16.2 Nonlinear effects in sound propagation

Nonlinear acoustics explores sound waves with large amplitudes, causing effects that violate the superposition principle. This field studies phenomena like wave distortion, new frequency generation, and amplitude-dependent sound speed, which become significant in high-intensity or long-distance scenarios.

Practical applications of nonlinear acoustics include medical ultrasound imaging, focused ultrasound therapy, and underwater sonar technology. These utilize effects like harmonic generation and acoustic streaming to enhance performance, though challenges remain in predicting and controlling nonlinear phenomena.

Fundamentals of Nonlinear Acoustics

Nonlinear vs linear acoustics

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• Nonlinear acoustics studies sound waves with large amplitudes causing nonlinear effects violating superposition principle
• Linear acoustics assumes small-amplitude sound waves based on linear wave equation
• Key differences include wave distortion, generation of new frequency components, and amplitude-dependent speed of sound in nonlinear regime
• Nonlinear effects become significant with high sound pressure levels, long propagation distances, or medium properties strongly dependent on pressure or temperature (shock waves in air, cavitation in water)

Phenomena in nonlinear acoustics

• Acoustic streaming induces steady fluid flow from high-intensity sound waves transferring momentum from sound to fluid (Rayleigh, Eckart, Schlichting streaming)
• Radiation pressure exerts time-averaged force by sound waves on objects or interfaces proportional to sound intensity used in acoustic levitation (manipulation of small particles)
• Acoustic saturation limits sound amplitude growth due to nonlinear effects balancing energy transfer to higher harmonics with energy input (maximum achievable sound pressure level for given frequency and distance)

Nonlinear Acoustics in Practice

Nonlinear effects on sound propagation

• Air experiences shock wave formation in high-amplitude sound generating harmonics and spectral broadening (sonic booms)
• Water enhances absorption of high-frequency components and induces cavitation at high intensities (underwater explosions)
• Biological tissues increase heating due to nonlinear absorption forming localized high-pressure regions (focused ultrasound therapy)
• Solid materials exhibit nonlinear elasticity leading to harmonic generation and dislocation movement causing microstructural changes (ultrasonic welding)

Applications of nonlinear acoustics

• Medical ultrasound enhances image contrast through harmonic imaging improving resolution via nonlinear beam forming (fetal imaging)
• High-Intensity Focused Ultrasound (HIFU) enables noninvasive thermal ablation of tissues with enhanced heating efficiency due to shock formation (tumor treatment)
• Nondestructive testing and evaluation utilize nonlinear effects for improved defect detection (crack identification in metals)
• Acoustic cleaning and sonochemistry harness nonlinear effects for enhanced chemical reactions (ultrasonic cleaning of jewelry)
• Underwater acoustics and sonar technology benefit from nonlinear propagation for long-range detection (submarine detection)
• Challenges include difficulty predicting and controlling nonlinear effects, increased modeling complexity, and potential unintended biological effects in medical applications (tissue damage in ultrasound therapy)