Sources of Aviation Noise
- Sources of Noise
- What Aviation Noise Sources Exist?
- Helicopter Noise
- What is a Sonic Boom?
- Sources of Non-Aviation Noise
Noise made by a helicopter, is very complex and is comprised of multiple forms of noise sources. Whetherboth a civilian or military helicopter, it generates noise that sounds very different from noise emitted made by airplanes. Airplanes are configured with fixed wings and helicopters are configured with rotary wings (typically a spinning main rotor and a tail rotor). As such, this rotary mechanical design (configuration) causes them to produce periodic noise associated with the speed of rotation.
This type of noise is referred to as "thickness noise," and is the result of a sound wave pulse created by the repetitive rotary motion of the air being displaced by the wing (blade) surface. The frequency of the pulses (how fast the pulses occur) depends highly on the blade-passing frequency (BPF). The blade-passage frequency depends on both the number of times per second the helicopter rotor spins around a revolution and the number of blades on the rotor.
Blade Loading Noise
Another source of noise results from the lift and drag forces on the rotary wings and is called "blade loading" noise. Blade loading noise is a function of the distributed blade pressure set of the blade-passing frequency and multiples of the blade-passing frequency (harmonics).
Blade Vortex Impulsive (BVI) Noise
The more aerodynamically involved noise phenomenon that occurs in descent is Blade Vortex Impulsive (BVI) noise. This noise results from the interactions of rotor blades with the shed vortex trails generated by the tips of the rotor blades. BVI noise has an highly impulsive nature and generates very directional noise pulses below the rotor plane.
Other contributors of helicopter noise include engine and gearbox noise. Noise that is usually secondary and not dominant relative to the other rotor related engine and gearbox noise are both composed of many frequencies (or pitches) and periodic in nature at the middle to high frequencies.
Factors that Affect Helicopter Noise
- The number of rotor blades
- The design of the rotors
- The engine type
- The helicopter's motion, such as
- Blade speed
- Speed of the helicopter
- The type of action the helicopter is executing
- The helicopter's surroundings, such as
Does the Impulsive Nature of Helicopter Noise Make It More Annoying?
Sounds composed of pulses are often considered more annoying than more constant sounds. This type of sound is called impulsive because the sound changes quickly. These impulsive sounds can be given "penalties," or higher effective noise levels, to make up for the added annoyance. Different types of military noises such as small arms fire, 25 mm cannon fire, large blasts, and helicopter noise have been tested to determine proper noise "penalties." These tests are conducted with subjects who compare control noises at different noise levels to the different types of impulsive noise. An example of a control noise is the sound of a passing truck. In one study, (see Schomer, 1995) the noises from small arms fire, 25 mm cannon fire, and large blasts all required significant penalties, as high as 15 dB. The helicopter noise evaluated in that study showed little to no penalty. The duration of a sound, the level, and how quickly the sound changes all have an affect on annoyance.
Larkin, Ronald P. Effects of Military Noise on Wildlife: A Literature Review. Champaign, IL: U.S. Army Construction Engineering Research Laboratory, 1996.
Schomer, Paul. Human and Community Response to Military Noise: Results from Field-Laboratory Tests of Small Arms, 25 mm Cannons, Helicopters, and Blast Sounds. Champaign, IL: US Army Corps of Engineering Research Laboratories, 1995.
For definitions of words used in this section go to the NoiseQuest Glossary of Terms.