Why so loud? It’s all about altitude and engine thrust….
The FAA claims that Nextgen has “reduced” aircraft noise. Unfortunately, what this really meant was that the decision to concentrate all the aircraft into narrow corridors over certain communities resulted in “fewer” people affected by aircraft noise. The big downside was that concentrated corridors of frequent planes meant dramatically more noise, more often, over specific neighborhoods. NextGen unilaterally chose winners and losers for noise. This approach was ‘damn the consequences’ for those souls who were chosen to bear the noise burden for the Northern California Metroplex.
Adding salt to the wound, is that the new concentrations were shifted to quieter suburban areas that did not have much air traffic previous to NextGen. The FAA admits they failed to take topography into account, which had big noise consequences for the East Bay. Oakland Airport arrivals from the north that were previously dispersed over a more than 6 mile-wide area, are now concentrated over a narrow, 2,000 foot-wide path over the highest areas of the East Bay. This new NextGen arrivals path can never use the FAA’s touted quiet gliding descent techniques because the aircraft must fly low and underneath departing San Francisco planes. The planes are forced to fly level, under power for over 23 nautical miles down the East Bay Hills which is the noisiest, dirtiest and fuel intensive way to fly.
Overnight, planes began flying as much as 1500 feet closer to homes, schools and the East Bay Regional Parks. When you consider that experienced sound levels double with each 1000 foot altitude drop, that’s a huge noise increase. Ambient noise levels and weather also play important roles, which we will get to later.
Why is it so loud? Mostly it comes down to the altitude of the plane overhead and the fact that the East Bay experiences almost all their overflights with either planes under high thrust departing from San Francisco or arriving planes forced to fly level under engine thrust instead of using quiet, gliding descents with engines near idle.
- Departing San Francisco and Oakland traffic under high thrust
Airplanes need to get to their flight altitudes as quickly as possible for several reasons. At higher altitudes, planes fly cleaner, use less fuel and can head in the direction they need to shorten the flight path. The sound of planes at flying altitudes are not noticeable in a typical urban or suburban area. However, the climb portion of a flight causes loud, adverse noise disturbance on the ground. Departing planes are heavy with fuel, passengers, and cargo. To get all this weight up means jet engines are near full power, which is really loud. Frequent departing flights concentrated over a home every few minutes is simply really, really loud. San Francisco Airport departures are generally higher in altitude than Oakland departures because those aircraft have a few more miles to gain altitude before flying over the East Bay.
The type of aircraft can also make a difference. Larger, heavier aircraft take more time to gain altitude, so are noisier because of their larger engines as well as being lower when they fly over. Also, the outline and design itself of different aircraft changes how loud a plane is as it flies. A good example is the infamous Airbus A300 series of planes that have a loud and annoying whine or howl when they fly. The noise is caused by air passing over circular vent holes under the wing, similar to blowing over a bottle – just on a much larger scale.
The very disturbing low frequency, thunder-like rumbling that lasts for such a long time is amplified and reverberates in hillside areas as well. It’s this type of noise that creates the rattling windows and vibrations often described by people living under NextGen flight paths. The FAA does not take into account this low-frequency sound in their noise calculations. Closed windows, earplugs or sound proofing cannot alleviate this type of noise impact, which might be why the FAA chooses to ignore it. Unfortunately, ignoring it does not make it go away.
- Arriving Oakland Airport traffic from the north is forced to fly level and low under power down the East Bay Hills for over 23 nautical miles
The concept of engine thrust affecting noise levels is important. NextGen touts using quieter “Optimized Profile Descents,” which is supposed to have arriving aircraft quietly glide into their descent with their engines near idle. The new NextGen Oakland Airport WNDSR flight path for arrivals from the north requires over 23 nautical miles of level flight, under power at altitudes commonly down to 3000 feet or less Above Ground Level. This is the noisiest and dirtiest way to fly an aircraft and creates a very harsh noise impact. WNDSR is designed for arriving Oakland planes to fly under the San Francisco departing aircraft. It was not designed to be efficient, save fuel or minimize noise impact. Arriving Oakland planes and departing San Francisco planes are stacked and can literally be flying on top of each other over our homes, schools and parks. From the get-go, WNDSR was designed and doomed to never be able to offer a quiet descent.
- Ambient noise levels
A big reason why aircraft noise is so loud is that our ears are always listening to new sounds in comparison to the noise around us. If we are in a fairly quiet, suburban home at about 45-50dB ambient noise levels, it is pretty quiet. When a loud plane suddenly roars overhead, this instantly creates a huge “signal-to-noise” ratio that we experience and why it seems extraordinarily loud and intrusive. It is this sudden, dramatic variation itself which affects how noise is perceived in addition to the actual decibel sound level.
- The Weather
Everybody talks about it, but we can’t do anything about this piece of the noise equation. Temperature, humidity and wind all affect how sound travels and, therefore, what we hear on the ground.
- Temperature – Because hot air is less dense than cold air, sound passes through hot air faster than it passes through cold air. For this reason, temperature gradients cause refraction (“bending” or deflecting) effects. When the air is colder on the ground than at altitude, sound waves bend towards the ground, which increases the sound level that we hear.
- Humidity – The attenuation (fading) of sound in the air is affected by the relative humidity. Dry air absorbs far more sound energy than does moist air. This is because moist air is actually less dense than dry air (water vapor weighs less than air, which is why clouds float). Stormy, rainy days allow loud sounds to travel further, which can create louder days when reverse flow wind conditions force arriving San Francisco traffic to fly low over the East Bay. In addition, cloud cover tends to bend sound waves downward toward the ground and that can also increase the sound heard on the ground.
- Wind – Wind is another factor that causes sound waves to bend in the direction the wind is flowing. Wind nearer the ground moves more slowly than wind higher up due to interference from hills, trees, and buildings etc. This wind gradient, with faster wind at higher elevation and slower wind at lower elevation, causes sound waves to bend downward to locations downwind of the airplane and to bend upward to locations upwind of the airplane. Waves bending downward means that a listener standing downwind of the source will hear louder noise levels than the listener standing upwind of the source.
All of these weather effects affect sound. In addition, the molecules in the air itself gradually absorb energy from the sound wave as it travels through the atmosphere. This absorbtion is why once planes are high enough in altitude, we don’t hear them.
With all of these factors affecting sound, noise management breaks down into two main categories:
- either reducing
the noise at its source (quieter airplanes)
- separating people from noise.
In other words, if we can’t get the airplanes quieter, we are left with getting them further away from people.
Now, it’s time to get loud with the FAA, and let them know why we need them to re-tool NextGen now!