Introduction to Acoustics

• Sound is a physical wave or mechanical vibration, a series of pressure variations in an elastic medium
• Airborne sound's medium is air
• Structure borne sound's medium is a wall ceiling
• Speech is a source of sound, specifically the human voice

Types of Acoustics

• Psychoacoustics, environmental acoustics, landscaping
• Electroacoustics involves generating and detecting audible sound
• Sonics (sonar) is sound navigation, usually for medicine

Elements of Acoustics

• Source
• Transmission
• Receiver

Architectural Acoustics

• Room acoustics involves the design of spaces
• Sound isolation is the intervention of architecture infrastructure like walls, ceilings, floors, and materials
• Background noise includes mechanical noise, traffic and water hammering
• Amplification is the electronic enhancement of music and sound

Music and Noise

• Music is produced by an instrument
• Noise is produced by impact, vibrating bodies, even speech

Types of Sound

• Wanted sound is heard perfectly at the right level/loudness without pain/strain
• Unwanted sound entails annoyance

Properties of Sound

• Amplitude is the level of sound
• Frequency: Higher pitch correlates with higher frequency (distance of soundwave)
• AM (amplitude modulation) has a higher range
• FM (frequency modulation) has a shorter range
• Propagation is the transmission of soundwaves through a medium (water, gas)
• Speed of sound is 343m/sec
• Diffraction of sound is how sound spreads out

Sound Theory

• Sound cannot travel in vacuum spaces

Basic Types of Waves

• Longitudinal Wave: pressure wave (left to right)
• Transverse Wave: Acoustic waves vibrate perpendicular to the wave's propagation (up and down). Ripples are an example

Compression and Rarefaction

• Compression: Region with high pressure
• Rarefaction: Region with lower pressure

Frequency

• Measured by cycle per second
• Hz is the unit of frequency
• 20-20K Hz: Level of sound audible to the human ear
• Presbycusis: The audible range decreases over time
• Frequency of speech ranges from 20-8000 Hz

Sound Categories

• 0-20 Hz: Infrasound
• 20 Hz-200 Hz: Bass sound
• 200 Hz-2,000 Hz: Medium sound
• 2,000 Hz-20,000 Hz: Treble sound
• 20M Hz: Ultrasound

Propagation and Attenuation

• Propagation: sound waves travelling via a compressible medium like air or water
• Attenuation: Reduction of sound, sound weakens with distance from source

Altering Sound

• Physical disturbance: sound can be altered/reduced
• Sound sensation: auditory sensation produced by a sound.
• Sound motion: propagation of sound waves is the disturbance in pressure moving through a medium like air

Speed of Propagation

• Reduced compression lowers the speed of sound.
• Velocity of propagation depends on the medium, travels through solids/structures
• Speed of sound at sea level: 344m per sec

Speed of Sound in Differing Mediums

• Sound travels faster in liquid than in air
• Liquid: 1440m per sec
• Wood: 3100m per sec
• Bricks: 3600m per sec
• Concrete: 3700m per sec
• Steel: 4900m per sec
• Glass: 5000m per sec
• Aluminum: 5800m per sec
• Velocity of sound varies through temperature; Higher altitudes reduce speeds
• Inverse Square Law: For doubling of the distance, sound diminishes by 6 decibels

Loudness of Sound

• Breathing: 10 dB
• Whisper: 30 dB
• Refrigerator sound: 40 dB
• Moderate to Quite: 60 dB
• Loud: 70 dB
• Very Loud: 80 dB
• Air Dryer: 90 dB
• Helicopter: 100 dB
• Extremely Loud: 110 dB
• Siren: 120 dB
• Aircraft: 130 dB
• Fireworks (140 dB) are the threshold of pain in the human ear
• Constant contact results in permanent hearing loss

Sound Pressure

• Local pressure from the ambient atmospheric pressure, produced when a sound wave propagates
• SI unit: Sound Pressure pascal for pressure
• Pressure is a component of sound wave along with velocity
• Audible range: 20 micro pascals to 20 pascals
• SPL meter: Measures sound level
• Decibel (dB spl): is the more useable unit
• The range from 20 micro pascals to 20 pascals is represented 0 dB spl to 120db spl

Sound Intensity

• Sound intensity at a point: rate at which energy flows, measured in watts per square meter
• The ear is not like a microphone, it picks up some sound easier than others
• The ear is particularly sensitive to sounds in the 2000-4000 Hz frequency range
• Our ears do not detect differences in sound

Sound and Safety

• The highest sound that can be heard safely is about times as intense as the lowest
• Danger zone: 80-150 dB
• Exposure to sounds above a certain level can permanently damage hearing
• Health and safety regulations require max level exposure to sound with protection

Sound Absorption

• Sound absorption, reflection, and transmission are the most important performances in acoustic design
• Measures the amount of sound energy absorbed by a material
• Sound absorption coefficients describe a material's ability to absorb:
  • 1- no absorption
  • 2- means 100% absorption