Effects of Grill Patterns on Fan Performance/Noise

Introduction

Fan grills are often underestimated in terms of their contribution to both fan noise and airflow. It would stand to reason that the more raw material present on the grill would cause a more restricted airflow, which in turn would cause a drop in airflow and a rise in noise. In our experience however, the shape and pattern of the grill can have a huge impact on noise due to the different types of turbulence that can be caused.

While the number of possible grill designs is essentially limitless, we will be testing some of the most common designs that come included with fans and chassis, as well as some popular after-market designs. A few of these grills can also found cut into window panels or stamped directly onto the metal body of a chassis itself by the manufacture.

The goal of this article is to determine exactly how much a grill design contributes to both the noise and airflow of a fan.

Test Grills

For our testing, we will be using six different patterns cut out of 3mm acrylic as well as three common metal grills that often come included with fans/chassis. The grill patterns for each are shown below:

Concentric Circles. Simple design, used widely in the industry.	Angled Slats. Often found on pre-installed fan mounts for acrylic chassis windows.	Honeycomb. Very common on chassis where the grill is stamped directly onto the chassis frame.
Swirl. Found on some chassis fans, common on large household fans. Fragile due to the thin arms.	Loose Swirl. Variation of the swirl with larger open spaces.	Turbine. Uncommon, but shows up sometimes on budget chassis.
Mesh. Very restrictive, often doubles as a loose filter.	Stamped Circles. Variation of the concentric circles with the grill slightly raised above the fan	Wire. Most common after-market grill with the grill slightly raised above the fan.

For all of our testing, we will be using an Antec 120mm Tri-Cool on both the low and high settings. This will give us both low and high airflow tests without the need to change the fan between tests.

Noise

We have found that the additional noise from grills is most prevalent on the intake side of the fan, so noise measurements will be taken in that configuration. The dBa meter was place at a 90 degree angle from the fan at a distance of two inches. This is much closer than dBa readings are normally taken at, but we expect the difference between some of these grills to be minimal so we wanted to make sure the changes were measurable. Because of this, our measurements will be in no way comparable to the manufacturer's specifications of noise.

Results are reported in both the raw dBa reading as well as a percentage over the reading of the fan with no grill.

*The ambient environment is 29 dBa

Low fan speed

Quietest	No Grill	35.5 dBa
	Wire	36.0 dBa (1.4%)
	Mesh	36.0 dBa (1.4%)
	Swirl	36.0 dBa (1.4%)
	Loose Swirl	36.4 dBa (2.5%)
	Stamped Circles	38.0 dBa (7.1%)	Difference in noise level starts to become noticeable
	Honeycomb	38.0 dBa (7.1%)	Grill starts to give a high-pitched "whining" noise
	Concentric Circles	39.8 dBa (12.1%)
	Angled Slats	40.5 dBa (14.0%)	Whining noise becomes very pronounced
Loudest	Turbine	50.2 dBa (41.4%)	Whining noise is more noticeable than the fan itself

High fan speed

Quietest	No Grill	55.0 dBa
	Mesh	55.3 dBa (0.5%)	Technically the quietest grill, but it raises the pitch of the fan noise making it more noticeable
	Swirl	55.6 dBa (1.1%)
	Wire	55.8 dBa (1.5%)
	Loose Swirl	56.3 dBa (2.4%)
	Stamped Circles	57.8 dBa (5.1%)	Whining noise starts to occur
	Honeycomb	58.0 dBa (5.5%)
	Angled Slats	61.6 dBa (12%)	Whining noise becomes very pronounced
	Concentric Circles	62.0 dBa (12.7%)
Loudest	Turbine	71.0 dBa (29.1%)	Whining noise is again more noticeable than the fan itself

From a noise standpoint, what exactly do these results mean? Basically, you should avoid concentric circle, slat, honeycomb and turbine style of grills if possible. The mesh, wire and swirl grills were all clearly the best performers in both the low and high airflow tests with dBa readings within 1% of each other. Likewise, the turbine grill is the loudest grill by a healthy margin; increasing dBa readings by 29.1-41.4%

The remaining question now is how much these grills are restricting the airflow of the fan. If the best grill in terms of noise is also is the most restrictive, it makes that grill much less desirable for use in a computer system.

Airflow

To measure the changes in airflow, we used an anemometer (Extech AN100) with a custom adapter to adapt from our 120mm test fan to the smaller 80mm diameter of the anemometer sensor. The adapter is 200mm long, giving us a smooth, gradual slope from the 120mm side to the 80mm side. Using this type of adapter means that our CFM readings will be nowhere near the manufacturer's specifications, but should do a good job at giving us comparison readings. It will also be more real-world as a fan installed into a chassis is never going to perform at the manufacture rated open-air CFM levels.

We will again be testing with the fan on both low and high fan speeds, as well as testing with the grill on both the intake and exhaust sides of the fan. The results are listed below largely in order of best to least airflow. Due to some variances between the intake and exhaust results however, they are not 100% in order for each configuration.

Results are reported in both the raw CFM reading, as well as a percentage of the airflow based on the CFM reading of the fan without a grill.

Conclusion

The wire grill is easily the top choice in terms of both noise and performance. There are a few points where it was not the top result, but in those cases it was close enough that it was within our estimated margin of error.

The mesh grill, while being one of the quietest grills, is also one of the poorest in terms of airflow. Interestingly, this grill is the only one whose results deviated largely between the noise and performance sections. This is likely due to the fact that it is a much different style than the other grills we tested. Instead of having relatively large frames and openings, the mesh grill has a lot of smaller openings, but placed very close together. So while it is very restrictive, it does not create the same type of turbulence noise that the other low-performing grill designs such as the angled slats and turbine.

And lastly, the turbine grill is absolutely horrible; dropping airflow by ~15% and increasing fan noise by as much as 41% when the fan was on high.

Taking both noise and airflow into account, here is our final list of grill designs in order of best to worse:

1. Wire
2. Swirl
3. Loose Swirl
4. Stamped Circles
5. Honeycomb
6. Mesh
7. Concentric Circles
8. Angled Slats
9. Turbine