Bouncing Off Walls: Industrial Fan Noise Directivity Specification

What is Industrial Fan Noise Directivity?

Industrial fan noise directivity is an indication of how the fan surroundings contribute to the sound level in the field. It is a small but important factor in specifying the right accessories, insulation, housing, and enclosures for noise reduction in your application.

Because proximity to reflective surfaces amplifies sound, where you put your fan makes a huge difference in how loud it sounds to the listener. That’s why we look for an Air Movement & Control Association (AMCA) industrial fan noise directivity as a component of your specification. AMCA Publication 303-79 (Application of Sound Power Level Ratings for Fans) outlines four directivity levels: Q=1, Q=2, Q=4, and Q=8.

The numbers indicate the directivity factor based on the number of reflecting surfaces within a specified distance, doubling with each additional surface. A June 19, 2017 article Understanding Fan Acoustics published in Processing Magazine explains why industrial fan noise directivity is relevant for specification:

“While dBA [A-weighted decibels] is often used for evaluating the overall sound level at a specific location since this site measurement includes all local noise sources, it cannot be directly used to determine the actual fan sound level.”

What the Directivity Ratings Mean

This is a very simple component to determine your industrial fan specification. There’s no special measurement – you just need to know where the application will be situated within your facility.

Directivity Q=1 (Q1)

This defines the most rarely occurring situation in an industrial setting in which the centrifugal fan is suspended in air with no reflecting surfaces (floor, walls, or ceiling) within the specified radius (usually five feet).

Directivity Q-2 (Q2)

Q2 indicates one reflective surface, the floor on which the fan is installed. That means the application is in an open area away from all of the walls. This is the assumed baseline if there is no directivity specified.

Directivity Q=4 (Q4)

This may be the most common industrial fan noise directivity rating specification. Q4 means you’re up against a wall. Literally. In this case, there are two reflective surfaces within the specified proximity, usually the floor and one wall, where the system is plugged in for power.

Directivity Q-8 (Q8)

Finally, there are times when your fan gets put in a corner. That doesn’t mean it’s behaving badly, but it does mean the noise level you’ll need to account for is higher, with three reflective surfaces (the floor and two walls) nearby.

How to Specify Industrial Fan Noise Directivity

The higher your specified industrial fan noise directivity rating, the louder the sound will appear to the listener. As mentioned, if you don’t specify then most fan manufacturers will assume Q2.

Be careful to check your application so that as you’re considering the adjustments necessary for actual noise level, you can either factor in adjustments from Q2 to your own conditions, or specify the correct rating for your situation. A specification including the directivity rating would read something like “85 dBA with Q2 directivity at five feet.”

Hear it from an Application Engineer

Senior Application Engineer Chet White demonstrates industrial fan noise directivity levels in this two-minute video.

When you need to specify a centrifugal fan or blower for your industrial process application, or if you need centrifugal fan or blower maintenance, repair or retrofit, reach out and connect with one of our application engineers to discuss the details of your project.

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