7.1 Concept of acoustic impedance

Acoustic impedance measures how much a medium resists sound waves. It's crucial for understanding how sound behaves in different materials. This concept helps explain why sound travels differently in air versus water, and why we hear echoes in certain spaces.

Calculating acoustic impedance involves density and sound speed. Different types exist, like specific and characteristic impedance. These concepts are key to grasping sound reflection, transmission, and absorption in various environments and applications.

Understanding Acoustic Impedance

Definition of acoustic impedance

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• Acoustic impedance quantifies medium's resistance to sound wave propagation measured as ratio of acoustic pressure to particle velocity
• Higher impedance slows sound propagation while lower impedance accelerates it (water vs air)
• Density and elasticity of medium significantly affect impedance values (steel vs rubber)
• Influences sound wave behavior through reflection, transmission, and absorption at boundaries between media (air-water interface)

Calculation of acoustic impedance

• Formula: $Z = \rho c$ where $Z$ is acoustic impedance, $\rho$ is medium density, and $c$ is speed of sound
• Measured in Pa·s/m or Rayl (kg/m²·s) representing resistance to sound flow
• Temperature impacts calculation by altering medium density and sound speed (warm vs cold air)
• Pressure variations in gases affect impedance values (atmospheric pressure changes)

Types of acoustic impedance

• Specific acoustic impedance varies with frequency and position in sound field representing local sound pressure to particle velocity ratio
• Characteristic impedance remains constant for a medium independent of frequency or position describing inherent acoustic property
• Both types measured in same units and describe resistance to sound propagation
• Specific impedance applies to complex sound fields while characteristic impedance used for plane waves in lossless media

Effects of acoustic impedance

• Impedance mismatch between media causes reflection and transmission at boundaries (air-glass interface)
• Transmission coefficient quantifies sound intensity transferred between media based on their impedance ratio
• Reflection coefficient measures sound intensity reflected at boundary calculated using impedance values
• Standing waves form due to impedance discontinuities (organ pipes)
• Impedance matching techniques minimize reflection and maximize transmission (acoustic treatments)
• Applications include room acoustics, musical instrument design, and ultrasound imaging