All sounds come from sound source. It takes energy to produce this sound, and the energy is transmitted through the air in sound waves-tiny, quick oscillations of pressure just above and below atmospheric pressure. These oscillations, or sound pressures, impinge on the ear, creating the sound we hear.
Sound Pressure Level (SPL)
SPL is a measure of the sound pressure of a given noise source relative to a standard reference valve. The standard reference value is typically the quietest sound that a young person with good hearing is able to detect.
The Decibel (dB)
Sound Pressure Levels are measured in decibels (dB). The logarithmic conversion of sound pressure to sound pressure level means that the quietest sound we can hear has a sound pressure level of 0 dB, while the loudest sounds we can hear without pain have sound pressure levels around 120 dB. Most sounds in our day-to-day environments are about 30 to 100 dB.
The “A” Weighted” Sound Level dB
The rate at which sound pressure oscillation reach our ears is known as frequency or pitch. The closer the vibrations, the higher the pitch and vice versa. Our ears are better equipped to hear mid and high frequencies, therefore we find mid and high frequency noise to be more annoying. People usually respond to sound most readily when the predominant frequency is in the range of normal conversation. By using filters such as the “A weighted” filter we are able to best match sound levels of our ears sensitivity.
Sound Exposure Level (SEL)
SEL is the most common measure of cumulative noise exposure for a single aircraft flyover. The SEL is a summation of the “A-weighted” sound energy over the duration of a noise event, where the duration is defined as the time when the sound level first exceeds a threshold level (normally just above the background noise level) to the time that the sound level drops back down below the threshold.
The Day-Night Average Sound Level (DNL)
The average noise level over a 24 hour period except the noise occurring at night (between 10 PM and 7 AM) which is artificially increased by 10 dB. This weighting reflects the added intrusiveness of night noise events attributable to the fact that community background noise typically decreases by 10 dB at night.
Aircraft Stage Classifications
The FAA classifies aircraft into four stages for clarification. Stage 1, being the loudest, Stage 2, Stage 3 and Stage 4. The noise levels for stage definition is measured at three points. These points are designed to measure noise levels for take-off, landing, and sideline. For more information click on the following link. Noise Levels for U.S. Certificated and Foreign Aircraft – AC36-1H Please note that non-emergency Stage 1 and Stage 2 jet operations are banned from APF.
For more information about Aircraft Noise Terminology, please contact Diane Terrill, Deputy Executive Director, at 239-643-0733.