question: Why do you hear helicopter sounds when you have your windows down while driving on a highway?
comment: Resonant frequency of the cabin. You can relieve this by opening another window a small amount, preferably on the other side of the car. The resonant frequency thing is the same reason why a whistle, or a flute works, too. It's just that a car cabin is so much larger that it's resonant frequency is much lower. So low, it's subsonic. (lower than 20hz).
passages: 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.
  
  Section::::Sources of helicopter noise.:Tail rotor noise.
   Perpendicular BVI occurs when the axes are perpendicular and in parallel planes. Due to its low unsteadiness, the noise effect of perpendicular BVI are less significant with respect to parallel BVI. It produces a continuous broadband noise characterised by a much lower intensity compared to the impulse (harmonic) noise, which caused by parallel BVI.
  
  Section::::Classes of blade vortex interactions.:Oblique BVI.
  
  Oblique BVI occurs between the vortex and the blade when the axes are oblique. In helicopter research field, oblique BVI is a common phenomenon that looks like an intermediate action of parallel BVI and perpendicular BVI.
   As a predominant source of noise, BVI phenomenon can be detrimental to blade structure integrity as well because of the unsteady fluctuation of aerodynamics, such as vortex buffeting and dynamic stall in the retreating blade. Therefore, BVI becomes a prime concern in the helicopter research field. In order to understand the BVI flow characteristic more closely and suppress the noise and vibration actively, it is important to predict the BVIs precisely. Recently, the tools for capturing BVIs can be divided into three parts, which will be described as follows:
  
  Section::::Means of BVIs prediction.:Wind tunnel test.
   The relevant OSHA regulation is 29 CFR Part 1926.601(b)(4) which requires "a reverse signal alarm audible above surrounding noise level", but only when the motor vehicle has "an obstructed view to the rear". The determination of the noise level is left to the employer. However, because equipment is moved from place to place, the loudest alarms are often selected.
  
  NHTSA requires electric vehicle warning sounds to alert pedestrians in electric and hybrid vehicles manufactured after 2018, for both forward and reverse travel at low speeds.
   BULLET::::- Bluff Body Noise – the alternating vortex shedding from either side of a bluff body, creates low-pressure regions (at the core of the shed vortices) which manifest themselves as pressure waves (or sound). The separated flow around the bluff body is quite unstable, and the flow "rolls up" into ring vortices—which later break down into turbulence.
  
  BULLET::::- Edge Noise – when turbulent flow passes the end of an object or gaps in a structure (high lift device clearance gaps) the associated fluctuations in pressure are heard as the sound propagates from the edge of the object (radially downwards).
   Section::::Sources of helicopter noise.:Broadband noise.
  
  Another form of loading noise, broadband noise consists of various stochastic noise sources. Turbulence ingestion through the rotor, the rotor wake itself, and blade self-noise are each sources of broadband noise.
  
  Section::::Sources of helicopter noise.:High-speed impulsive (HSI) noise.
   Transonic speeds can also occur at the tips of rotor blades of helicopters and aircraft. This puts severe, unequal stresses on the rotor blade and may lead to accidents if it occurs. It is one of the limiting factors of the size of rotors and the forward speeds of helicopters (as this speed is added to the forward-sweeping [leading] side of the rotor, possibly causing localized transonics).
  
  Section::::Condensation clouds.
   Literature distinguishes different classes of BVIs in helicopter rotors depending on the impacting vortex axis with respect to the blade span. Generally, It can be divided into four distinct types, which will be described as follows:
  
  Section::::Classes of blade vortex interactions.:Parallel BVI.
  
  Parallel BVI occurs when the vortex and the blade axes are nominally parallel. It is the BVI phenomenon that produces the largest-amplitude impulse (harmonic) noise, due to that the unsteady vortex moves towards to the downstream.
  
  Section::::Classes of blade vortex interactions.:Perpendicular BVI.
   Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. The result of these differing sound levels is key in noise pollution considerations, for example from roadway noise and aircraft noise, and must be considered in the design of noise barriers. This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways as well as noise barriers.
   In 2010 Awaaz Foundation tested noise levels of helicopters after several leading Industrialists applied for permission to build private helipads atop their residential buildings in densely populated areas of Mumbai. After the data was presented in court. permission to use private helipads in all urban areas of India were banned by the MoEF.
   BULLET::::- Download
  
  Section::::Music video.
   Section::::Description.:Occurrence.
  
