How to Convert the Noise Criteria to Decibels

Heating and ventilation systems in school classrooms produced background noise up to 100 times the recommended levels, according to a 1999 study conducted for the Florida Department of Education. Today, indoor background noise is still a concern for architects designing attractive, functional buildings that provide a safe environment for occupants. Noise Criteria curves, developed in 1957, define allowable background sound levels at various frequencies, broadly matching the range of human hearing. Conversion of NC levels to decibels is possible for any specified frequency.

Things You'll Need

• NC curve chart
• Ruler

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Instructions

• 1

  Check the frequency in hertz (cycles per second) for which you require the conversion; this should lie between 63 hertz and 8,000 hertz. Make a note of this value.

• 2

  Check the applicable NC level. This will lie between NC-10 and NC-70. Note the level.

• 3

  Read along the bottom axis of the NC curve chart until you reach the frequency that you noted. Place the ruler vertically on the chart in line with this frequency value.

• 4

  Find the NC curve that corresponds to the applicable NC level. Mark the point where this curve intersects the ruler.

• 5

  Reposition the ruler, placing it horizontally across the chart through the point that you marked. Mark the point at which the ruler intersects the vertical axis. Note the sound pressure level, or intensity level, of this point. For example, if the frequency is 250 hertz and the NC level is NC-40, the sound pressure level is 50 dB.

• 6

  Check each step of your work. Record the result, which is the NC level converted to sound pressure level in dB.