Most people recognize the sound of a helicopter when they hear it, thanks to the distinct “whoop-whoop” noise of the blades. When a helicopter flies low enough, it may even produce harmful levels of noise. The blades are partially responsible for the noise, but there is much to consider.
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TLDR – Helicopters are so loud due to blade-vortex interaction. The air pressure decreases above and increases below the blades as the main rotor spins, providing upward thrust. A vortex is created to even out the air pressure, which produces sound vibrations when it hits the next blade.
Why Do Helicopters Make a Chopping Sound?
The distinctive sound that helicopters make comes from a concentrated vortex of air generated by the main rotor. Researchers at the German Aerospace Center determined that most of the sound produced by a helicopter are sound vibrations caused by the vortex hitting the blades.
The main rotor of the helicopter generates thrust. As the rotor rotates and the blades spin, the air pressure above the blades drops while the air pressure below the blades increases. This difference in air pressure pushes the helicopter up.
Air flows around the blades to even out the air pressure. The air above the blades is warmer due to the decreased air pressure and becomes trapped underneath the cooler, low-pressure air from below the blades. The warm air trapped underneath cooler air creates a concentrated vortex.
When the vortex hits the next blade, the blade vibrates, which is called “blade-vortex interaction.” The vibrations of the blades produce a loud sound associated with helicopters.
How Loud Is a Helicopter?
According to the Helicopter Association International (HAI), the sound of a helicopter flying at 500 feet is about 87 decibels. At 1,000 feet, the sound drops to 78 decibels. For comparison, a vacuum cleaner is about 75 decibels while a power lawn mower is about 90 decibels.
The sound of the blade-vortex interaction is much louder for passengers and pilots. The perceived noise when riding in the helicopter is typically around 97 decibels.
How Do You Reduce Helicopter Noise?
Helicopter noise reduction methods are typically included during the design phase to address the most common sources of helicopter sounds. However, there are operational techniques for reducing helicopter noise during flight.
For example, helicopters make more noise when turning compared to flying straight, as the change of direction alters the air vortices around the blades. This also occurs when decelerating, turning into the advancing blade, and steep descents.
Minimizing turns and avoiding decelerating when turning can help decrease the noise of the blade-vortex interaction. Shallow approaches can also limit the noise level compared to steep approaches.
To reduce the noise for pilots and passengers, every person on board normally wears a special aviation headset. These headsets are noise-canceling, and they are also equipped with a communication system so everyone on board is able to communicate effectively.
The chopping sound is not the only noise produced by a helicopter. The sound of the engine, the tail rotor, and other factors contribute to the sound of the rotorcraft. Manufacturers use various helicopter noise reduction techniques to address these issues. For example, the helicopters used during the raid on Osama Bin Laden were modified to make them quieter. The pitch and speed of the blades were adjusted to limit the noise caused by the blade-vortex interaction.