You Can’t Cheat Physics 101: Noise

Photo by Wouter Breedveld

This is the second installment of You–Can’t-Cheat-Physics-101

If you read the first You–Can’t-Cheat-Physics you will see our premise is pretty simple. We explore and discuss aspects of equipment design and selection that would be really cool to have if it didn’t like violate certain fundamental laws and theories of physics, thermodynamics and what-not.

Again still want to reserve the main caveat here. This is based on our current knowledge of physics which while impressive, as always, again I repeat AS ALWAYS has been found to be wrong given enough time. Can’t see why that wouldn’t change in the future. Sorry scientist guys.

In fact,I’m pretty sure it was a scientist guy, Issac Newton, who said “If I have seen further it is by standing on the shoulders of giants.” I guess my point is WE ARE the shoulders of future generations who will see further than us.

That means in 200 years from now the cyborg-versions-of-us, (and the machines that have taken over the world) will look back on us, and our level of technology and understanding and think of us as “quaint”.

But right now its all we got, so it’s what we have to work with, if that’s okay.

The first topic was on trying to cool things below the ambient temperature or that whole pesky thing called ‘entropy’.
Next up for skewering is sound, or more accurately noise.

Sound is what we sense with our ears. It only gets labeled noise when the sound is loud, unpleasant, unexpected, or undesired. The second movement Adagio from Mozart’s Piano Concerto no. 23 in A, KV 488 is pretty sweet. But if it was coming from your neighbors apartment at 4 in the morning, you might just call it noise.

But what I will be talking about is industrial fan noise. If that was coming through your neighbour’s wall at 4 in the morning you would definitely call it noise. Yep, not a lot of wiggle room on that one.

In fact industrial fan noise is pretty well annoying any time of the day.

Okay where do we get caught in the wanting to cheat physics part?

Well mostly with process guys or purchasers of fans thinking that silencing the open inlet or outlet of their noisy fan (say by 20 dBA) will reduce the overall level around the fan by the same amount.

John, my noise specialist friend, has a nice way of conveying this. He says, “Think of noise escaping from a fan like water coming from a pail with a bunch of different diameter holes in it. Big holes, lots of water; smaller holes, less water.”

So if the casing TL is only 15 dBA, then that’s the reduction you get. And less if you don’t have a good shaft seal.

Then there’s the flex connection breakout at the fan discharge, duct breakout noise, not to mention the motor and belt noise. Low noise motors are usually rated at 82 dBA unloaded. When they are driving the fan they can easily exceed 85 dBa, a very common benchmark point.

On top of that one has all the other noise sources around the fan to consider.

So when someone puts a silencer on the inlet (FD Fan), or on the outlet (ID fan) which is the equivalent of sealing the biggest hole in the “thought-experiment bucket” the little holes will still keep pouring water out.

That doesn’t mean a silencer is a bad idea. It just means, one should understand the other contributing factors, and what the likely overall noise reduction outcome will be, before choosing how big to make the silencer. And also if you are going to treat the fan what other areas you might need to look at.

So remember look at all the ‘holes’ and it’s best to take care of the big ones first, but don’t neglect the impact of the little ones either.

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