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The Basics of Fan Sound

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The Basics of Fan Sound Fan Application ® product FA/121-03 application guide A technical bulletin for engineers, contractors and students in the air movement and control industry. The Basics of Fan Sound Fan sound is a very important consideration in the amplitude or amount of sound energy reaching our selection and application of fans. In spite of this, fan ears. Pitch is the relative quality of the frequency sound continues to be one of the most content made up of pure tones as well as misunderstood topics in the air handling industry. broadband sounds. We typically use the pitch to identify the source of a sound. However, both the In an effort to provide a better understanding and loudness and pitch may vary depending upon point of reference on how fan sound is developed, where we are located relative to the sound and the rated, applied and controlled; this is the second surrounding environment. article in a series of four articles covering this topic. What is fan sound? Part 1 Understanding the Development of Fan Sound Data and the Product Line Rating Process (FA/120-02) Fan sound represents a characteristic combination of frequencies made up of many different Part 2 The Basics of Sound (FA/121-03) individual components. It is a by-product of many Part 3 Radiated Sound (FA/122-03) different aerodynamic mechanisms going on inside the fan. Some of these include vortex shedding, Part 4 Sound Criteria, Attenuation Techniques and eddy formations, turbulence and discrete tones Preventive Measures to Limit Sound Problems such as the blade frequency. There are also various (FA/123-03) combinations of mechanical sound coming from This article discusses the nature of sound, sound drives, motors, bearings etc. All of these terminology, different methods of rating fans for logarithmically combine to form a sound spectrum sound and typical calculations that most people recognizable to the ear as being a fan. take for granted due to computerization. This concept is illustrated in Figure #1. The lower What is sound? sound spectrum is one-third octave band sound level data from a twenty-four inch airfoil fan. Note We are all aware that energy comes in many that there are individual peaks that are prevalent at different forms; light, heat, electrical, nuclear, sound various frequencies. These peaks correspond to the etc. However, unlike the others, sound is sound contribution of individual components such characterized as a form of energy resulting from as the blade frequency, motor, drives or even a vibrating matter. As the matter vibrates, it creates panel resonance from the scroll. It is these tones waves in the surrounding medium (air, water, metal that our ears characterize as fan sound. Please etc.) that have alternating compressions and remember, fan sound comes from a number of rarefactions. In air, this represents a very small sources, aerodynamic as well as mechanical. change in the barometric pressure to which our ear drums react. Our ears distinguish one sound from By convention, fan sound is presented on an octave another by its loudness and pitch. Loudness is the band basis in accordance with ANSI standards. This ® P.O. Box 410 • Schofield, WI 54476 • 715.359.6171 • Fax 715.355.2399 Copyright © 2003 Greenheck Fan Corp. Greenheck Product Application Guide makes dealing with sound less Figure 1 Fan Sound Power Levels (dB) Installation Type B, Free Inlet, Ducted Outlet burdensome by using eight 24 Airfoil Single Width numbers versus twenty-four to 4100 CFM, 1.4 SP, 870 RPM, 60% WOV 90 define the frequency spectrum. Full Octave Band 1/3 Octave Band However, when one-third octave 85 band sound values are 80 logarithmically added together into octave bands, the resulting 75 sound level is higher but the 70 individual peaks are not 65 graphically identifiable in the new Sound Power Level (dB) spectrum. This does not mean that 60 they are no longer there. Your ear 55 will still hear the tones, but the method of presentation smooths 50 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 8300 8000 10000 over the appearance of individual Frequency component contribution in the independent of the distance from the source or the data. The octave band data can be further simplified environment in which it is located. As an example, a to a one number system such as LwA or dBA, sones sixty watt light bulb consumes sixty watts no matter etc. However, with each simplification in the where it is screwed in. Fan sound power is the method of presentation, the identification of specific same. components is further reduced. Fan sound power is determined through tests Defining fan sound conducted in accordance with AMCA Standard 300, It is easier to understand