Measuring Aircraft Noise

What is noise to one person may not be noise to another. So measuring noise specifically aircraft noise is not an easy task. Here’s how we do it:

• A-weighted – Sound is measured in decibels using a sound level meter with a filter that simulates human hearing. This filter and the human ear hear sound in the speech-important, middle frequencies better than sound in the lower and higher frequencies. The filter is called the “A-weighting” filter and the sound
  measured with this filter is called the A-level (AL) and is measured in dBA (A-weighted decibels).
  
  
• Sound Exposure Level (SEL) – When our sound level meter is measuring the AL, think of the sound falling on the microphone like rain. The maximum rate of rainfall is the maximum AL. Now think of the sound level meter as a bucket. After the noise event, like an aircraft overflight, has passed, the rain collected in the bucket (having passed through the microphone) is the noise dose or Sound Exposure Level (SEL). Loud noise events create large buckets of sound energy and quieter events create smaller buckets.
  
  
• Now what do we do with these “buckets” of noise (the Leq and DNL) – The buckets are collected over a 24-hour day and are poured into a large container. The volume is averaged over the 24 hours of the day to arrive at what is called the Equivalent Sound Level (Leq). When the buckets collected during the nighttime hours are multiplied by ten (10) as a penalty (because people are more sensitive to sound at night) and then the volume is averaged, we arrive at the Average Day-Night Sound Level, or DNL.

The DNL is the value that you will most often encounter when looking at the overall noise exposure from aircraft and it is the first value used to determine your home’s eligibility in the Sound Solutions Program.