6.1 Atmospheric effects on sound propagation

Atmospheric effects play a crucial role in outdoor sound propagation. Temperature gradients, wind patterns, and air composition all influence how sound travels through the air. These factors can cause sound waves to bend, scatter, or get absorbed, dramatically altering how we perceive noise outdoors.

Understanding these effects is key to predicting and controlling outdoor noise. By considering temperature inversions, wind refraction, and atmospheric absorption, engineers can better design noise barriers, plan urban layouts, and mitigate environmental noise impacts in various settings.

Temperature Gradients & Sound Propagation

Impact of Temperature Gradients on Sound Refraction

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• Temperature gradients in the atmosphere cause sound waves to refract, altering their direction of propagation
  • Positive temperature gradient (temperature increasing with height) causes upward refraction of sound waves
  • Negative temperature gradient (temperature decreasing with height) leads to downward refraction of sound waves
• The speed of sound increases with rising temperature, affecting sound wave refraction
  • In a positive temperature gradient, sound waves traveling upwards encounter higher temperatures and travel faster, tilting the wavefront upwards
  • The opposite effect occurs in a negative temperature gradient, with sound waves refracting downwards
• Temperature inversions, where warm air sits above cooler air, can create a "sound duct" that traps sound waves near the ground
  • This "sound duct" effect enhances sound propagation over long distances (several kilometers)
  • Temperature inversions are common at night or early morning when the ground cools faster than the air above it

Factors Influencing the Effect of Temperature Gradients

• The effect of temperature gradients on sound propagation is most pronounced at low frequencies and over long distances
  • Cumulative refraction effects become more significant as sound waves travel farther
  • Low-frequency sounds (infrasound) are more susceptible to refraction than high-frequency sounds
• The presence of a thermocline, a sharp change in temperature gradient, can cause significant refraction of sound waves
  • Thermoclines are common in bodies of water, where a layer of warm water sits above colder, denser water
  • Sound waves crossing a thermocline experience abrupt changes in speed and direction
• Other factors, such as humidity and air pressure, also influence the speed of sound and can affect refraction patterns
  • Higher humidity increases the speed of sound, while higher air pressure decreases it
  • These factors combine with temperature gradients to create complex sound propagation patterns in the atmosphere

Wind Effects on Outdoor Sound

Wind-induced Refraction of Sound Waves

• Wind causes sound waves to refract due to changes in the effective speed of sound
  • Sound waves traveling with the wind experience a higher effective speed, while those traveling against the wind have a lower effective speed
  • Downwind propagation of sound waves results in downward refraction, enhancing sound levels near the ground
  • Upwind propagation causes upward refraction, reducing sound levels near the ground
• The wind gradient, or the change in wind speed with height, determines the extent of refraction
  • A strong wind gradient leads to more significant refraction effects compared to a weak gradient
  • Wind speed typically increases with height due to reduced friction near the ground
• The combined effects of wind and temperature gradients can lead to complex sound propagation patterns
  • The dominant factor (wind or temperature) depends on the specific atmospheric conditions
  • In some cases, wind effects may dominate, while in others, temperature gradients may be more influential

Wind Turbulence and Sound Scattering

• Wind turbulence causes scattering and diffraction of sound waves, affecting sound propagation
  • Turbulent eddies in the wind can scatter sound waves in various directions
  • Scattering leads to fluctuations in sound levels and reduced coherence of the wavefront
• The intensity of wind turbulence depends on factors such as wind speed, surface roughness, and atmospheric stability
  • Higher wind speeds and rougher surfaces (forests, urban areas) generate more turbulence
  • Unstable atmospheric conditions (daytime heating) promote turbulence, while stable conditions (nighttime cooling) suppress it
• Wind turbulence can also cause amplitude and phase fluctuations in the received sound signal
  • These fluctuations can affect the intelligibility of speech or the localization of sound sources
  • Turbulence-induced fluctuations are more pronounced at higher frequencies and over longer distances

Atmospheric Absorption of Sound

Mechanisms of Atmospheric Absorption

• Atmospheric absorption is the conversion of sound energy into heat as sound waves propagate through the atmosphere
  • This process leads to a reduction in sound intensity over distance
• Classical absorption is caused by the viscosity and thermal conductivity of air
  • Viscosity-related absorption is due to the friction between air molecules as they oscillate
  • Thermal conductivity-related absorption occurs when sound waves compress and expand air, causing temperature fluctuations
• Molecular absorption is caused by the relaxation of vibrational and rotational modes of air molecules
  • Oxygen (O2) and nitrogen (N2) molecules absorb sound energy and convert it into internal energy
  • The relaxation time of these molecules determines the frequency-dependent absorption characteristics

Factors Affecting Atmospheric Absorption

• The amount of atmospheric absorption depends on several factors:
  • Frequency: Higher frequencies experience greater absorption than lower frequencies
  • Temperature: Higher temperatures lead to increased absorption
  • Humidity: Lower humidity levels result in higher absorption, especially at high frequencies
  • Pressure: Absorption is generally proportional to air pressure
• Atmospheric absorption is frequency-dependent, leading to changes in the frequency spectrum of sound over long distances
  • High-frequency content is attenuated more rapidly than low-frequency content
  • This results in a "low-pass filter" effect, where distant sounds have a muffled or bass-heavy character
• The ISO 9613-1 standard provides a method for calculating atmospheric absorption coefficients
  • The standard takes into account frequency, temperature, humidity, and pressure
  • Absorption coefficients can be used to predict sound attenuation due to absorption over a given distance
  • Example: At 1 kHz, 20°C, 50% relative humidity, and sea-level pressure, the absorption coefficient is approximately 0.5 dB/km

Atmospheric Refraction & Sound Propagation

Refraction and Snell's Law

• Atmospheric refraction is the bending of sound waves due to changes in the speed of sound caused by variations in temperature, wind, or other atmospheric conditions
• Refraction occurs when sound waves encounter a change in the medium's properties, causing the wavefront to change direction
  • Example: Sound waves passing from cool air to warm air will refract towards the cooler region due to the lower speed of sound in the cooler air
• Snell's law describes the relationship between the angles of incidence and refraction when sound waves pass through a boundary between two media with different sound speeds
  • The angle of refraction depends on the ratio of the sound speeds in the two media
  • Snell's law: $sin(θ1) / v1 = sin(θ2) / v2$, where $θ1$ and $θ2$ are the angles of incidence and refraction, and $v1$ and $v2$ are the sound speeds in the respective media

Sound Propagation in a Stratified Atmosphere

• In a stratified atmosphere with varying temperature or wind gradients, sound waves follow curved paths due to continuous refraction
  • The sound speed gradient determines the curvature of the sound path
  • A positive sound speed gradient (increasing with height) leads to upward refraction, while a negative gradient results in downward refraction
• Atmospheric refraction can lead to the formation of acoustic shadow zones and focus zones
  • Shadow zones are regions where sound levels are significantly reduced due to the upward refraction of sound waves
  • Focus zones are areas where sound levels are enhanced due to the convergence of refracted waves
• The presence of a sound speed gradient can result in the formation of sound channels, such as the SOFAR (Sound Fixing and Ranging) channel in the ocean
  • Sound channels allow sound to propagate over long distances with minimal attenuation
  • The SOFAR channel is formed by the combination of pressure-induced and temperature-induced sound speed gradients in the ocean
  • Sound waves entering the SOFAR channel are refracted back towards the center of the channel, enabling long-range propagation (thousands of kilometers)