Diagnosing and addressing noise issues are more complex than one might think. As we’ve shown, there are many different sources of noise which can introduce error into measurements. There are two distinct steps which need to be taken when addressing a noise issue: determining the type of noise you are experiencing and then finding the source of that noise.
DETERMINING THE TYPE OF NOISE
When you notice noise in your measurements, you should first consult the instrument maker. They will usually have experienced field service personnel on staff who have seen many different scenarios. Oftentimes, they can identify the type of noise drawing on their own experience and knowledge of the instrument’s sensitivities. You may also want to consult the instrument manual as they will often have troubleshooting tips for self-diagnosis.
If the above doesn’t work or is impractical, the most surefire way to diagnose the type of noise is by conducting a site survey. The site survey should include acoustic, EMI, and vibration measurement equipment. This will provide you with quantitative data to continue your analysis.
Another method for determining noise type is by attempting to correlate the occurrence of noise to a time of day or event in the area. If the noise levels are constant, this method may not be helpful. But if you carefully observe the noise levels throughout the day, you can often determine what times the noise is worse. You can then begin the process of correlating this to events in the lab or surrounding area. For example, if the source of noise is foot traffic, you can often observe that noise levels decrease dramatically in the early morning or in the evening when few people are in the building. Other questions include: Does a train pass nearby at regular intervals? Does a nearby machine operate at certain hours? This is a trial-and-error process, but if you ask the right questions it can point you in the right direction.
FREQUENCY OF COMMON NOISE SOURCES
Some instruments have the capability to analyze the frequency of noise in the measurements. If your instrument has this, or you are otherwise able to distinguish the frequency of the offending noise, this can be a great indicator of the type of noise you are seeing. Below is a table which describes the frequency levels of some common noise sources.
|Frequency (Hz)||Likely Source|
|0.5 – 1||Tide, Ocean|
|0.5 – 6||Foot Traffic|
|1 – 5||Building Sway|
|5 – 10||Seismic|
|5 – 40||Machinery (pumps, etc.)|
|5 – 100||Vehicle Traffic|
|30 – 50||Air handlers / HVAC|
|60, 120, 180…||Electronic line noise|
|60 – 7000||Human Voice|
|300 – 300 MHz||Radiofrequency (RF)|
DETERMINING THE SOURCE OF NOISE
Once you’ve spent some time characterizing the noise that the instrument is experiencing, it is time to try to identify the source. If you’ve been able to determine the type of noise, you can limit your investigation to only those sources which emit that type of noise. A good first step is to brainstorm all the possible sources for the noise and create a list of the possibilities. As your investigation proceeds, you can systematically eliminate these sources.
It makes sense to start your investigation close to the instrument. Noise sources nearby are more likely to cause trouble for instruments as there is less structure to mediate and interfere with the transmission to the instrument. This is especially true of items which are directly connected to the instrument or the instrument’s support structure. Try disconnecting various accessories from the instrument and observe how that affects the noise levels. Start with pumps and electronics that control fans, as those are likely offenders. If it is impossible to disconnect these items and run your measurements, try changing their structure by adding mass to them, placing damping material under them, or weighting the cable that connects them to the instrument. If this causes a change in the noise that you are seeing, then you can deduce that you have found the offending noise source.
Once you’ve eliminated possible noise sources in the room, expand your investigation to include the adjacent rooms. Remember that rooms on upper and lower floors should also be considered adjacent. If you notice machinery or equipment that are possible sources of noise, ask to have those pieces turned off and unplugged so that you can compare noise levels with and without them running. Ask around – if there are other people using precision instruments in your building, they may have noticed noise issues as well.
If you’ve been able to eliminate the various sources of noise within the building, it is time to take a trip outside. Look for streets and parking lots nearby which can be sources of vehicle traffic. Keep an eye out for construction that is occurring in the area – see if you can correlate the noise in your measurements to when the construction site is active. Other external sources of noise include high winds and proximity to large bodies of water.