Quality Control

FAN NOISE & YOU

In modern society we are exposed to noise from all sorts of sources. Is it any surprise that we try to find a nice, quiet sanctuary more and more often?

When we relate this to fans for moving air, we have a dilemma. The more air we move and thus the faster we move it, the more noise we create. Modern technology has come a long way in reducing the noise created by airflow devices, but there is a limit. Moving air will always generate some noise.

Gas flow dynamics technology has developed to a point that further noise level reductions are unlikely for everyday axial flow fans (propeller type fan blades) as used in Suncourt products. There are alternatives to axial flow fans, but generally the cost of these alternatives make the application unattractive for the products of Suncourt and others manufacture due to high cost and packaging limitations.

But what is noise in the first place? As is the case with light, noise is a form of energy, which is expressed in a unit of energy, the Watt. But, since sound energy is expressed as the number of Watts per unit of surface area (m²), we need a more usable unit to express what our ears really sense.

So the dB was developed. The dB is a measure, not a unit. While this is not a textbook on sound, it is nice to know that the dB is a measure developed on a logarithmic scale. This allows us to express sound levels on a narrow defined scale rather that go from very small numbers to huge long numbers.

What this logarithmic scale does is shown below:

If the energy level is 10, the dB measure is 10 dB

If the energy level is 20, the dB measure is 13 dB

If the energy level is 100, the dB measure is 30 dB

So, 20 dB is not twice as loud as 10 dB. Because a logarithmic scale is used, every 3 dB increase in sound level doubles the perceived sound level for humans.

Remember, 30 dB is only a number. To give the dB a true measure of energy, it should be expressed with reference to the number of Watts of energy. The expression is dB re xW. This qualifier is normally not given when speaking of dB's. More correctly we would give a Sound Pressure Level (SPL), referred to the human ear. It is the sound that falls on our ears that we are interested in. The sound that falls on our ears is a measure of energy per cm². (The eardrum is approximate 1 cm²).

To make a long story short the dB measure gives a good relative measure of noise levels that human perceive. Since the human ear does not perceive the energy level of noise the same for different frequencies of sound, the dB is corrected for the typical sensitivity of the human ear. The human ear is most sensitive at sound frequencies of 3000 to 4000 Hertz (Hz). At low sound levels, the human ear is more sensitive to high-pitched tones than to low-pitched tones. At very high sound levels the human ear is virtually equally sensitive for all tones. The corrections are based on curves developed by Fletcher & Munson and Robinson-Dadson for pure tones and the scale based on these curves is normally labeled dBA.

While other sound level measures are being used, based on either more sophisticated or sometimes "iffy" scientific criteria, such as Phones or Sones or NC Curve corrected, the dB, referred to the Watts per area of surface gives a true traceable measure of energy.

Are you still glad you asked?

Below is a table that gives noise levels common in everyday life. You may compare the sound of Suncourt products to this table and see what you may expect. Of course, remember that the sound levels for our Inductor fans are given without being installed in a duct system. Generally, this will attenuate the sound level even further. For particularly sensitive people, sound levels can be attenuated to even lower levels by installing a Suncourt DucTMuffler.

Suncourt fan products are quiet and will not normally interfere with everyday life. Our published noise levels are measured in dBA at a distance of 6 feet, the industry standard.

The design criteria applied to Suncourt fans are: A quiet motor bearing system, a well balanced motor and fan impeller as well as providing the required air movement at as low a RPM as possible.

There is a benefit for slight background sound of the nature produced by Suncourt fans. The fan typical sound frequencies fall in the range that is referred as "white noise". White noise has the ability to mask environmental noises and is often prescribed to combat insomnia. Many devices are sold that generate white noise such as rain, falling water or wind sounds. A small fan is just as effective in providing a helpful background to mask sounds like a creaking house, traffic, ticking clock, etc.

Sound Level in dBA Source
140 Threshold of pain, gunshot, siren 
135 Jet taking off, rock concert
120 Chain saw, jackhammer, snowmobile
100 Tractor, power saw
90 Inside acoustically insulated tractor cab
82 Inside coach section of typical passenger jet
80 Busy office, typical Home Center store on busy day
75 Average radio, vacuum cleaner
60 Normal Conversation
52 Typical desktop computer
55 to 45 Suncourt fans, installed, depending on model. The DB8GT is 68dBA
45 Rustling leaves, soft music
30 Whisper
15 Threshold of hearing

 

Fans and Resonant Frequencies

Larger fans, 10" diameter and up, may, on rare occasions, be sensitive to resonance. The RPM that the fan runs at can vary from 1200 RPM in free air to 3000 RPM when boosting an existing airflow. Thus the actual RPM of the fan can be anywhere in this range. Rarely, but is does happen, there are RPM's that cause resonant vibration. This is most common in applications with low back-pressure where the fan runs at or near peak RPM or when boosting a high velocity existing airflow. There is a simple solution to this problem that the installer can do on the models with Aluminum fan impellers.

A slight change in the pitch of the fan blade will change the RPM at which the fan runs in your particular application. The fan blade pitch (angle) can be changed (increased) by bending the fan blades vanes slightly and equally.

After making a vane pitch correction, not to exceed 3/16", spin the fan blade by hand to make sure all blades have been adjusted the same. Uneven pitch will cause unbalance and vibration.

If you cannot make this adjustment yourself, please contact Suncourt for a solution.