Loudness of a fan is not always its dB<base href="http://www.atruereview.com/" /> <link rel="shortcut icon" href="http://www.atruereview.com/images/favicon.ico" /> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <link href="http://www.atruereview.com/templates/rt_versatility_iii/css/template_css.css" rel="stylesheet" type="text/css" /> <link href="http://www.atruereview.com/templates/rt_versatility_iii/css/style10.css" rel="stylesheet" type="text/css" /> <link href="http://www.atruereview.com/templates/rt_versatility_iii/slimbox/slimbox.css" rel="stylesheet" type="text/css" /> <link href="http://www.atruereview.com/templates/rt_versatility_iii/css/rokmoomenu.css" rel="stylesheet" type="text/css" />  <link rel="shortcut icon" href="http://www.atruereview.com/images/favicon.ico" /> <style type="text/css"> td.left div.moduletable, td.right div.moduletable, td.left div.moduletable-hilite1, td.right div.moduletable-hilite1, td.left div.moduletable-hilite2, td.right div.moduletable-hilite2, td.left div.moduletable-hilite3, td.right div.moduletable-hilite3, td.left div.moduletable-hilite4, td.right div.moduletable-hilite4, td.left div.moduletable-hilite5, td.right div.moduletable-hilite5, td.left div.moduletable-hilite6, td.right div.moduletable-hilite6, td.left div.moduletable-hilite7, td.right div.moduletable-hilite7, td.left div.moduletable-hilite8, td.right div.moduletable-hilite8 { width: 175px; 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While this is partially right, it is not the full truth. This article is intended for anyone who wants to learn what I researched online about how sound is measured and what I have learned from trying different fan types.</p> <p align="left"><b>How to measure in decibel (dB)</b></p> <p align="left">Let me first explain what a dB is and how it is measured.</p> <p align="left">Decibel is a measurement unit that was named after Alexander Graham Bell, and means 1/10th of a bell. dB expresses differences in intensities between two levels of sound. The most common level units for measuring dB are sound pressure level, voltage, and power.</p> <p align="left">Decibels are based on a logarithmic scale. This means that as decibels increase and the value of "loudness" increases exponentially. </p> <p align="left">The value of a log is the exponent in the equation NUMBER = 10^exponent; thus, LOG(2) = .3 and 10^.3 = 2.</p> <p align="left">To get a feel for the difference in power (sound) of a fan, you can use the equation dB = 10 * LOG(Power 1/ Power 2). Hence if you have a 3dB difference between two fans then the power is doubled 10 * LOG(2) = 3dB. This does not mean that the sound you will hear is doubled.</p> <p align="left">Sound is normally based off of a reference level of 20dB. 0dB indicates that there is no change between the device and the reference level. There is still sound at this level though normally the sound is so low that most people cannot detect it.</p> <p align="left">To calculate a sound level use this equation:<br> sound level = 20 LOG(measured/reference)</p> <p align="left">A fan with a 30dB rating would equal:<br> 20 LOG(FSnd/ 20) = 30dB<br> LOG(FSnd/20) = 1.5dB<br> we are using Base 10 Logs so 10 ^ 1.5 = 32 times greater than the reference level<br> </p> <p align="left">For more information you can check out these sites:</p> <p align="left"><a href="http://www.modrec.com/about/MRT_decibel.pdf">http://www.modrec.com/about/MRT_decibel.pdf</a><br> <a href="http://www.phys.unsw.edu.au/%7Ejw/dB.html%20">http://www.phys.unsw.edu.au/~jw/dB.html </a></p> <p align="left"><b>dB Isn't Everything</b></p> <p align="left">Now that we all know how a decibel works, I can try to explain why all dB's are not equal.</p> <p align="left">Each fan, due to its bearing system, has a different frequency. The human ear unfortunately does not respond the same to all frequencies. Human ears normally have a greater response from a 1kHz to a 4kHz range. Remember, you can't hear the high pitched dog whistle that irritates your dog. </p> <p align="left">If you have a ball bearing fan that is producing 3kHz (this is an imaginary scenario for I have no idea of the actual frequency) and a sleeve fan operating in the range of 500Hz, then their dB's could be the same, say 32. But the ball bearing fan will sound louder to your ear because of the frequency. </p> <p align="left">Someone you know has a loud stereo system in which they pound their bass. You sit in his/her car and your head is vibrating from the power of the system. Most of these tones are in the low hertz range where you will not be able to hear them at all. Now if you go to a rock concert and listen to the guitar at the same loudness level, your ears will ring for a day.</p> <p align="left">Another thing to remember, we are all exposed to different scenarios in life; therefore, our hearing will differ from another person. For instance a person working with a jack hammer 8 hours a day, will be tone def to the sound of the jack hammer. To the rest of us, that sound will cause our ears to ring. </p> <p align="left">This is the reason that I mention in my articles that a sleeve bearing fan is quieter than a ball bearing fan. In dB measurements, they are equal. 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