Environmental Noise Measurements
General community noise measurements
Many environmental noise measurements are made for planning purposes, to check noise levels against local noise ordinances or to investigate noise nuisance complaints between neighbors. Results are usually taken on property boundaries or at specific positions laid down in the appropriate measurement protocols (or ordinances). When nuisance complaints are being investigated it may be necessary to make measurements inside houses or apartments to assess the noise levels between party walls. Different noise limits may apply at certain times of the day and night to reflect people's sensitivity to disturbances during the day. Simple sound level meters such as the CEL-254 or CEL-320S can be used for this work.
using the CEL-320S for community noise measurements
Many measurement protocols require that readings are taken over an extended period of time that can be as short as a few hours or as long as a whole week. Instruments used for these sorts of recordings must be able to store noise readings at regular intervals called "periods". An overall noise reading, or "cumulative" value, for the whole of the measurement run enables the result to be compared against any limits that have been applied to the source of noise. Specific SEL data collected about significant noise events can be useful to correct regular periodic Leq values and to establish exactly when the event occurred.
significant noise event recording for SEL
Transportation noise measurements
Noise sources under investigation in environmental noise measurements include all forms of transportation such as aircraft, highways, railways, quarries, industrial and commercial properties. When traffic noise measurements are required it is often necessary to measure the 1 hour Leq or L10% values and then process them into longer intervals such as 12 hour Leq or 18 hour L10%. The correct measurement position may be in front of the property or at the side depending on the position of the offending noise source. An instrument that can measure the overall level and shorter periods during the measurement can show the effect of different traffic flows during the day. Suitable instruments for this work are the CEL-360 or the CEL-480 or CEL-490 meters.
measuring road traffic noise
A typical example is the measurement of community noise which is usually carried out for a full 24 hour interval to give a single value for the whole day. Periods of 1 hour are typically measured and accumulated during the day on the basis of the hourly equivalent continuous noise levels. These are sometimes called the hourly Leqs or HNL, and they are combined to produce the 24 hour Day Night Level or LDN. All readings measured during the day time between 0700 and 2200 are included "as-is" but a penalty of 10 dB is added to all readings between 2200 and 0700 before they are combined together for the whole 24 hour interval. The purpose of the night time penalty is to reflect most people's extra sensitivity to noise levels when they are trying to go to sleep. The CEL-490 meter and the dual range versions of the CEL-500 series with the log mode are able to directly record the LDN results.
example of 24 hour measurement results
The collection of shorter noise levels logging, or (profiles), during a recording can be a useful addition to the understanding of the temporal variation during the overall measurement. These "time history profiles" can range from 1 second to 1 minute when periods of 1 hour are typical for many measurements that may last 12 or even 24 hours. The time history profile function acts as a digital level recorder that is built in to the sound level meter and can be used to highlight the occurrences of short duration events during the course of the longer term recording. If a tape recording is made during these events then an audio log of the source of the noise can be useful when measurements are made in an unattended manner. Some analyzers can be connected to video cameras to obtain both an audio and video recording of the possible noise source. The CEL-360, CEL-480, CEL-490 or the CEL-573 instruments have the capability to measure profiles. The CEL-593 is an ideal instrument to link to an audio tape recorder for control of the recording times based on noise level exceedances.
use with an audio tape recorder
Long term noise measurements
Adequate attention must be given to protecting the noise meter during the long term measurement against the ravages of the weather. This also includes the microphone and preamplifier which may have to be positioned in exposed locations by means of suitable extension cables. The protection for the microphone should be capable of withstanding heavy rain storms and high winds without becoming waterlogged. Bird spikes on top of the protection system help to keep the protective material from being pecked away. The windscreen material needs to be transparent to the noise signal but impervious to rain. Eventually it will be eroded by the sulfur dioxide dissolved in the air and will deteriorate and need replacing. The meter itself can be located in a lockable case with its own battery power supply or mains unit if available. During the noise measurement the kit case is usually padlocked to a non moveable post or other suitable fixture. Cases such as the CEL-6760, CEL-6723 or CEL-6627 are available depending on which CEL noise meter is chosen for the measurements.
measuring noise levels around a coal mine
Short term noise measurements
For short term measurements the instruments can be hand held or mounted on a suitable tripod instead of having to consider all of the requirements of waterproof cases and full microphone protection systems. The majority of community noise measurements are taken in terms of the overall 'A' weighted level but significant additional information can be acquired if measurements are made in octave or third octave bands using real time noise analyzers. In this manner all of the detail contained in the frequency domain is realized together with the overall 'A' level necessary for compliance purposes. Certain noise sources will have their own noise "signatures" which can only be seen when carrying out this type of analysis and comparing the spectral results against the frequency response of known local possible sources of the problem. The CEL-450, CEL-490, CEL-573 and CEL-593 real time analyzers are able to store all of the frequency information simultaneously with the overall 'A' weighted level.
spectrum of noise source
A wider variety of noise parameters are often required to adequately describe the total noise climate. These include the noise equivalent level (Leq), the maximum level (Lmax), the minimum level (Lmin), the percentile noise level (L10% or L90%) and the day night level (LDN). The relationship of these results during measurements can often give a good understanding of what is happening at the measurement site. For example, if there is a wide difference in the maximum level but the background level stays fairly constant throughout the run it can be concluded that short duration events were causing the problem but that the underlying noise climate remained constant.
graph of different noise parameters