{"id":50,"date":"2018-06-08T13:53:07","date_gmt":"2018-06-08T13:53:07","guid":{"rendered":"http:\/\/www.audiodevelopers.com\/minnetoska\/?page_id=50"},"modified":"2018-06-29T13:02:15","modified_gmt":"2018-06-29T13:02:15","slug":"how-to-measure-noise","status":"publish","type":"page","link":"http:\/\/www.audiodevelopers.com\/minnetoska\/how-to-measure-noise\/","title":{"rendered":"How to Measure Noise"},"content":{"rendered":"

Overview<\/h2>\n

As already noted in the article on “Our Story”, a noise-free environment is a quality of life issue.\u00a0 This article describes how to take valid noise measurement to verify whether noise-makers in your community are exceeding the thresholds set by the state of Maryland and are infringing on your right to a noise-free environment.\u00a0 In the next two sections, we go over the necessary equipment and discuss the properties of sound to introduce the equation used to calculate sound levels at any location. \u00a0 In the third section, we show how you can use Google maps to accurately assess sound levels at various locations on your property by taking measurements inside your own home.\u00a0 And finally, we provide some examples to show how easy this process can be.<\/p>\n

The Equipment<\/h2>\n

The section on noise measurement in COMAR 26.02.03.02.D.3 specifies the use of a Type II sound level meter.\u00a0 The ANSI specification that defines the Type II sound level meter is ANSI S1.4-1971.\u00a0 Additional specifications related to measurement of transient sound signals are in ANSI S1.43, which was originally published in 1997 and was last reaffirmed in 2007.<\/p>\n

Type II meters range from $15 to $1000.\u00a0 The wide difference in price range for Type II is due to certification.\u00a0 The low-cost Type II meters are designed to the Type II standard, but are not certified to meet the Type II requirements. \u00a0So if you are a government agency or just want absolute assurance that you have a Type II noise meter, you will need to buy a “certified” Type II meter, and you can find a good selection in the $350 range, complete with calibration certificates.\u00a0 The certified Reed instruments R8050 is available from a number of online sources:<\/p>\n

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But what about those budget meters, or even the cell phone apps that claim to measure sound accurately?\u00a0 In the 45 years since the original specification was published, sound level meters have become a popular product, available from a wide range of vendors.\u00a0 In fact, searching Amazon for the string \u201csound level meters\u201d returns over 2300 results.\u00a0 Just how good are these meters, and are they good enough for justifying a complaint that the neighbors are making too much noise?<\/p>\n

The Minnetoska POA has three meters:\u00a0 a generic digital meter purchased online for about $18, a “logging meter” purchased for about $60, and a Radio Shack 33-2050 built in the late 90’s.\u00a0 We took these 3 meters to Scantek, Inc, in Columbia, Maryland, and had them calibrated with certified equipment.\u00a0 This was not a full ANSI Type II certification, but rather just a check at 94dB with a 1KHz tone, which is the primary standard used for response calibration.\u00a0 The results are shown in the table below.<\/p>\n

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The Type II specification requires accuracy to plus or minus 2 dB, and all of the low-cost meters we tested easily met that accuracy.\u00a0 And this really isn’t a surprising result:\u00a0 in the last 20 years electronic devices have evolved considerably, and thermometers, voltmeters and other mass-produced test equipment have become low-cost commodities that do what they are supposed to, with good accuracy.\u00a0 So, if you want assurances that your sound meter is accurate, you can buy that certified Type II meter for $350 and have it calibrated yearly.\u00a0 But for noise measurements with “reasonable accuracy”, those $18 devices from many online vendors will probably be accurate enough.\u00a0 And if you have any doubts about the accuracy of the low cost devices, you can take them to Scantek to check the calibration (which they did for us at no charge).\u00a0 The meters that we had calibrated are shown below:<\/p>\n

\"\" \u00a0\"\" \u00a0\"\"<\/p>\n

Another possibility for low-cost sound measurement is using a cell phone application with an external microphone.\u00a0 There was a study in the Journal of the Acoustical Society of America in 2014 that assessed software programs that used cell phones as noise meters.\u00a0 There was a follow-up study in 2016 that assessed performance of the same apps using an external microphone (see https:\/\/blogs.cdc.gov\/niosh-science-blog\/2014\/04\/09\/sound-apps\/).\u00a0 The initial study showed that while some of the cell phone apps met the Type 2 standard, most of the cell phones with internal microphones did not meet the accuracy requirement, particularly the older phones.\u00a0 However, by using a low-cost external microphone, the measurement accuracy of the cell phone applications were within \u00b1<\/span>1 dB of the reference system.\u00a0 These microphones are available for $16 from Parts Express (iMM-6), and each comes with a calibration file that is unique to that device.\u00a0 With the external microphone, you can use your cell phone as a noise meter with very high assurance that it will provide the Type II accuracy required by the Maryland noise standard.<\/p>\n

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Properties of Sound and Some Math<\/h2>\n

The Maryland Noise Law specifies the allowable noise levels at your property line.\u00a0 However, taking noise measurements of your neighbors at your property line can be difficult and embarrassing.\u00a0 Standing on your property line with a noise meter waiting for your neighbors to act up is no fun and is a sure way to create neighborhood tensions.\u00a0 But there are easier ways to take accurate measurements from a distance, and in this section, we’ll show how the properties of sound allow us to predict the noise levels at any location by measuring from a “comfortable location”.<\/p>\n

First, let’s look at a typical sound level chart to better understand the measurement scale.\u00a0 In the chart below, the areas that are associated with physical damage and mental health issues are the areas in red.\u00a0 This scale doesn’t show dirt bikes or unmuffled ATV’s, but they would usually fit in between the jackhammer and discotheque, in the 95dB to 105dB range, when measured at 50 feet.\u00a0 The areas in orange are the sound levels that are common problems for neighbor noise disputes.\u00a0 The Maryland noise limit for residential areas in the daytime is 65dB.<\/p>\n

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Notice that the previous chart shows both sound and distance for each source.\u00a0 That’s because you need to measure any source at a specific point or distance.\u00a0 For example, the jackhammer sound intensity is specified at a distance of 15 meters.\u00a0 And for the Maryland noise law, the sound is measured at any point on your property line.\u00a0 The graphic below helps explain the relationship of sound intensity and distance, and we will use this information to derive a simple equation that lets us calculate noise intensity from a source at any distance.<\/p>\n

As shown in this graphic, sound energy propagates in all directions and as a result, sound intensity follows the inverse square law.\u00a0\u00a0 Therefore, every doubling in distance will result in four times the attenuation.\u00a0 Expressed in decibels, the sound will be attenuated by 6dB for every doubling in distance.\u00a0 This relationship is an idealization, because terrain features and vegetation can create echoes, reverberation and attenuation that cause variations in sound intensity.\u00a0 However, for open areas such as the Minnetoska subdivision, the inverse square law provides a reasonably accurate estimate of sound intensity at the locations of interest.<\/p>\n

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This \u201c6dB for each doubling of distance\u201d relationship is discussed in the Occupational Safety and Health Administration (OSHA) Technical Manual (TM), Appendix B, section B.6.\u00a0 With some simple math, it can be shown that\u00a0 doubling of distance\u201d relationship can be used to calculate the sound intensity at any point if you know the intensity at a given point.\u00a0 The equation presented in the OSHA TM is the following:<\/p>\n

Lpd2 = Lpd1 + 20 * log(d1\/d2),<\/p>\n

where:<\/p>\n