fan sound using the “Reverberant Room Method for Sound Testing of graphical format illustrated in Figure #1. Using this Fans”. Test results are provided in sound power figure as a reference, each of the rating parameters levels in dB referenced to ten to the minus twelve will be discussed. It is important to understand the watts in each of eight octave bands. This is the concepts presented since they apply equally well for sound produced by the fan at its source and is all fan sound data, not just for this specific example. independent of the fans environment. Sound power versus sound pressure Sound pressure levels represent the energy a The difference between sound power and sound microphone or our ears would receive and depends pressure is critical to the understanding of this upon the distance from the source as well as the subject. Most industrial and commercial ducted fan acoustical environment of the listener (room size, catalog data is presented in sound power levels in construction materials, reflecting surfaces, etc.). each of eight octave bands. However, some Sound pressure levels are provided in dB referenced commercial and residential non-ducted catalog to the microbar (.0002) or twenty micropascals. sound data is presented on a sound pressure basis Installation type using a single number rating system such as dBA or Published sound ratings are presented at the fan sones. inlet, fan outlet or total sound power for the Energy for light, heat, electrical and most fan sound following installation types. It is important to know is provided referenced to the watt. The power the installation type that best matches the actual produced by a light bulb, a heater or a fan are an application because sound levels are not the same indicator of the power produced by the source for each installation. ® 2 Fan Application No. FA/121-03 Greenheck Product Application Guide Installation Type Configuration and commercial fan equipment. A third single A Free Inlet, Free Outlet number system is used for non-ducted propeller B Free Inlet, Ducted Outlet fans and power roof ventilators called the sone. A C Ducted Inlet, Free Outlet sone is a term of loudness perceived by the ear D Ducted Inlet, Ducted Outlet related to a frequency of 1,000 Hz. The sone is a sound pressure term at a distance of five feet from Fan designation the fan and is linear to the human ear. The size and design of the fan must be identified. Calculating sones from sound pressure level is Fan rating outlined in ANSI Standard 3.4. A loudness index is The AMCA Certified Ratings Program has a seal for obtained from a graph or calculated using a formula “air” and a seal for “air and sound”. A sound rating in the standard. The total loudness is calculated cannot exist by itself. It must have a corresponding from another formula. (See example 1.) The aerodynamic rating because sound is a function of application of sones is outlined in AMCA the fan rating point. Publication 302. Loudness/ amplitude Frequency Because sound loudness is referenced to very small Frequency is the number of pressure variations per numbers and there also is a very wide range, it is second expressed in Hertz. One cycle per second much more convenient to use the decibel. The equals one Hertz. The human ear can perceive decibel is a dimensionless number expressing in sound between 20 Hz. and 20,000 Hz. However, fan logarithmic terms the ratio of a quantity to a sound is dominant between 50 Hz. and 10,000 Hz. reference quantity. As an example, one dB Therefore, there is no reason to deal with represents the threshold of hearing. frequencies outside of this range. Sound Power (dB) = This frequency range for test purposes has been 10 log (sound power [watts]/10-12) divided into twenty-four individual bands called one-third octave bands. Three one-third octave Sound Pressure (dB) = bands when logarithmically combined together 20 log (sound pressure [microbars]/.0002) form an octave band. An octave band is the interval Sones between any two frequencies having a 2:1 ratio. As An alternative loudness descriptiont is sones. Sones an example, the center frequency for the first octave follow a linear scale, that is, 10 sones are twice as band is 63 Hz. The center frequency for the second loud as 5 sones. Use the following formula to octave band is 125 Hz, third is 250 Hz. and so on up convert sones to decibels. to the eighth octave band with a center frequency of 8000 Hz. The abscissa of Figure #1 illustrates the dBA = 33.2 Log10 (sones) + 28, Accuracy +/- 2dBA relationship between band numbers and frequency LwA and dBA respectively are sound power and for both one-third and full octave bands. sound pressure rating systems for most industrial Example 1 Calculation of sones AMCA octave band no.
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