How to Calculate SEL

A sound-exposure level (SEL) is a time-weighted measurement of the impact of loud sound, or noise events, on humans or the environment. It is used as an objective measure for comparing the intensity and duration of loud sound events. The intensity measure is based on the physics of sound pressure and is known as decibels. The intensity measure may be weighted for the impact of sound relative to humans (A-weighted, dBA). It may exclude extreme frequencies of less than 60 hertz and greater than 8,000 hertz (C-weighted, or dBC), or unweighted, including all frequencies. The duration of a noise event is measured in hours or fractions of hours. The equation for SEL combines these two measurements to arrive at a value for noise exposure over time.

Things You'll Need

• Calculator
• Spreadsheet program

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Instructions

Manual Calculation of SEL using dBA

• 1

  Rock bands can have sound levels at 108-114 dBA, which is about 16 times as loud as a normal conversation.

  Take the measurement of sound intensity for human hearing levels (dBA) and divide it by 10. Let n = dBA, where n = noise. Then use that figure as an exponent of 10. The expression for that part of the calculation would be 10^(n/10). Let this number be N. For example, if the dBA measurement was 60, then the value of N = 10^6.

• 2

  Multiply the number N obtained in Step 1 by the hours of exposure. Use decimal expressions of time if the time was less than or more than a whole number. For instance, if the time was 40 minutes, the number you would use for time would be 40/60 = 0.66.

• 3

  Divide the product of time and N by 8. Then, take the logarithm (base 10) of this number. Lastly, multiply this number by 10. The entire equation looks like: SEL = 10*log ((nT) / 8). The equation is divided by 8 to arrive at the 8-hour SEL. Cumulative SEL calculations can be arrived at over time, where different noise levels exist and different time exposures occur. In that case, add up the series of exposures and make an equation like this: Cumulative SEL = 10*log ( ((nT) /8 )sub1 + ((nT) /8 )sub2 + ((nT) /8 )sub3 + ....... ), where the number of repetitions of ((nT) / 8) depends on the number of observations within the total time window of observations.

Calculation of 8-hour SEL using an Spreadsheet Computer Program

• 4

  Automating the SEL calculations may be needed if you do them routinely.

  Make your own SEL calculator. There are programs available on the Internet in spreadsheet format, and you can make your own to suit your specific needs. You can adjust the formulas to give you 8-hour, 24-hour or weekly SEL values.

• 5

  Open your spreadsheet program and create a new document. Create a spreadsheet that has three columns. The first column can be labeled "Event," the second "Time" and the third "dB." Place these labels in cells A3, B3 and C3. Events will need to be a series of contiguous events for which you have dB measurements. You can identify "Events" as you wish to relate to your data set. Time has to be measured in decimals, so if one event occurred over 80 minutes, the number would be 1.33 for time.

• 6

  Create cells that have the label "SEL" and another that calculates the eight-hour SEL value. In cell A7, write in the label "SEL." In cell C7, enter the following formula (for three events and data in the indicate cells above): =10*LOG10((((B3)*10^(C3/10))/8) + (((B4)*10^(C4/10))/8) + (((B5)*10^(C5/10))/8)). Note that there are not spaces around the "+" mark in the formula for the cell, and make sure that you include all of the parentheses. There also is not a period at the end of the formula. You can expand or contract the spreadsheet as needed -- just make sure that the formula covers all of the cells where data exist. Enter your data and the spreadsheet and the answer will be given in cell C7.