Triggers and Alerts for Environmental Noise & Vibration

The advent of cheap 3/4/5G data modems and data-plans means that equipment such as noise and vibration monitors can now be set out in the environment to collect and report back on a long-term basis with little intervention. This just leaves the responsible person to deal with the data, either by looking at regular intervals at the website portal, or by responding to ‘alert’ messages sent by the system according to various ‘triggers’. But what is going on with these triggers and alerts and how should you set them up properly?

Internet Connection

The first thing to decide is how the equipment will be communicating with the internet. You may have access to a wired connection with power and ethernet, in which case, you will have a strong, reliable, and free connection to the web and will be able to make use of the bandwidth for all the instruments features. Second to this, you may be near enough to a building to use Wi-Fi and once again, you will have free access with unlimited data transmission.

Most equipment will be out of reach of these technologies, in which case data modems will be the solution. These now come in 3, 4 or 5G versions and it is the SIM Card that will determine which of these is used. SIM cards also come in two flavours; mobile phone, and data and this is also a very important choice to make, as you will see…

Email, SMS and Apps!

Most systems use their web portal to distribute alerts, and sending emails in this way is simple, and free. If you want SMS messages, however, then a web-to-text service is needed, and these can end up being expensive at around 3-4p per message, which can soon rack up if you set your triggers too keenly.

The alternative to this is to let the instrument send the SMS via a mobile SIM, which means you can make use of mobile phone data-plans, including unlimited free messages. The problem here is that you are then locked to one network, so your coverage may not be good enough on-site. Data-SIMS conversely, will roam to find the best signal in a particular area, but they will not send text messages.

The best way to get your alerts on a mobile phone with notifications is to use the App for your equipment, which should have a messaging section, which can pop up notifications on your home screen and it’s all free!

How Much Data?

The other issue to be aware of using any Data modem is that there will be limits to the amount of data you can send on a given monthly plan. You will not normally have any issues with the numbers, even if you are sending 1/3 octave bands every second, but as soon as you start trying to send audio recordings regularly, then the data is quickly gobbled up and the pricing may escalate alarmingly! If you simply collect the data and audio, you need and test it before you leave the system to its own devices, you should be fine.

Trigger Happy

Finally, those triggers. The theory sounds great – you set a level and receive a message when it is exceeded. The reality, however, involves choices and a bit of thought! Firstly, what parameter are you going to use for the trigger? Is it simply the sound pressure level (L_Ap) or would it be better to use a 5-minute L_Aeq? What about a 1-hour L_Aeq? Just because the standard you are using might require time history intervals of 3 hours, this does not mean your triggers need to be the same. Don’t forget the triggers are there to inform you of the situation on-site, so set the triggers based on what you want to know as an indication of how likely the site is to exceed the limits set by the overseeing authority.

A good example is table E2 in BS 5228-1:2009+A1:2014 (Code of practice for noise and vibration control on construction and open sites – Part 1: Noise). The averaging time varies from 1 hour to 10 hours, so what would you set?

This might suggest that you should set your triggers to match each of the time periods mentioned, when actually what you are more interested in is how likely is it that the levels of site are approaching 55dB at night, 65dB in the evening and 70dB during the day. It may be that you set these at, say 53dB, 63dB and 67dB respectively. Perhaps a 5- or 10-minute L_Aeq would do the trick? Whatever you start with, you should try out to see how many alerts you get at those levels. Then tweak it until you get a sensible response.

Never forget that these alerts are there for a reason, so you will need a very clear system in place to monitor the alerts and, most importantly, take action as prescribed!

Don’t Mix Your Triggers

Don’t forget that you can also set triggers to start a measurement and store the data or equally there may be triggers to record audio or wav files for vibration. Just make sure that you don’t mix up your triggers and set each of them according to what you want from that function – they probably won’t all be the same!

Data recording triggers are less of an issue in modern sound meters due to the size of memory available, although if you need to capture very short time periods, this may still be relevant. This type of triggering will almost always be based on the L_p (Instantaneous sound pressure level) as this is the only parameter that will react quickly enough to capture the event.

Audio Triggering Pitfalls

When looking to record audio files based on ‘event’, you will need to set triggers separately for that. These audio triggers will probably also need to be based on the L_p for the same reason as above. We have seen users set an audio trigger based on a 10-minute L_eq and then wonder where the audio was – it may have happened any time during that 10-minutes! Some sound meters will allow you to set a pre-record time of, say 5 seconds, so that you end up with audio included from before the trigger level was exceeded – that way, you don’t miss the start of the event.

Not Forgetting Vibration

Vibration comes with its own considerations, which are very much application dependent. If you are monitoring to ensure the operation of sensitive equipment using VC curves, then you must make sure to measure in 1/3 octave bands and have the correct levels set for each band. In this case, the alert system must be robust and may even include an audible and visible alarm on-site.

Trigger Settings Galore

There may be other factors to consider, which will be settings on your instrument such as rising or falling level and slope.

The rising or falling level will activate a trigger depending on the level you start at. A rising trigger is most common and will cause an alert when the level starts below your chosen level and then rises above it. A falling trigger will do the opposite and fire when the level falls. You can see this in the graph below.

Slope can be positive or negative, much the same as rising or falling trigger levels, but this depends on the rate of rise or fall of the sound level and will usually be set in dB/second or PPV/second. It is designed to catch sound that have a rapid onset in an environment where this does not usually happen. It could be, for example a generator starting up in a rural area.

Conclusion

Although it all sounds simple in theory, there are many factors to consider when setting up a monitoring station to send you alerts, based on triggering events. The setup can be a little involved and if you get it wrong, you may well be bombarded by hundreds of expensive text messages or experience a clogged-up email inbox. Don’t forget that however you set the triggers, you need a clear plan to act on the alerts when they arrive.

Bear in mind that there are often separate ‘trigger’ settings for alerts, data-recording and audio recording, so be careful to select the correct one and make sure they are set appropriately for the required action.