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Noise is one of the most common causes of complaints about the indoor climate, and unfortunately, standards and requirements are not always met in practice. In order to design fully functioning, yet quiet indoor climate solutions that ensure a high degree of comfort regarding sound, all components must be of good quality and used correctly. It might be an exaggeration to say that it all starts with understanding decibels (dB), but it is not that far from the truth.
When in the midst of discussing heating, ventilation and air conditioning (HVAC) it may seem like the unit decibel (dB) often comes up. Decibel (dB) gets thrown around, which seems fair because noise is something really needed to consider when designing HVAC solutions. But do you sometimes find dB and dB confusing? Tony Olsson, Product Manager Acoustics at Swegon, will explain.
Starting from the basics, decibel is a logarithmic unit used to express a ratio. It is unlike “meters” or “kilograms” not a unit of a certain measurement, it only expresses how much larger or smaller one quantity is in relation to another.
Sound power and sound pressure
In the world of HVAC, decibel can represent different things. Sound power (Lw) and sound pressure (Lp) are the most common expressions, but it can be difficult to sort out what is what. Both of these are expressed in decibel, despite the fact that they refer to different things:
- Sound Power (Lw) is how much acoustic energy a source emits, referenced to 10^-12 watts. Sound power is a fundamental property of the noise source. It can be described using a radiator, the wattage doesn’t change whether one is standing close to the radiator or far away.
- Sound Pressure (Lp) is how that energy is perceived at a certain location by an ear or microphone, referenced to 20 µPa. Sound pressure is what is actually heard. This type of decibel depends on the environment, distance from the source, room acoustics and much more. The radiator can work as an example here too, 500 W on the radiator appears colder as the distance to it increases
It is essential to understand the distinction between the two when designing HVAC solutions, this because manufacturers are obligated to report sound power levels (Lw), and often decide to also report Sound Pressure (Lp). This can be confusing.
An example might do
Imagine installing an air handling unit (AHU) in two different rooms:
- Room A has bare concrete walls.
- Room B is lined with acoustic panels.
Both AHUs are rated at 70 dB Lw, but with the different acoustic settings in both rooms, the one in room B might result in only 55–60 dB Lp while the one in room A could reach over 65 dB Lp, if the rooms are of the same size and the sound is measured at same distance in both rooms. This example describes how Lp varies depending on its context, while Lw remains constant.
So, manufacturers are obligated to report Lw because it is a fundamental property of the product and it’s the only true metric for comparison of products. Some chose to also report Lp because it can give the customer a better understanding of how the product will behave in common spaces. However, going back to the above – Lp is context-dependent which is why Lp also can be complex and misleading.
Sound source with Lw 36 and Lp at different distances from sound source.
Worth knowing to not make mistakes
- It is important to specify distance and/or environment when quoting Lp, otherwise the value might be misleading.
- Confusing Lw for Lp may lead to under- or over-design in acoustic treatment, subsequently causing the solution to fail to meet the demands set for the building.
- Comparing dB values from different sources without context (e.g., free field vs. reverberant field) leads to inaccurate conclusions.
In sum
In HVAC, not all dB values are the same. Lw describes how loud something is at the source while Lp describes how it will feel in the room. Understanding this distinction helps avoid mis-communication between designers, manufacturers and building owners, and ultimately, a clear understanding of decibel leads to quieter and more comfortable indoor environment.
Find out more in our acoustic calculation software and get familiar with our acoustic products on our website.
