Cycling Wind Noise and How We Can Help
(The Physics of Cycling Wind Noise: From Aeroacoustic Generation to Mitigation)
For many cyclists, the pleasure of the open road or trail is often accompanied by an unwelcome companion: relentless wind noise. This isn't a minor annoyance; it significantly impacts safety, communication, and riding enjoyment. But what exactly is this pervasive noise, and how can we effectively combat it? The answer lies in understanding the complex physics of air movement around your ears, particularly illuminated by the work of aeroacoustics pioneer Ffowcs Williams.
Wind Noise Physics: Pseudo-Sound vs. Acoustic Sound
The prevailing roar you hear while cycling is primarily a phenomenon known as 'pseudo-sound'. This concept, thoroughly explored in Ffowcs Williams' "Hydrodynamic Noise" paper (1969), describes large, localized pressure fluctuations that are not propagating sound waves in the traditional sense.
"The hydrodynamic pressure fluctuations are essentially the non-radiating components of the flow, representing the 'pseudo-sound' that does not propagate to the far-field as true acoustic energy." - J.E. Ffowcs Williams, (1969), Hydrodynamic Noise, Annual Review of Fluid Mechanics 1, pp. 197 - 222.
Imagine the air around your head like a chaotic, turbulent river. As this 'river' hits obstacles, such as your ears or helmet straps, it creates swirling eddies and unpredictable currents. The rapid and intense pressure changes from these eddies and currents, which directly affect your eardrum, are what we call 'pseudo-sound'. Because your ears are in the middle of this turbulent flow (i.e., near field), these pressure changes can feel stronger than actual sound waves nearby. Basically, you're feeling the wind's turbulent kinetic energy directly on your eardrum. Research by Arndt et al. provides a basis for understanding the pressure spectra and variation with speed.
Navier-Stokes equations describe the physics of the shear layer that forms over the concha cavity of the ear, while the Poisson Pressure Equation calculates the resulting pseudo-sound.
The kinetic energy spectrum, denoted as E(k), follows the famous Kolmogorov k−5/3 law. This law describes how the kinetic energy is distributed across different scales of turbulence (where k is the wavenumber, or inverse of eddy size). This relationship is a consequence of the energy cascade, the process where energy is transferred from large eddies to smaller ones.
The hydrodynamic pressure fluctuation spectrum, denoted as Ep(k), follows a k−7/3 law. This steeper scaling is a direct result of the pressure field being a quadratic function of the velocity field, a relationship explicitly shown by the Poisson pressure equation (pseudo-sound).
While 'pseudo-sound' is a dominant component of cycling wind noise, turbulence interaction with your ears also produces some propagating sound waves. When turbulence impacts your ears and helmet, it creates fluctuating forces. These forces act as 'acoustic dipoles', generating sound waves, a concept explained by Curle's extension of Lighthill's acoustic analogy. Powell's theory of 'vortex sound' provides further insights into the interaction between turbulent flow and solid surfaces. Essentially, the turbulent kinetic energy interacting with the ears drives both the hydrodynamic pressure fluctuations and the 'dipole acoustic waves' that constitute cycling ear wind noise.
Hydrodynamic wind noise is predominantly lower frequency due to its association with large, energy-containing eddies at the integral length scales of the turbulent flow (per Kolmogorov's theory); higher frequency 'dipole wind noise' arises from the interaction of smaller, faster-fluctuating turbulent eddies with solid surfaces. Where the two sources overlap, there can be an additive effect, resulting in an elevated overall noise intensity at the cyclist's ears.
Pioneering work by Kristiansen and Pettersen in their 1978 study provided the first reported scientific explanation for the origin of noise heard by humans exposed to atmospheric winds. Their research demonstrated that this noise is predominantly low-frequency, often perceived as a rumbling tone, and arises from fluctuating aerodynamic pressures. They also found that both the noise level and its dominant frequency vary significantly with changes in wind velocity and angle of incidence, and notably, observed substantial inter-subject variability in perceived noise levels, which they attributed to the unique anatomical characteristics of individual head shapes.
Kristiansen, U. R., & Pettersen, O. K. Ø. (1978). Experiments on the Noise Heard by Human Beings When Exposed to Atmospheric Winds. Journal of Sound and Vibration, 58(2), 285-291.
Why Wind Noise is More Than Just Annoying
Loud and intrusive ear wind noise when cycling presents several issues:
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Compromised Safety: The constant roar masks environmental sounds. The approaching sound of a car. Shouted warnings or the subtle click of a shifting gear can be drowned out, increasing the risk of accidents.
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Impaired Communication: Riding with friends can become an exercise in shouting and misunderstanding. Ineffective communication impedes group rides, jeopardizing safety, and diminishes the shared experience.
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Rider Discomfort / Fatigue: The volume of wind noise can be fatiguing over long rides, detracting from the peaceful and meditative aspects of cycling. It can lead to a sense of exhaustion or simply feeling worn out.
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Noise Exposure >85 Decibels: Cycling at speeds >15 mph can produce ear-wind noise exposure >85 dB.
The Cat-Ears Solution: Taming the Turbulence
Recognizing that the primary culprit is turbulent flow at and around the ears, effective solutions must focus on taming this turbulence. This is where Cat-Ears come in. Our products are made with porous pile materials that interact with the airflow to reduce both velocity and turbulence intensity. This ability of porous pile materials has been extensively researched and validated by Kudo, Nishimura, and Nishioka in their studies from 1999 to 2010.
"It is remarkable that the near-wall airflow is very calm for the pile-fabric (material). We see that the separated shear layer is thicker and weaker for the pile-fabric compared with the smooth case. In fact, the streamwise position where the shear fluctuations become maximum is located at about x/d=2.5 for the pile-fabric while it is about x/d=0.4 for the smooth surface." - Kudo, Nishimura, Nishioka, (1999), Aerodynamic Noise Reducing Techniques by Using Pile-Fabrics., 5th AIAA/CEAS Aeroaoustics Conference., AIAA-Paper.
Flow Visualization in our Wind Tunnel (~20 mph to verify the Turbulence Distribution).
Pile fabric / material dissipates kinetic energy, breaking down larger eddies into smaller, quieter ones. Cat-Ears / AirStreamz address the physics of how wind noise is created at the source, allowing you to hear more of what truly matters on your ride. With our unique products, you can reclaim your senses on the road, enhancing safety, improving communication, and making every mile more enjoyable.
Our products are road, trail, and wind tunnel tested. We use professionally calibrated mini microphones properly placed inside the ear canals. Full signal analysis is performed with MATLAB and Sigview Spectral Analysis Software. Hot-wire anemometers are used to quantify airflow velocity and turbulence intensity around the ears, as well as to perform cross-correlation analysis. Yaw and pitch tested, Cat-Ears products have always been the highest-rated.
Blocking or deflecting wind is not as effective due to flow separation over and around the barriers, which creates turbulent eddies similar to wake turbulence. The Flow Visualization above is a Cat-Ears AirStreamz on the left and an actual wind-blocking product on the right. By taming the turbulent flow and the chaotic pressure fluctuations it creates, our products address the root cause of wind noise more comprehensively than a mere physical obstruction. Blocking or deflecting can impact ambient sound perception. Cat-Ears and AirStreamz are acoustically transparent.
At Cat-Ears, we imagine, solve, design, and lead. Always with unyielding integrity.