SOUND WAVES

Sound Waves

• Sound is a longitudinal mechanical wave.
• Sound waves propagate through a medium.
• Sound is a sensation in the human brain, responding to pressure fluctuations in the air.
• When an object moves through the air, it disturbs nearby air molecules.
• These disturbances travel at the speed of sound through the air.

What is a Wave?

• A wave is a vibration or disturbance in space.
• A medium is the substance sound waves travel through.

Types of Waves

• Mechanical waves need a medium to propagate.
  • Water waves
  • Sound waves
• Electromagnetic waves do not need a medium to propagate.
  • X-rays
  • Radio waves
  • Light

Direction of Propagation

• Waves can be categorized into transverse and longitudinal.
• Transverse waves: The particles of the medium vibrate perpendicular to the direction of the wave.
  • E.g. water wave, electromagnetic waves traveling in a medium.
• Longitudinal waves: The particles of the medium vibrate parallel to the direction of the wave.
  • E.g. sound waves
  • Two main sections:
    • Compression: high molecular density and pressure.
    • Rarefaction: low molecular density and pressure.

Describing Waves

• Wavelength (λ): Distance between identical points on successive waves
• Amplitude (A): Maximum distance a particle is displaced from its equilibrium position.
• Velocity (v): Speed at which the wave propagates through the medium.
• Period (T): Time it takes for two successive maxima (or minima) to pass a point.
• Frequency (f): Inverse of the period (f = 1/T).

Wave Speed

• Wave speed (v) = wavelength (λ) x frequency (f).

Example

• A harmonic wave travels along a rope. The oscillator completes 40 vibrations in 30 seconds.
• A given maximum travels 425 cm along a rope in 10 seconds.
• The wavelength is 0.319 m.

Sound Waves (continued)

• Sound waves are a special type of longitudinal wave.
• Sound travels as pressure variations.

Energy Conversion (Hearing)

• Air pressure waves vibrate the eardrum.
• Middle ear converts vibration to mechanical energy and then hydraulic energy.
• Hydraulic energy stimulates sensory cells in the inner ear.
• Sensory cells create electrical impulses that travel to the brain.

Propagation of Sound Waves

• Sound waves are characterized by frequency, wavelength, speed, period, and amplitude.
• Sound waves can travel through solids, liquids, and gases.
• Sounds propagate as alternating pressure waves, causing compression and rarefaction.
• The particles of the transmitting matter vibrate in the direction of propagation.
• Stimulating frequencies lead to hearing.

Sound vs. Light

• Sound is a mechanical, longitudinal wave; light is an electromagnetic, transverse wave.
• Sound needs a medium to travel; light does not.
• Loudness is measured in decibels (dB).

Perception of Sound

• Hearing is the sense of detecting sound.
• Sound is detected by the ear.
• The ear transduces sound into nerve impulses.
• These impulses are processed by the brain to create the perception of sound.

Frequency Range of Sound Waves

• The human ear can hear sounds with frequencies between 20 Hz and 20 kHz.
• Different animals and applications, have a wider frequency range too.

Power and Intensity of a Sound Wave

• Power (P) is the rate of energy production (measured in watts).
• Intensity (I) is power per unit area (measured in W/m² or decibels).
• Power and intensity increase with pressure (p).

Intensity of a Sound Wave

• Intensity is the amount of energy in sound waves.
• More intense sounds are generated by sources producing greater pressure maxima and minima.
• Sound intensity determines loudness.

Pitch of a Sound Wave

• Pitch is the sensation of a sound's frequency. High-frequency sound waves have a high pitch; low-frequency sounds have a low pitch.
• The higher frequency, the shorter the wavelength; the lower frequency, the longer the wavelength.

Loudness vs. Pitch

• Loudness is dependent on the amplitude of the wave; pitch is dependent on the frequency of the wave.
• Loudness- Amplitude dependent
• Pitch - Frequency dependent

Wavelengths of Sound Waves

• Short wavelengths correlate with high frequencies and high pitch.
• Long wavelengths correlate with low frequencies and low pitch. Examples of different sounds and their correlating frequencies are included.

Noise-Induced Hearing Loss (NIHL)

• NIHL occurs when structures in the inner ear become damaged due to loud noises.
• Loud noises damage the cochlea in the inner ear.
• This results in permanent hearing loss.

Wave Interaction with Matter

• Transmission: Waves passing through a medium.
• Reflection: Waves bouncing off a surface.
• Scattering: Waves being dispersed in various directions.
  • Occurs when sound encounters small objects or variations in density.
• Diffraction: Waves bending around an obstacle or through an opening.
• Refraction: Waves changing direction as they pass from one medium to another.

Reflection

• When a wave reaches a boundary, part is reflected and transmitted.
• The degree of reflection depends on the dissimilarity of the two media.
• The laws of reflection apply to flat surfaces.

Principles of Sound Waves

• Sound waves generate vibrations in objects, which then creates new sound waves.
• Speech is generated by vibrations of the vocal cords.

Refraction

• Sound waves change direction or bend when passing from one medium to another at an angle other than 90 degrees.
• The speed of sound changes with the medium's temperature; warmer air means faster sound.

Standing vs Traveling Waves

• Standing Waves: Result from the superposition of two waves of equal amplitude and frequency traveling in opposite directions.
  • Characterized by stationary nodes and antinodes.
• Traveling Waves: Continuously move in a single direction.
  • Longitudinal or transverse
• Waves can interfere constructively or destructively when they overlap.

Harmonics- Closed Pipe

• Harmonics occur as multiples of the fundamental frequency in closed pipes.
• Only odd harmonics of a closed pipe can be observed; these produce sound.
• The shortest length is a quarter of the wavelength.

Harmonics- Open Pipe

• Harmonics occur as multiples of the fundamental frequency in open pipes.
• Both odd and even harmonics are observable from open pipes.
• The fundamental frequency is half of the wavelength.

Doppler Effect

• The Doppler effect is the change in frequency of a wave in relation to an observer moving relative to the source.
• Moving sound sources towards a listener have a higher pitch and vice versa.
• The speed of sound in air is around 343 meters per second at normal atmospheric pressure and temperature.

Mach Number

• The Mach number is the ratio of the object's speed to the speed of sound.
• Mach 1 means the object is traveling at the speed of sound.
• Over Mach 1, the object breaks the sound barrier.

Applications of Sound

• Echoes: Used to determine distance (submarines, ultrasound).
• Stethoscope: Used to listen to internal sounds in the body (heart, lungs).
• Ultrasound: Used for medical imaging.
• Example: Ultrasound machines are healthier than X-rays as the frequency of Ultrasound waves are of a lesser frequency, reducing damage to the fetus.

Ultrasound Interactions

• Reflection: When a sound wave bounces off a boundary.
• Refraction: When a sound wave changes direction at a boundary.
• Attenuation: When a sound wave loses intensity as it travels through a medium
• Scattering: When sound waves are dispersed as they hit variations in density
• Absorption: When the wave is converted into heat.
• Diffraction: When a wave bends around an obstacle.

Doppler Ultrasound

• Based on the frequency shift between an emitted ultrasound beam and the received echo.
• Reflects off moving blood or other structures to image movement.

Ultrasound in Medical Therapy

• Applying ultrasound to certain body areas and frequency ranges can reduce pain.
• Used in specific intensities to destroy, or break down, structures (e.g. gallstones, tumours).

Ultrasound Techniques

• Ultrasonic Cavitation: Breaking down fat cells using ultrasound vibrations.
• Doppler Imaging: Used for imaging blood flow within vessels.