- by Peter Koelewijn, 23-04-2018
In part 1 and 2 of this series of blogs, I have covered what sound is and what acoustic comfort is. When sound is judged to be unpleasant, loud or disruptive to hearing, it is called noise. Super yachts, cruise ships, and even commercial vessels, all are subject to certain noise level requirements. In this blog I am going to explain different types of noise.
A couple of years ago I attended the sea trial of what one would call a super yacht. As the vessel cruised towards its destination the coastline slowly moved away from me. On my way to the galley I passed the engine room and for a moment I stood still before the door. Muffled sounds and squeaky noises where audible through the massive door. Although I had no business there, curiosity set me to it and I walked inside. As I opened the door a wall of sound was flowing over me!
It seemed like this enormous wall of noise was only one sound, like one thundering roar. But when I listened carefully, different components could be heard. And then I noticed that it was not one sound, but a blend of noises mixed together. Engines where pounding, pumps running and air thundered through gigantic ducts above the room. The same goes for HVAC. Walk inside a fan room and notice all different processes with each their own noise. If you want to isolate the noise problem you first need to identify its origin. In mechanical engineering there are three different noise types, namely:
When a fan, pump or compressor is running, electrical energy is transferred into work. The machine is doing its job. Bearings are rolling, pistons pumping and screws rolling. As the gears move there is contact between materials causing friction and therefore it causes air pressure difference in the direct vicinity of the machine.
The product of a running fan is transportation of air through ducts and pipes. Air is a gas consisting of different molecules. They all have their own mass and can collide with each other causing friction and turbulence. Just as the solid components of a compressor, only on a micro level.
As the name already suspects, these noises are caused by vibrations. But aren’t all noises in its base caused by vibrations? Yes, that is correct. It is even so that its origin most of the time is mechanical or air flow noise. However we define this type separately because it can cause a lot of trouble. An example of this type are resonating ducts.
If you have a noise problem somewhere in the cabin or public space you could listen very carefully and try to notice where it comes from. But unless you have the ears of a dog, it is almost impossible to filter out the correct noise type. In our previous blog we discussed how noise can be defined by its frequency and loudness. In fact we can use the same properties to define the type of noise. With a spectrum analyzer you can display the loudness as a function of the frequency. Just as displayed below.
Image 1: Spectrum analysis
This is an example of a fan. The blue line represents a normal operating fan. The red line is the same fan but with broken bearings. Notice that it will peak on the lower frequencies. So when you see a peak in loudness on a low frequency, it probably is a mechanical noise. Disruptive flowing of air due to turbulence will peak in a higher frequency. With a spectrum analyzer you can pinpoint the type of noise that is causing the problem. In our final blog we will discuss the solutions for all three noise types.
Peter Koelewijn | Sales Manager
Peter Koelewijn has been working at Heinen & Hopman since 2001. Starting out as a mechanic, he gained valuable field experience mounting ducts and pipes for various shipbuilding projects. Five years later he switched to engineering, where he worked his way up from draughtsman, to engineer to site manager, leading teams of HVAC mechanics at one of the largest shipyards in Germany. The last couple of years he has been working as sales manager, combining knowledge from the field and the office to find the best solutions for our customers.