- by Peter Koelewijn, 02-03-2018
What defines the comfort level of an air conditioning system? Chances are high the first thing that comes to mind is temperature, which is understandable. Air temperature is one of the main properties of climate as air conditioning is all a matter of thermodynamics, but there is another property which definitely contributes to the level of comfort. That is sound.
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 these series of blogs we are going to dig deeper into the topic of noise and noise control.
According to the dictionary the phrase ‘comfort’ can be described as a pleasurable state of welfare. A status in which one effortlessly feels good. But also – and this is probably best applicable on HVAC – a status in which one experiences no discomfort. When a system is not evidently audible everyone is happy. The complaining starts when the presence of the installation is noticed. But then again, comfort is not the same as luxury.
If we define acoustic comfort as a status where one doesn’t notice the running HVAC system, the comfort level will depend on the situation we are in. Every situation has its own noise criteria. What can be comfortable in one situation will not automatically be experienced as such in the other. Not that it is a complete grey area; some smart people already gave us guidelines for noise requirements, such as the Det Norske Veritas (DNV) comfort class document. The tables underneath are taken from this document. Let’s take a look at them.
Table 1: Noise requirements Cargo ship
Table 2: Noise requirements Yacht
The first row indicates the comfort rating, where one is the highest class and three is more forbearing. Now notice the differences in cabins / sleeping rooms. At level one, the requirements for a cargo ship are 50 decibels against 35 decibels for a luxurious yacht. That is a major difference as we will see when we handle decibels below.
Looking at these tables one could say; the quieter the system the better, because comfort rating one has lower decibel levels than rating two and three. But this is not entirely true. The most silent installation is not always the most comfortable. One sound can mask another more irritating sound. An example of this is the public spaces on a ship or an office environment. The ambient sound of the HVAC system will conceal background noises which are present in public areas, offices or even the noise of engines running. Now how can one sound which is louder and covers a quieter sound, still contribute to the comfort level? This has something to do with how we experience sound.
Some noises are more irritating than others. For example the soft beep of your alarm clock is more annoying than the sound of music coming from a hi-fi system, even if the latter is louder. There are two properties of sound that contribute to the degree of comfort. One is the volume of a sound; the other frequency. To discuss these we are going to dig a little deeper into the matter.
A decibel is the degree of pressure increase – or the amplitude – which corresponds with the loudness of a sound. The higher the pressure, the louder the volume. Converting to the human ear, it is indicated as the decibel unit.
The other property is frequency, which is the number of periods per second. A high frequency is perceived as a high sound. A low frequency is perceived as a low sound.
So every sound you hear is a combination of intensity (loudness of a sound) and frequency (the tone height of a sound). This combination defines how disturbing a noise is. For example, a sound with a volume of 30 dB and a frequency of 1000 Hz will be more annoying than a sound of 40 dB and 125 Hz.
Image 1: Spectrum of human hearing
In reality you will never find a source of sound with only one single frequency. No, every air handling unit, fan or electro motor produces noise containing multiple frequencies, all with their own intensity. To get a clear view of this a tool called the octave-band has been established. In this diagram the sound intensity is displayed against the frequency. Each frequency covers a specific range of frequencies, starting from 31 Hz to 16.000 Hz.
Image 2: Octave band
There are special instruments which can measure the sound level in every frequency. This a powerful instrument as every sound will have its own characteristic. In my next blog I will go further on this topic as we will handle three different sorts of noises which can be found in our branch.
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.