Don’t waste your time with daytime noise measurements

Image by Peter Dargatz from Pixabay

Last week I got an urgent call to measure noise from installations as soon as humanly possible, in a project which have had some problems with too loud ventilation. The problem had just been mitigated (hopefully) but they still needed a protocol to give the green light to the building. I basically booked the next possible flight and went to Stockholm. Usually, I hate doing these one-day trips. They can often be exhausting. This time however, it wasn’t a complete disaster. I had planned extreme margins both before and after the measurement. Still, I learned a couple of valuable lessons on why these one-day trips are a bad idea anyway.

I got up at 4:00, walked my dogs and headed off to the airport around 04:30 to catch the first flight of the day. I had barely sat down on my seat until I fell asleep. I even missed the in-flight breakfast serving but woke up just in time to grab a box and get some energy in my stomach. After touchdown, I headed off to the rental car premises and picked up a very nice BMW 330e which I had found quite interesting, because it seemed to be a fun car with extreme amounts of oomph when you engage hyper sport mode with petrol and electric in a nice harmony. At least it felt as if I arrived quicker to the site. The drawback was that the battery was empty in no time at all…

Around 0800 I parked outside the site. The project was supposed to fulfil Sound Class B, which is 4 dB stricter than normal requirements. In practice, it means a pretty much dead silent dwelling. My return flight was the last one of the day, taking off around half past nine. Thus, I had plenty of time to measure properly. I got introduced on site and got to work. I basically had one of the houses all to myself. Before long, I ran into trouble. Many of the rooms I tested resulted in too high noise levels. There were some heavy machines nearby, with a front-loader, an excavator and a truck and when they were close it was impossible to get meaningful results. But I waited until they were out of sight and almost inaudible. That usually does the trick, but not this time! I measured again and again, popped a couple of balloons to normalize the results. The reason is that empty rooms are very reverberant and that increases the sound pressure level from installations (and other sources) substantially. But even with normalization, I couldn’t “save” the project. Things were starting to look dark.

Meanwhile, the whole building started to look dark as well. No worries, I have a headlamp in my kit, so I fired it up. By now, I had almost decided to give up. But now, the clock was approaching 16:00 and the heavy machines started to drop off in the distance. The workday was over. When most of the workers had left, I did a final run of measurements, just to be sure. Lo and behold; I just managed to get approved results without margin. It turns out that the heavy machines, even though I subjectively rated them to be unimportant when they were far away and behind other buildings (in practice inaudible), had messed everything up. They produced low frequency noise that clearly got the eigen-frequencies of the room started. It is quite possible to have a lot of low frequency sound energy in a room that is perceived as silent. With a quiet building all alone, I re-did all my measurements in less than an hour. Super-fast and with significantly higher quality. No background noise issues at all. There is another important factor here. If you have high quality, undisturbed data going into your sonometer, you will significantly speed up the evaluation. Probably by hours, if you can simply use the on-screen results and not bother with a computer program to exclude intermittent bangs or noises. This easily translates into several hours saved.

Moral of the story, don’t waste your time with sensitive day-time measurements. This was the last time that I ever attempted to perform installation noise measurements during daytime. Next time, I will arrive on site after lunch, rig everything and then be ready as soon as the workers leave. Then, I will measure throughout the night if necessary. Uncomfortable yes, but not as uncomfortable as spending 2-3X the time and come out with a lower quality result. This is especially important if you need to measure higher sound classes. They won’t stop the construction site because of you.