midterm-ACOUSTICS

Page 1

Acoustics

• Overview of the study of sound and its properties.

Page 2

Understanding Sound

• Author: Malcolm J.W. Povey, 1997.

• Sound theory rooted in classical physics, specifically Newton's laws of motion.

• Key Point: Velocity of sound is related to compressibility and density.

• Laplace's Contribution: Differentiated between adiabatic and isothermal compressibility.

• Sound Theory Evolution: Rayleigh’s advancement of wave theory; Strutt’s result on sky color relates to sound scattering based on wavelength.

Page 3

Wave Theory: The Basics

• Sound Definition: Sound originates from vibrations (e.g. speaker cone, piano string).

• Molecular Disturbance: Air molecules create dense (compressions) and less dense (rarefactions) areas leading to energy transfer.

• Wave Nature: Energy transfer occurs parallel to the direction of vibration.

Page 4

Wave Theory: The Basics (Continued)

• Air Pressure Measurement: Microphone detects changes in air pressure.

  • Positive voltage during compressions (higher pressure).

  • Negative voltage during rarefactions (lower pressure).

Page 5

Sound Theory

• Sound sensation stems from air vibrations and pressure fluctuations.

• Vibrations: Mechanical in nature; air has mass and stiffness affecting sound travel.

• Simple Harmonic Vibrations: Pure tones characterized by one frequency.

• Measurement Units: Sound pressure (Pascals) and sound intensity (Watt/m²).

Page 6

Types of Sound

• Categories: Audible, inaudible, pleasant, unpleasant, soft, loud, noise, music.

• Frequency Ranges: Human hearing: 20 Hz to 20,000 Hz.

  • Infrasonic Waves: Below 20 Hz, used in earthquake detection, animal communication.

  • Ultrasonic Waves: Above 20,000 Hz, utilized in medical imaging and navigation.

Page 7

Amplitude and Frequency

• Amplitude: Measures maximum deviation in periodic motion.

  • High amplitude correlates with loud sounds; low amplitude indicates quiet sounds.

• Frequency: Measured in Hertz (Hz); represents cycles per second.

  • Relevant frequency range for human hearing: 20 Hz to 20 kHz.

• Wavelength Characteristics: High-frequency waves with shorter wavelengths have lower energy, while low-frequency waves with longer wavelengths contain high energy.

Page 8

Longitudinal and Transverse Waves

• Longitudinal Waves: Sound travels as compressions in gases and liquids.

• Transverse Waves: Created in solids; particle movement is perpendicular to wave direction.

• Slinky Analogy: Demonstrates wave properties through compressions and expansions.

Page 9

Transverse vs. Longitudinal Waves

• Transverse Waves: Medium particles move perpendicular to wave direction.

• Longitudinal Waves: Medium particles move parallel to wave direction.

Page 10

Mechanical Sound Waves

• Transmission Mechanism: Sound through air involves particle displacement creating a chain reaction.

• Mechanical Waves Characteristic: Require a medium to transmit energy; cannot propagate in a vacuum.

• Pressure Waves: Sound waves exhibit compressions and rarefactions, translating to low/high-pressure patterns.

Page 11

The Human Ear

• Frequency Sensitivity: Range: 20Hz to 20kHz, peaking at 3-4 kHz for speech.

• Sound Measurement Unit: Decibels (dB) ranges from 0 dB (threshold of hearing) to 120 dB (threshold of pain).

• Sound Filters: A-weighted filters account for human ear sensitivity variations across frequencies.

Page 12

The Four Types of Noise

1. Continuous Noise: Persistent noise from machinery.

2. Intermittent Noise: Fluctuating noise levels from passing trains or aircraft.

3. Impulsive Noise: Sudden bursts (e.g., construction).

4. Low-frequency Noise: Background hums often difficult to control.

Page 13

Sound Properties and Their Perception

• Velocity of Sound: Pressure disturbances propagate through medium via particle interaction.

• Speed Calculation: Speed defined as distance traveled by a wave per unit time.

Page 14

Factors Affecting Wave Speed

• Properties Influencing Speed: Inertia and elasticity of the medium.

• Material Examples: Steel (high elasticity) vs rubber band (flexible).

• Phase of Matter Order: Solids > Liquids > Gases in speed of sound transmission.

Page 15

Speed of Sound Measurement Example

• Measurement using data logger between two microphones.

• Speed comparison between different materials: Air, Water, Steel (343 m/s, 1493 m/s, 5130 m/s).

Page 16

Inverse Square Law

• Intensity of sound diminishes as distance from the source increases.

• Applies to sound, light, and gravitational forces.

Page 17

Inverse Square Law General

• Non-restricted point sources act according to inverse square law.

• Measure intensity at various distances.

Page 18

Inverse Square Law, Sound

• Sound intensity decreases with distance; sound waves obey this law under specific conditions.

Page 19

Measured Sound in a Room

• In real environments, sound intensity does not drop off as sharply due to reverberation.

Page 20

Sound Level Changes Rule

• Doubling the distance from a point source decreases intensity by 6 dB.

Page 21

Sound Reinforcement Model

• Simplified assumption of sound drop-off; amplification needs considered.

Page 22

Numerical Example of Amplification Needs

• Calculating sound levels at different distances; importance of amplification in achieving audible levels.

Page 23

Sound Pressure

• Definition and measurement of sound pressure in pascals or N/m²; logarithmic scale representation (SPL).

Page 24

Sound Pressure Level (SPL)

• Abbreviation for Sound Pressure Level expressed in decibels.

Page 25

Sound Pressure Level – The 6dB Rule

• Explanation of SPL distance effect; sound perceived louder as distance decreases.

Page 26

How Perceived Loudness Works

• Relationship between decibels and perceived loudness; increase of 10 dB perceived as double the loudness.

• OSHA noise exposure limits emphasize sound pressure impact.