Despite the prevalence of ear wind noise, a singular, comprehensive explanation remains elusive due to the multifaceted nature of the phenomenon.  Research is fragmented across diverse disciplines, including acoustics, fluid dynamics, and psychoacoustics, each addressing specific aspects without integrating the complex interplay.

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Complexity of the Problem:

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• Intricate Ear Geometry: The human ear, with its complex structure, poses significant challenges for accurate modeling and measurement.

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• Turbulent Flow Complexity: Turbulent flow, especially around complex shapes, is notoriously difficult to model and predict.

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• Near-Field Effects: The near-field region around the ear, where wind noise originates, is dominated by complex interactions between hydrodynamic and acoustic phenomena.

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The field of aeroacoustics is complex, and the distinction between hydrodynamic and acoustic pressure is crucial for a full understanding of the sound generation by flow.  At Cat-Ears, we are bridging existing knowledge gaps by systematically investigating the generation, propagation, and perception of cycling related ear wind noise.

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According to J.E. Ffowcs Williams (1969), pseudo-sound refers to the locally sensed, non-propagating pressure fluctuations within a turbulent flow.  These pressure changes are caused by the movement of turbulent eddies and, while audible if a microphone is placed nearby, they differ from true sound waves as they do not radiate outwards at the speed of sound but instead remain associated with the slower-moving turbulent structures, decaying quickly with distance from the source of the turbulence. 

 

​​When cycling, airflow interacts with the ear, creating complex velocity fluctuations in / around the concha cavity.  Since this airflow is largely incompressible at typical cycling speeds, the resulting local, non-propagating pressure fluctuations are governed by the Poisson Pressure Equation.  Stemming from the Navier-Stokes equations related to fluid motion, this equation mathematically links the spatial derivatives of the turbulent velocity field within and near the concha cavity to the instantaneous pseudo-sound pressure fluctuations.​

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Pseudo-Sound Simply Stated:

 

There is a direct correlation between the level of turbulence in the shear layer over the concha cavity and the resulting ear wind noise.  Here's a breakdown of that relationship:

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• Turbulence as Source:  Wind noise originates from turbulent airflow interacting with the head and outer ear (pinna).  When airflow separates from surfaces like the temple, cheekbone, or edges of the pinna, it creates unstable shear layers.  These shear layers break down into turbulent eddies.

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• Turbulence Creates Pressure Fluctuations:  This turbulent flow, particularly within the shear layers over the concha cavity, is characterized by chaotic fluctuations in air velocity.  These velocity fluctuations directly cause local, non-propagating pressure fluctuations, often referred to as hydrodynamic pressure or pseudo-sound.  The Poisson equation mathematically links these turbulent velocity fluctuations to the pressure fluctuations.

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• Pressure Fluctuations are Perceived as Noise:  These pressure fluctuations, generated very close to the ear canal entrance (in the near-field), exert forces on the eardrum, which the brain perceives as wind noise.  The intensity and characteristics of the turbulence influence the magnitude and character of these fluctuations.

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• Correlation Strength:

>  Higher Turbulence Intensity / Velocity:  Higher wind speeds lead to more intense turbulence (greater velocity fluctuations) in the shear layers.  This, in turn, generates more substantial pressure fluctuations, resulting in higher perceived wind noise levels.  Computational Fluid Dynamics simulations confirm that separation and vortex shedding are the primary drivers of these instantaneous pressure fluctuations.

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>  Turbulence Scale:  The size of the turbulent eddies (turbulence scale) also plays a role, particularly influencing the frequency content of the noise.  Larger eddies associated with lower frequencies contribute significantly to the characteristic "rumbling" sound of wind noise.  Research suggests that the scale of the turbulence might be more critical than the intensity of the incoming turbulence for LF noise generation.

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In summary, the turbulence level (encompassing intensity, velocity fluctuations, and scale) in the shear layer over the concha cavity is fundamentally linked to the generation of pressure fluctuations that constitute ear wind noise. Stronger, more energetic turbulence generally leads to louder wind noise.​

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Hotwire measurements in the shear layer over the concha cavity provide essential data about the velocity field and turbulence characteristics, which are direct inputs and crucial for understanding and potentially solving the Poisson pressure equation in that region, especially in the context of validating and informing CFD simulations.  They bridge the gap between the theoretical relationship described by the Poisson equation and the real-world turbulent flow.

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