirement under that directive to assess noise generated by other activities, such as those that may arise from gardening, industry construction work, sports and leisure activities, pop concerts and the like. It is worth noting too that it has not been shown that the use of an Lden index for the determination of noise change is entirely appropriate with regard to its use in projects where noise impact assessment is being considered. The Lden index is a relatively crude 24hour average which normally uses annual road traffic, air traffic and rail data and its use is most common in strategic noise maps.
The Lden index is widely used as part of the Environmental Noise Directive (and the preparation of Strategic Noise Maps) and represents the annual average noise levels at a height of 4 metres above the local ground level. The height of the Lden assessment point depends on the application and will typically range from 1.5 to 4 metres above the local ground level (refer to ANNEX I of END) . Using AADT (Annual Average Daily Traffic) figures, road surfaces, gradients, vehicle speeds, site geometry and topography, noise maps are derived as part of each member states noise mapping requirements and obligations.
With all noise predictions and noise mapping, there are always assumptions and limitations. While Strategic Noise Maps are readily available via many web based public platforms, it would be very important to consider the use of the data, its accuracy and the reliance on such data and its intended purpose. Agencies such as the Irish EPA for example are very clear on the limitations of use and state that it should be noted that the main focus of noise maps is for strategic management of environmental noise, based upon a notional annual average day. They should not be seen as representing what may be measured directly at any location within the map.
The Irish EPA have prepared a Guidance Note for Noise Action Planning for the Environmental Noise Regulations. This document was produced initially in 2006, revised in July 2009 and updated in June 2019. The objective of the original Guidance note (July 2009) was to provide practical information, advice and guidance to designated Action Planning Authorities on the development of noise action plans under the Environmental Noise Regulations, and reporting of the plans to the EPA.
For more details on the EPAs guidance notes see:
http://www.epa.ie/pubs/advice/noisemapping/
