Putting 65 CNEL in Perspective

Data from the Orange County’s Noise Abatement Office at John Wayne Airport proves that the averaging of  loud airplane noise with the quiet moments in between planes - to generate a  statistic called the Community Noise Equivalent Level (CNEL) - is a misleading indicator of annoyance.  While, airport advocates claim that residents should not be impacted when noise averages less than 65 decibels CNEL, county data shows a different picture.

For the year, July 1997 - June 1998, John Wayne received 2,645 noise complaints from residents.  Over 98 percent of  John Wayne complaints came from individuals subjected to less than 65 decibels CNEL.

Most complaints came from areas where noise level is only half of the 65 CNEL average.  The largest concentration of protests came from Newport Beach, and especially the Balboa Island and Corona del Mar areas of the city.  County instruments show that the average aircraft noise in this area calculates to be approximately half of 65 decibels CNEL.

Another cluster of complaints came from the Tustin/Orange area where a monitoring station recorded an average 56 decibels CNEL   On the CNEL scale, 55 is exactly half as loud as 65, and 56 is just over half as loud.  In other words, residents are disturbed by half of what officials consider the threshold of noise annoyance.

Why are people living 4 to 6 miles from the airport complaining about noise that the county says should not bother them?  For good reason.  The CNEL averaging technique is misleading.  It may be a fair measure of a steady freeway hum or constant noise from a piece of machinery but it does not adequately consider the impact of loud bursts of noise as planes fly over.

Here are some facts to illustrate the point.  Instruments installed by the county at a Back Bay monitoring station, designated as RMS-6,  recorded an around-the-clock average 57 decibel CNEL.  But, whenever planes go over, the instrument’s readings jump.  Data shows that FedEx A310 cargo planes produce 88 decibels of Single Event Noise Exposure Level (SENEL).  American Airlines MD-80’s produce 91 decibels, SENEL.

County staff and consultants equate 57 CNEL average noise level with “normal conversation”.  The reality is that planes produce noise spikes of  “power lawnmower level” and stop the conversation.

The calculations are even more complex because "peak noise" from airplane events normally are 8-10 dB less than the calculated SENELs. Therefore, a conversation at RMS-6 - where the ambient non-airplane noise aveages 40-50 dB - may be drowned out by a plane that is momentarily as loud as 80 dB.  The calculated SENEL for the event may be around 88 dB and the CNEL for the day, computes as 57 dB.

Averaging is a common but misleading way to measure aircraft noise impacts.  An angry parent near the Federal Express air cargo hub in Greensboro, North Carolina summed it up this way. “Telling our small children, who are awakened at night, crying in their beds because they can't sleep, and are frightened by the roar of overhead jet engines, that the noise is not so bad if you average it out over a 24 hour period, won't get them to sleep at three in the morning.”

Click here for more on the meaning of decibel noise levels.

This is an old webpage being retained for historical reasons.

IT WILL NOT BE UPDATED.