Sound Advice
Volume 11, Issue 3, 2008

Noise Monitoring Developments

When a project calls for a noise monitoring effort, usually the major issue is compliance with an ordinance or conditional use requirement that specifies the maximum permissible noise levels that can intrude onto a nearby sensitive property.

One option for conducting such a noise monitoring effort is to install an unmanned sound level meter with an extended performance power supply, and then program the device to record statistical noise data over several hours (or even days) to meet the client’s goal. When the initial study period is completed, the data can then be downloaded remotely or by a technician visiting the site with a laptop computer. With a cleared memory and a recharged power supply, the unit is good to go for another specified measurement period.

The disadvantage with the unmanned approach is that even if the sound level meter is accurately programmed to record relevant data and the measurement goes according to plan, the data itself provides the consultant with no clue as to what caused any spikes in noise level that may be detected in the post-monitoring phase of the work. 

Using only data gathered by an unmanned monitoring device, the person or entity responsible for the noise source under investigation may find themselves facing unfair allegations of noise ordinance or conditional use permit violations that come with court action and fines. Having recorded data “blind” in this manner, any sudden loud noises tend to be attributed to the noise source being studied.  But other innocent, unrelated sources could have caused the spikes. A bird landing on the microphone windscreen, people talking in the vicinity of the sound level meter oblivious to the measurement going on, a loud vehicle nearby, an airplane or helicopter overhead, a siren, or even a homeowner mowing his lawn nearby, can cause a huge increase in noise level that was not generated by the noise source under investigation. 

Now two major manufacturers of sound level meters, Brüel & Kjær and Larson Davis, have each introduced a new handheld device which addresses this “What created that noise level?” question that acoustical consultants face when analyzing the data they’ve collected. Of course, the new generation of sound level meters comes with all the latest methods for the capture and accurate reporting of sound levels and statistical data. The new technology they have added is the ability to create digital audio files.  This means that the consultant analyzing the data obtained from an unmanned sound level meter can now access and listen to the actual noise recorded when the spike in noise level occurred, and thus have much better insight as to its cause.

Another, even better approach is to set up not only a sound level meter but also a video camera to attempt to identify noise sources. However, there are significant difficulties with this approach.  First, the more complicated the set-up, the more likely it is that some glitch may occur, requiring the technician to go out to the site and make adjustments as necessary. Second is the security of the monitoring equipment set-up. (Amazingly, in the past we have had sound level meters stolen right from under the noses of the guard service we hired to prevent theft or damage.) Third, even if the equipment set up works perfectly and is not stolen or tampered with, there is no guarantee that the camera will be pointing in the right direction to capture on film the spike-causing noise events as they occur. Fourth, there are no video cameras available that can record enough data for a long-term monitoring study. And fifth, going through all that video would constitute a daunting and time-consuming task for any consultant. Another ongoing technical problem would be the equipment power supply; with mains power seldom available we must rely on batteries and manufacturers’ claims regarding battery life expectancy.

Even with all this technology now available, the low-tech approach is still the most effective:  a technician sitting close to the sound level monitor, annotating the occurrence of loud noises, the decibel reading from the meter, what time the events occurred and what caused them. A pair of human eyes and ears on the situation can monitor not only the noise, but also the equipment, and can address any technical issue as it occurs. Also, the presence of a technician on site can act as a deterrent to theft or tampering with the equipment. Of course, this type of assignment can be quite challenging work for the technician, who has to write fast enough to keep up with the noise spikes as they occur.

In terms of cost to the client, the use of manned or unmanned long-term monitoring may be a wash. For manned monitoring, there is an hourly charge for a technician/guard to be present at the monitoring site for long periods. For much of the time, our staff member may be doing nothing other than listening and monitoring the equipment. But when the loud events occur, the technician's experience and training enables him to make precise notes on what is happening, removing any guesswork or frustration at the post-monitoring stage. For unmanned monitoring, the data gathering phase is less expensive, requiring only the equipment set-up and retrieval. However, the post-monitoring analysis of the data obtained in this way is more expensive. The consultant must analyze all of the audio and/or video data, identify any spikes, and do his best to identify their cause(s), which can be time-consuming without the anecdotal information that a technician at the site could have provided.

Whether to use unmanned or manned noise monitoring largely depends upon the needs of the project, and that’s where our expertise as acoustical consultants comes in. If you have a monitoring project you’d like to discuss with us, you can feel comfortable knowing that we have conducted many of these studies. We will use our expertise, experience and common sense to create the most cost-effective strategy for obtaining and analyzing the most accurate data for your project.

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