How to Measure Fan Noise
Fan Noise – Some Sound Basics
Sound is a physical occurrence. Sound is also what we hear. Interestingly, “sound happens” whether or not anyone is around to actually hear it. Fortunately, sound is governed by natural laws so we can measure, predict, and use it to our advantage just like the fan performance is governed by the Fan Laws.
So, what’s the difference between sound and noise? Simply stated, noise is unwanted sound. Noise is relative to the ear of the listener. I may like classic rock & roll music, but to someone else—it’s just noise.
Determining how noisy a fan is and what to do about it is not as straightforward as one might think. Aside from the design of the fan, speed of the fan, pressure the fan develops, volume of air going through the fan, there are external factors such as the environment in which the fan operates. For example, a fan operating inside an uninsulated building is going to seem much noisier with its sound bouncing off the walls like reflections off a mirror than the same fan operating in the middle of a Texas cattle ranch.
There are four (4) very common terms used in acoustics:
Decibel (dB) = one-tenth of the logarithmic unit (Bel) by which noise is measured and evaluated
dBA = A convenient single number low frequency weighted dB scale for judging the loudness of a sound that more closely correspond to the threshold of hearing of the human ear.
Sound Pressure Level (Lp) = the logarithm of the ratio of the actual pressure to a common reference value of 0.0002 microbars
Sound Power Level (Lw) = the logarithm of the ratio of the actual power of the sound to 10(-12) watt reference.
Sound Power Level (Lw) is like the sound fingerprint of a fan. It cannot be changed—it is constant. Sound Pressure Level (Lp), on the other hand, changes depending on the environment in which the fan operates.
In order to determine fan noise, Sound Power Level (Lw) is what the acoustic engineer needs—something that is constant all the time no matter what the fan’s operating environment might be.
However, there is a problem measuring the Sound Power Level (Lw) of a fan. In general terms Sound Power Level (Lw) cannot be directly measured. The only practical way to arrive at a Sound Power Level (Lw) is to take Sound Pressure (Lp) readings at each of the eight octave band center frequencies in a laboratory setting. Then we would back calculate the sound power level at a specific distance used for the sound pressure level measurement:
Fan Sound Power Level (Lw) = Fan Sound Pressure Level (Lp) + Delta (Room Effect)
Once we have this measurement, we can apply silencing techniques, insulated wraps, sound enclosures, or whatever is required in order to reduce the noise of the fan. We can even add noise from other equipment to these values to determine the combined noise effect and then how to silence that down to an acceptable level.
There is an old axiom that holds true when it comes to reducing fan noise: “Quiet is expensive.” However, you are always better off to start with a quiet fan to begin with than to try and silence a noisy fan.
Also, it’s important to keep in mind we are dealing with logarithms here, so an increase of only 3 dB will effectively double the intensity of the noise. Which fan would you want? Fan A that is 81 dBA at a distance of 15 feet or Fan B that is 84 dBA at the same distance? Not much difference decibel-wise, but in reality, Fan B is will sound 2 times louder than Fan A!
We at Eldridge work with fans and sound every day when we are engineering and solving industrial and marine ventilation and noise control problems. Let us help you solve your fan and airborne noise problem.