By Jim Davis    2024-10-20   (Last Updated October 21, 2024)

Helicopter Noise

Comparison to fixed-wing planes

Compared to a jet or airplane, most people will say helicopter noise is more annoying. The aerodynamics of a helicopter rotor blade change continuously as it spins around the rotor, while the wing of a plane has a more steady loading. This causes an annoying “thump-thump” sound in a helicopter, while the plane/jet produces a more continuous sound. Helicopter noise is also at lower frequency, which tends to travel further through the atmosphere.

If this weren’t bad enough, helicopter flight profiles make things much worse. Helicopters mostly flow lower to the ground than jets/planes. They takeoff/land from office buildings, hospitals, parking lots, … closer to communities. When hovering, nearby communities are subject to the same noise for a longer duration.

Let’s take a look at some of the noise sources.

Rotor noise

The dominant source of helicopter noise are the main and tail rotors. These rotors spin at very high speeds, causing the blade tips to move near the speed of sound. A typical main rotor blade tip is moving about 500 miles per hour in hover. In forward flight, the advancing blade tip moves 500 MPH plus the aircraft speed.

The tail rotor, while much smaller, does contribute significantly to overall rotor noise. The tail rotor has many of the same noise phenomena as the main rotor, all of which are described below.

High-speed impulsive noise

High blade tip speed gives rise to “high-speed impulsive noise” (HSI noise). This noise is created when the blade tips near the speed of sound and shock waves are created. The noise is primarily emitted forward and in the rotor plane. In combat zones, this noise is an advance warning to the enemy that a helicopter is approaching.

For more information about HSI noise try the resources below.

• NASA Techical Memorandum 84553
• Blade Shape Optimization for HSI Noise Reduction and Performance Improvement of Helicopter

Blade-vortex interaction noise

Rotor blades emit vortices (mini tornados) in flight. When a blade encounters a vortex emitted by another blade, it undergoes sharp aerodynamic changes that alter performance and can create substantial noise. This blade-vortex interaction (BVI) noise is directed forward and down.

Vortices travel down with main rotor downwash, hence BVI noise is more pervasive when a helicopter is descending into these vortices. Engineers have studied various tip shapes in hopes of reducing this noise: swept, tapered, parabolic, anhedral, … tips. Limited improvement has been found, and the modifications often adversely affect performance and/or vibrations. More sophisticated “active” blade modification (actually moving things during flight) have been investigated, but thus far have not proved practical.

Thickness noise

Periodic changes in blade aerodynamics as blades rotate around the rotor cause pressure variations in time. These variations send sound waves from the front and trailing edge of each blade. The magnitude of this sound depends on the airfoil shape, primarily the thickness.

Thickness noise can include a broad range of frequencies, and typically increases significantly with angle of attack and rotor speed. This noise is mainly emitted in the plane of the rotor.

Loading noise

Loading noise is caused by the periodic pressure variations on the blade surface, as it spins around the azimuth. This noise increases with thrust—it worsens when carrying heavy loads or climbing aggressively. This is primarily low-frequency noise and directed below the rotor.

Engine and mechanical noise

Within or nearby a helicopter, you may hear noise sources besides the rotors. The first thing you typically hear within a helicopter, before the (main) engines are started, is the auxiliary power unit (APU). The APU is a smaller engine that’s used to start the main engine(s) and power various accessories. It typically produces a humming sound that can be annoying to occupants.

Once the main engine(s) are started, engine noise will typically dominate. As engine speed and rotor speed increase, the gearbox may become audible. Occupants or bystanders may hear bearings or gear meshing. This sound can worsen with insufficient oil/lubrication.

Once the rotor produces sufficient thrust for takeoff, rotor sounds typically dominate, although the engine will still be audible to occupants.

Reducing noise

The main factor impacting noise is rotor speed. Far-field acoustic levels decrease dramatically as rotor speed is reduced, as described in Helicopter Main-rotor speed effects on far-field acoustic levels.

As eluded to above, the shape of rotor blades also plays a role, but generally much smaller. Thinner blades, swept blades, … can reduce some of the noise sources described above.

A tail rotor can even be ducted, which can dramatically reduce its noise level. This both reduces main noise sources and partially shields noises that are created. It’s no silver bullet, however, as ducted tail rotors add cost, weight, and aircraft drag.

Pilots can reduce apparent noise by flying higher above communities and avoiding loud climbs or descents near communities.

Standards

Commercial Helicopters

Standards for helicopter noise in takeoff, overflight, and landing vary by location. In the US, the FAA sets standards in Part 36 of the Federal Aviation Regulations. The International Civil Aviation Organization also sets standards. Many locations further constrain noise or even set curfews based on community complaints. The unpredictability of the latter makes it difficult/impossible for helicopter manufacturers to guarantee a new helicopter can operate around specific communities.

Helicopters flying near communities tend to generate complaints, regardless of standards. Such helicopters are often required to alter flight path or time or flying characteristics in certain areas. This can significantly slow down and even prevent certain applications.

Some have argued that noise reduction techniques won’t help with this. The complaints may often be generated by the mere presence of the noise, not necessarily the extent of it. Unless it can be made imperceptible, it’s unclear how much noise reduction will diminish these impacts.

Military helicopters

Strictly speaking, military helicopters are exempt from FAA noise requirements. However, they may be subject to some restrictions including community rules if they frequent civilian airspace. The Department of Defense sets unique noise requirements based on a helicopter's mission. For example, an attack helicopter that might be used to take out enemy radar sites may have strict requirements on forward-propagated noise. It’s mission is all about getting into enemy territory without detection.

Unfortunately military helicopters are typically heavier and have more powerful (louder) engines than a comparable commercial helicopter. This can make noise abatement that much more challenging.

Infrasound

The strongest sound emitted by helicopters is typically infrasound, i.e. sounds below 20 Hz that humans can’t hear. Just because we can’t hear it doesn’t mean we can ignore it. Due to its low frequency, infrasound can propagate long distances. It can pass through walls and vibrate structures, potentially causing damage if resonant. Infrasound can also harm sensitive wildlife, primarily animals that use low-frequency sound for communication.

Stealth helicopters

The 2011 raid on Osama bin Laden’s compound revealed a top-secret helicopter. One of the two so-called “stealth” helicopters involved crashed. It was destroyed, but the tail section remained in tact for us to get a glimpse of the technology.

The helicopter was apparently was a highly modified UH-60 Black Hawk helicopter. It had a special coating, presumably to limit radar signature. However, many have speculated this coating could also be sound-absorbing. The tail rotor appeared completely different from a standard Blackhawk, apparently with 5 blades. This would allow it to operate at a lower rotation speed which, as we said, substantially reduces noise.