1.3 Sound wave characteristics and behavior

Sound waves are fascinating phenomena that shape our auditory world. They travel through various media, each with unique properties affecting propagation speed and behavior. Understanding how sound waves move and interact is crucial for grasping acoustics fundamentals.

Reflection, refraction, and attenuation play key roles in how we perceive sound. These processes explain why sound behaves differently in various environments, from echoes in large rooms to muffled voices through walls. The Doppler effect adds another layer, explaining pitch changes in moving sound sources.

Sound Wave Propagation and Behavior

Propagation of sound waves

Top images from around the web for Propagation of sound waves

• 

  6.4 Sound – Introduction to Oceanography View original

  Is this image relevant?

• 

  15.5: Waves - Physics LibreTexts View original

  Is this image relevant?

• 

  6.4 Sound – Introduction to Oceanography View original

  Is this image relevant?

• 

  15.5: Waves - Physics LibreTexts View original

  Is this image relevant?

1 of 2

Top images from around the web for Propagation of sound waves

• 

  6.4 Sound – Introduction to Oceanography View original

  Is this image relevant?

• 

  15.5: Waves - Physics LibreTexts View original

  Is this image relevant?

• 

  6.4 Sound – Introduction to Oceanography View original

  Is this image relevant?

• 

  15.5: Waves - Physics LibreTexts View original

  Is this image relevant?

1 of 2

• Longitudinal wave characteristics
  • Compression and rarefaction alternate regions of high and low pressure propagate through medium
  • Particle displacement parallel to wave direction oscillates back and forth along propagation axis
• Speed of sound in different media
  • Air: approximately 343 m/s at 20℃ varies with temperature and humidity
  • Water: approximately 1480 m/s at 20℃ faster due to higher density and incompressibility
  • Solids: varies widely, generally faster than liquids or gases (steel ~5000 m/s, wood ~3300 m/s)
• Factors affecting sound propagation
  • Temperature increases speed of sound in gases (air ~+0.6 m/s per ℃)
  • Density higher density typically increases speed (mercury vs water)
  • Elasticity of the medium more elastic materials transmit sound faster (steel vs rubber)
• Impedance
  • Resistance to sound wave propagation measures how easily sound travels through medium
  • Relationship: $Z = \rho c$, where $\rho$ is density and $c$ is speed of sound determines reflection and transmission at boundaries
• Transmission between media
  • Impedance mismatch greater difference leads to more reflection (air-water interface)
  • Energy transfer and reflection at boundaries partial transmission and reflection occur at interfaces

Reflection and refraction of sound

• Reflection
  • Occurs when sound waves encounter a boundary bounce back from surfaces
  • Angle of incidence equals angle of reflection follows law of reflection
  • Specular vs diffuse reflection smooth surfaces produce specular, rough surfaces produce diffuse
• Refraction
  • Change in wave direction due to speed change bends as it enters new medium
  • Snell's law: $\frac{\sin \theta_1}{\sin \theta_2} = \frac{v_1}{v_2}$ relates angles of incidence and refraction to wave speeds
  • Temperature gradients causing sound refraction in air creates mirages and sound shadows
• Diffraction
  • Bending of waves around obstacles or through openings allows sound to "bend" around corners
  • Huygens' principle each point on wavefront acts as new source of wavelets
  • Relationship between wavelength and obstacle size more pronounced for wavelengths similar to or larger than obstacle
• Interference
  • Constructive and destructive interference waves add or cancel based on phase
  • Standing waves and resonance form in enclosed spaces (musical instruments, room modes)

Sound wave attenuation factors

• Attenuation
  • Reduction in amplitude over distance sound becomes weaker as it travels
  • Causes: geometric spreading, absorption, scattering energy dissipates and spreads out
• Absorption
  • Conversion of sound energy to heat materials dampen sound vibrations
  • Porous materials and their effectiveness (acoustic foam, fiberglass)
  • Absorption coefficient measures fraction of incident sound energy absorbed
• Factors influencing attenuation and absorption
  • Frequency dependence higher frequencies generally attenuate more rapidly
  • Material properties (density, porosity, stiffness) affect absorption characteristics
  • Thickness of absorbing materials thicker materials typically absorb more effectively
• Transmission loss
  • Measures sound reduction through barriers or partitions
  • Mass law for single-layer partitions doubling mass increases TL by ~6 dB
• Reverberation time
  • Sabine formula: $T_{60} = \frac{0.161V}{A}$ relates room volume to absorption
  • Relationship to room acoustics and absorption longer RT in reflective spaces, shorter in absorptive

Doppler effect in acoustics

• Doppler effect principle
  • Apparent change in frequency due to relative motion perceived pitch changes
  • Formula: $f' = f\frac{c \pm v_r}{c \pm v_s}$ calculates observed frequency
• Scenarios
  1. Stationary source, moving observer pitch increases as observer approaches, decreases as recedes
  2. Moving source, stationary observer pitch increases as source approaches, decreases as recedes
  3. Both source and observer moving combined effect of both motions
• Applications
  • Traffic speed measurement police radar guns use Doppler shift
  • Medical ultrasound (blood flow measurement) detects flow velocity and direction
  • Radar systems measure target speed and direction
  • Astronomical observations (redshift/blueshift) determine celestial object motion
• Limitations and considerations
  • Medium motion effects wind can influence Doppler shift in air
  • Relativistic Doppler effect at high velocities requires special relativity corrections