  A helicopter normally encounters this condition when attempting to hover out of ground effect above the hovering ceiling for the aircraft, hovering out of ground effect without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed drops to nearly zero.
  
  Section::::Description.:Detection and correction.
  
  The signs of settling with power are a vibration in the main rotor system followed by an increasing sink rate and possibly a decrease of cyclic authority.
   The shape of the nose, windshield or canopy of an aircraft affects the sound produced. Much of the noise of a propeller aircraft is of aerodynamic origin due to the flow of air around the blades. The helicopter main and tail rotors also give rise to aerodynamic noise. This type of aerodynamic noise is mostly low frequency determined by the rotor speed.
  
  Typically noise is generated when flow passes an object on the aircraft, for example, the wings or landing gear. There are broadly two main types of airframe noise:
   Frequency spectrum analysis of the audio recordings indicated an increase in the amplitude of frequencies associated with main rotor vibration towards the end of the recording. The recording ended abruptly with three unusual, probably structure-borne, sounds.
   BULLET::::- Aeroacoustics — Is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. A notable example of this phenomenon is the Aeolian tones produced by wind blowing over fixed objects.
   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.
  
  Section::::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.
   BULLET::::- AMCA Publication 311 - "Certified Ratings Program - Product Rating Manual for Fan Sound Performance" explains in detail the certification procedures for both ducted and non-ducted fans under which the use of the AMCA International Certified Ratings Program Seal for sound performance is granted.
  
  BULLET::::- AMCA Publication 511 - "Certified Ratings Program - Product Rating Manual for Air Control Devices" describes the certification procedures under which the AMCA International Certified Ratings Seal is granted for sound, air performance, air leakage, water penetration and wind driven rain for air control products.
   Vortex-induced vibration (VIV) is an important source of fatigue damage of offshore oil exploration and production risers. These slender structures experience both current flow and top-end vessel motions, which give rise to the flow-structure relative motion and cause VIV. The top-end vessel motion causes the riser to oscillate and the corresponding flow profile appears unsteady.
   Many older LEDE ("live end, dead end") control room designs featured so-called "Haas kickers" – reflective panels placed at the rear to create specular reflections which were thought to provide a wider stereo listening area or raise intelligibility. However, what is beneficial for one type of sound is detrimental to others, so Haas kickers, like compression ceilings, are no longer commonly found in control rooms.
  
  Section::::See also.
  
  BULLET::::- Binaural fusion
  
  BULLET::::- Franssen effect
  
  Section::::Further reading.
  
  BULLET::::- Floyd Toole "Sound Reproduction", Focal Press (July 25, 2008), Chapter 6
   BULLET::::- Engine and other mechanical noise
  
  BULLET::::- Aerodynamic noise
  
  BULLET::::- Noise from aircraft systems
  
  Section::::Mechanisms of sound production.:Engine and other mechanical noise.
  
  Much of the noise in propeller aircraft comes equally from the propellers and aerodynamics. Helicopter noise is aerodynamically induced noise from the main and tail rotors and mechanically induced noise from the main gearbox and various transmission chains. The mechanical sources produce narrow band high intensity peaks relating to the rotational speed and movement of the moving parts. In computer modelling terms noise from a moving aircraft can be treated as a line source.
   Section::::Ambient Devices.
   Section::::Trucks.:Emergency vehicles.
  
  Many fire trucks, ambulances, and other large emergency vehicles operate air horns as a means of warning vehicles to clear the right-of-way.
  
  There are also electronic horns for emergency vehicles, which produce a similar easily recognizable sound. These are typically integrated into the same system as the vehicle's electronic siren, and sound through the same speakers. In the last several decades, electronic sound systems with more widely varying frequencies have been chosen as common supplemental warning systems.
  
  Section::::Locomotives.
   BULLET::::- Sound Redirection: One of the obvious ways to reduce the received sound level of an observer is to place the observer out of the path of the highest amplitude sounds. For example, if we mark off a circle around a jet engine and make sound power level observations along that circle, we would expect that the sound is loudest directly in line with the jet's exhaust. Observations perpendicular to the exhaust would be significantly quieter.
   There seems to be evidence that events similar to the hole tone occur on aircraft landing gear covers with circular holes. In Australia, there is the "Tenterfield Fox Whistle" and the "Traditional Fox Whistle" that appear to operate as hole tones.
  
  Section::::Monopole-like whistles.:Corrugated pipe whistle.
  