Technical paper: What you need to know about fan noise

Dr. Sergey Karadzhi, Research & Development Engineer, Systemair, writes on the importance of sound in fan development, active sound reduction and what to keep in mind when choosing the best fan for your projects.

Why noise levels determine the quality of a fan

Fans can be found everywhere in our life. It does not matter where you go. Most likely, fans are working nearby. Sometimes it disturbs you and reduces your productivity level and overall comfort. Sometimes you hear no noise from fans, yet they can still influence you. For example, low-frequency noises, not detectable by human ear, could lead to fatigue and exhaustion or even cause partial deafness. Therefore, during fan development, we pay special attention to fan noise and consider noise levels a key parameter in the first steps of the design process.

Fan noise levels could indirectly determine how well the fan is designed. If we will not consider additional sound absorption and look at a poorly designed fan, it features many areas with a separated flow and low efficiency. For example, if a fan has an efficiency of only about 60%, with 40% accounting for losses. These losses result in heat, but a part is emitted as a noise. And the less efficient the fan, the more noise it emits.

A holistic approach to fan noise reduction

However, fans are only a part of a ventilation system. Even if the fan is very silent as a separate element, it is essential to consider noise during the design of the entire ventilation system. First, it is crucial to minimise aerodynamic losses in the system. The reason is that any pressure loss in the system is a vortex, and any vortex creates noise. Moreover, you need a bigger fan for higher pressure losses, which creates more noise. Secondly, the fan should be appropriately selected, meaning the type of fan and working point must be near the maximum efficiency. If the fan is working far from the design point, separation of flow appears in the blade channels, and any separation of flow causes additional noise, which could be avoided.

In addition, if the fan is working on the left edge of the aerodynamic curve, it can have increased low-frequency noises and vibrations. Finally, recommendations for optimal fan installation should be fulfilled, ensuring a uniform profile on the fan’s inlet. In other cases, uneven distribution will lead to unstable fan work, increasing noise levels and reducing longevity.  Of course, other issues can lead to noise increase, such as bad balancing of the impeller and poorly designed system elements, among other factors.

Active sound reduction using the interaction of two fans

Different developers try to use active noise reduction for fans. In short, active noise reduction is when you are recording the noise from the source, shifting the phase and emitting noise with shifted phase to cancel the source noise.  It is a highly complex method because phase shift depends on distance, so that you can reduce noise only in certain areas.

There are cases where such an approach has been successfully implemented. For example, in spaceships, where ventilation is also critical because there are not so many sound sources, noise from ventilation are typically very disruptive to astronauts, especially when they sleep.  In this case, active noise reduction can be used as it eliminates noises only in certain areas, such as in the sleeping quarters of the astronauts.

Active noise reduction can be used in ventilation, but some issues must be considered.  For example, if the active noise reduction method is used for a fan in the channel, it can happen that after reducing noise on the outlet of the fan, fan inlet noise increases. In addition, microphones and loudspeakers can change their parameters with time, and this will lead to a decrease in sound reduction.

When to use active noise reduction methods

In the case of typical ventilation applications, it is usually too expensive to use active noise reduction because of the additional electronics required. That is why passive methods are commonly used, such as tube silencers. However, installers should also be careful when applying this method because sound reduction for silencers, which you can see in the catalogue provided by the fan manufacturer, is usually measured using a broadband noise source. In reality, the noise spectrum coming from the fan is very different. Thus, the modal structure of the sound field is different and sound reduction from a silencer in the real world will be lower than when using a flat or spherical wave source.

I was considering a situation where two fans stand near each other in my work. This situation is common in tunnels, where several tunnel fans are standing near each other to create additional thrust in the tunnel, usually in case of fire. If two fans are identical, which is typically the case, we can consider one fan as a source and another as a sound emitter for active noise reduction. Sound waves on blades passing frequency for one fan can be shifted to cancel the sound levels of another fan. In this case, we can reduce costs because no additional sound source is needed. Yet, this method is still rather complicated, and you should check each case’s interference picture. Thus, although it is not an approach that will be widely used, it can be helpful in specific projects.

These days, more computational methods are successfully implemented to predict fan sound levels. I would expect a very high level of implementation of these methods in the development process of fans in the nearest future in all leading companies. For axial fans, a good way of sound reduction is, of course, using skewed blades, and for radial fans in scroll casing, I would mention different shapes of the tongue and its sound absorption in it.

What should you keep in mind when choosing your fans?

When looking at the fan, the easiest thing to check would be the impeller blade quantity and quantity of guide vanes or support plates. If the number of blades/plates is divisible or multiple of the number of motor poles, it is the first sign that this fan can have increased tonal noises. In addition, for axial fans, it is essential to check the gap between blades and casing and for radial fans, the gap between the inlet nozzle and impeller. Big or wrong gaps indicate the fan with poor aerodynamics and increased noise levels.

After this, customer can also check the efficiency levels of the fan in the catalogue. If the fan has low efficiency, you can also expect higher fan noise levels. In most cases, sound levels are also mentioned in the catalogue, but it is always preferable to check how it was measured. The given values are trustworthy if the company has an ISO-accredited lab or the measurements were made at some reliable certification centre, such as AMCA. Otherwise, it would be better to check some samples to be sure.

In summary, remember to check:

• The impeller blade quantity
• Quantity of guide vanes or support plates
• For axial fans, check the gap between blades and casing
• For radial fans, the gap between the inlet nozzle and impeller
• The efficiency levels of the fan in the catalogue
• If the fan was measured in a reliable and trustworthy manner through an ISO-accredited laboratory or certification centre
• Samples of the products themselves