How Loud are Fans and How to Address Noise

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Industrial fans usually aren’t as loud as Packers fans at Lambeau Field, but they’re often loud enough to require mitigation to stay below a specified level.
Noise is a factor with rotating industrial machinery like industrial fans. Sound level depends on the type, size, and speed of the fan, and remedies depend on your specs. Looking for more information?
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How loud are industrial fans? There are four factors that go into that. And the short answer to the question is, they’re pretty loud. These are rotating pieces of equipment, so you’re going to have a noise that comes off of them as they’re rotating. But the four things that go into how loud it is are the type of fan, the size of the fan, and the speed at which that fan is moving. And the fourth thing is the perception of the hearer.
The fan that we’re standing next to looks kind of small. It’s smaller than me here, it doesn’t look like this one would be that loud, but when it gets cranking up to 3600 RPM based on what the motor is rated for, it will be approximately 98 decibels at 5 feet, based on Q=2 conditions, that’s free-field conditions.
In most manufacturing settings, what you’re going for through an 8-hour workday, is about 85 decibels at 5 feet. Your conditions may vary, we’re just using that as an example in this short video.
So if we want to get this fan that would be 98 decibels at 5 feet if you had one side open. So let’s look at the two sides. You have an inlet here and an outlet here. Typically a fan is either ducted on one or the other, or it’s ducted on both. So if you are ducted only on the inlet, and this outlet is open, then if I’m standing 5 feet back this way, I’m going to hear 98 decibels coming out of this outlet. If you need 85 decibels at 5 feet, you need to address this sound problem. So the way that you’re going to handle that off of an open outlet or an open inlet condition is by putting a silencer on it. A silencer is just an extension that’s going to duct to this that is going to muffle the noise sort of like a muffler in your car so that your perceived hearing from the outlet of that silencer will be less than 85 decibels.
So what if we have ducting on the outlet and on the inlet? Now we have what’s called a radiated noise level. And so the radiated noise level is this rotating machinery and what comes off of the housing, what comes off of what’s in here as it’s moving. And so this is independent of whether something is connected on the inlet or the outlet, it’s what energy the fan is putting off in terms of noise. For this specific fan at 98 decibels at 5 feet, you have an open outlet and an open inlet. If you close them both off, it’s going to reduce our radiated noise level to about 80 decibels at 5 feet. Now maybe that works for your condition. But maybe you need something that’s 70 decibels at 5 feet. Well if this radiated noise level is going to be 80 decibels at 5 feet without doing anything to address it, we have to address it. This fan is being addressed by sound insulation.
So the way to address radiated noise level is by increasing the thickness around the wheel that’s rotating inside of it. It may be that you need very little difference in noise level, and so you maybe increase the thickness of the housing by a little bit. It could be in this case that you need a lot of noise reduction, and so you put a 4 inch sound insulation jacket on it like this fan, which will reduce our expected radiated noise level by about 14 decibels, getting you below that threshold that you need for your application.
That’s the fan. The last noise source to consider is the motor. Typically motors are going to be below that 85 decibels at 5 feet that we talked about. But it’s possible if you need something like 65 decibels at 5 feet in your shop, that you’ll need to build a sound enclosure that houses the entire fan so that you also reduce the motor noise.
So to recap: fans are loud machines. Oftentimes you need to reduce the noise that’s being put off by the fan. If you have an open outlet or an open inlet and the other side is ducted and you need to address the noise, you address it by a silencer. If having both sides ducted or having a silencer and the other side ducted still isn’t enough, you need to address it by increasing the thickness around your housing by either sound insulation or increasing the thickness of your housing material. If that still isn’t enough, your motor is probably adding to the problem of noise and you need to build an enclosure around the entire unit.
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