each sources of broadband noise. High-speed impulsive (HSI) noise HSI noise is caused by transonic flow shock formation on the advancing rotor blade, and is distinct from loading noise. The source of HSI noise is the flow volume around the advancing blade tip, hence it cannot be captured by examining only the acoustic sources on the surface of the blade, HSI noise is typically directed in the rotor plane forward of the helicopter, like thickness noise. Tail rotor noise While most noise from a helicopter is generated by the main rotor, the tail rotor is a significant source of noise for observers relatively close to the helicopter, where the higher-frequency noise of the tail rotor has not yet been attenuated by the atmosphere. Tail rotor noise is particularly annoying to the human listener due to its higher frequency (as compared to the main rotor) which places it directly in the band in which the human ear is most sensitive. Methods of noise reduction Almost all helicopter engines are located above the aircraft, which tends to direct much of the engine-noise upwards. In addition, with the advent of the turbine engine, noise from the engine plays a much smaller role than it once did. Most research is now directed towards reducing the noise from the main and tail rotors. A tail-rotor which is recessed into the fairing of the tail (a fenestron) reduces the noise level directly below the aircraft, which is useful in urban areas. In addition, this type of rotor typically has anywhere from 8 to 12 blades (as compared to 2 or 4 blades on a conventional tail rotor), increasing the frequency of the noise and thus its attenuation by the atmosphere. In addition, the placement of the tail rotor within a shroud can prevent the formation of tip vortices. This type of rotor is in general much quieter than its conventional counterpart: the price paid is a substantial increase in the weight of the aircraft, and the weight that must be supported by the tail boom. For example, the Eurocopter EC-135 has such a design. For smaller helicopters, it may be advantageous to use a NOTAR (from NO TAil Rotor) system. In this yaw-control method, air is blown out of vents along the tail boom, producing thrust via the Coandă effect. Some designs have been done to reduce the rotor noise itself, for example the Comanche military