When I started in this business 20 years ago, noise wasn't really an issue. Now, it's becoming a more prevalent concern in our industry. In fact, the World Health Organization classifies noise as a type of pollution. The equipment that we provide institutions such as hospitals, universities, and pharmaceutical companies, must be as quiet as possible. These buildings are located in urban areas, so as design engineers and fan manufacturers, we have to create equipment that meets very strict criteria.
Fan manufacturers (including ourselves) test equipment not only for air performance, but for sound. We send the equipment to a test lab for certification. In our case, we send it to the Air Movement and Control Association (AMCA). The results will give the subjective noise of the equipment.
It's up to the design engineer to decide what's a permissible sound level for the particular project the equipment is being used for. It could be a simple case of choosing the right type of fan, since various fans have different sound characteristics. (Some are noisier than others.) Propeller fans tend to be noisy, while centrifugal fans with airfoil wheels tend to be quieter. When a project requires that you stick with a particular design of fan and there is a sound issue, you can decide which options to take advantage of in order to reduce the sound.
One option is to choose a fan that's one or two sizes bigger in order to reduce the speed, and consequently, the noise. You can usually reduce it by a few decibels, which might make enough of a difference. Otherwise, another common and straightforward approach is to use some form of sound attenuation. (These are essentially silencers.) A good analogy that's somewhat similar is the muffler you have on your car.
Attenuators come in all shapes, sizes, and materials. They are essentially a double-wall barrel with a perforated liner on the inside and acoustical media between the inside and the exterior wall. These silencers go either on the outlet or the inlet of the fans and they can actually reduce the equipment noise anywhere from 10 dB all the way up to 30 dB.
Like with most things, the more you spend and the bigger the attenuator, the more noise reduction you can achieve. Some attenuators are very small, weighing a couple hundred pounds. Others are so big and heavy that they need additional structural support off of the fan to be able to support the weight.
The best advice I could give for any end user, design engineer, or architect is to address the sound issue before you buy the equipment. Know what the sound requirements are for your application beforehand. Once the equipment is onsite and up and running, it will be very difficult and expensive to fix noise issues if neighbors around the building start to complain.
Of course, it's impossible to have perfectly noiseless equipment, since these are mechanical objects. However, we try to make our products as quiet as possible, just like any other fan manufacturer.
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