One thing to note regarding data centers is the difference between sound pressure level and sound power level.
Unfortunately, it may not be an easy thing to understand.
To that end ...
... The difference between sound pressure and sound power can be tricky to get your head around. For predicting noise levels from noise polluting sources like plant, it is very important.
As a consultant, I'm often verbally quoted noise levels by clients, i.e. this unit has a level of 60 dB'. Unfortunately, based on that alone, I can't do a great deal, without making a lot of assumptions. If you tell me it's 140 dB I know it's going to be very loud and problematic! But at 60 dB, depending on the situation, it's difficult to assess the risk.
What is a sound pressure level (SPL)?
A sound pressure is the pressure deviation from atmospheric pressure caused by a sound wave, in pascals. The sound pressure level (SPL) is a logarithmic measure of the ratio of a sound pressure over a reference sound pressure (corresponding to the hearing threshold of a young, healthy ear), quoted as a dB. If these two pressures are the same, we have an SPL of 0 dB.
A sound pressure level is what we can physically measure using a sound level meter. Most noise level parameters in a report are based upon an SPL, albeit they are mostly adjusted in some way, i.e. weighted to a single number (dB(A)), or a level difference such as a Dw.
A sound power level (SWL) is theoretical. A sound power is in Watts (W), a sound power level like above, is in dB, a logarithmic ratio of the sound power over a reference sound power. W for Watts, hence SWL (as SPL is already taken!) I'll explain more shortly.
Why do we need sound power levels (SWL)?
The sound pressure level (SPL) depends on distance, the position of the source and the environment, i.e. reflections from the ground, or if inside the surfaces of the room and therefore the reverberation time and volume of the room. So if we measure the SPL of a fan unit inside a plant room, and move this fan outside, the SPL is unlikely to be the same because of more sound energy escaping into the atmosphere. Similarly, if the unit is in the corner of a room, condensing sound radiation by the surfaces close around it, and then move this unit into the centre of the room on the floor, where it radiates more hemi spherically, the SPL will be different. Simply moving further away from a sound source, will reduce the SPL, particularly noticeable when outside.
If we take a large plant enclosure, with many different machines from different manufacturers, who all give us noise data as a sound pressure level, but all measured at varying distances, some in a lab, some outside, some in an anechoic chamber (or very often not referencing how it was measured at all which is effectively useless data as it could be measured at 1m or 10m!), we'll have a hard time predicting the sum of noise levels from our plant enclosure.
The sound power level (SWL) helps us around this. It is not dependent on distance, position or environment. This is the crucial difference. It is a theoretical value; it is not directly measurable. A noise source will have the same sound power irrespective of where it is placed. It gives us a level playing field to directly compare two sound sources. Predicting the noise levels from our plant enclosure is now much simpler, we can apply the same calculation to all of our equipment. ...
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