🔇Noise Control Engineering Unit 5 – Sound Transmission and Insulation

Fundamentals of Sound

• Sound is a mechanical wave that propagates through a medium (air, water, solids) by causing particles to oscillate and transfer energy
• Characterized by frequency measured in Hertz (Hz) determines pitch and wavelength (distance between two consecutive compressions or rarefactions)
• Amplitude of sound waves corresponds to loudness or intensity measured in decibels (dB)
  • Logarithmic scale where 0 dB is the threshold of human hearing and 120 dB is the threshold of pain
• Speed of sound varies depending on the medium (343 m/s in air at 20°C, 1,480 m/s in water, 5,000 m/s in steel)
• Human audible frequency range spans from 20 Hz to 20 kHz
  • Infrasound below 20 Hz and ultrasound above 20 kHz
• Sound pressure level (SPL) quantifies the local pressure deviation from the ambient atmospheric pressure caused by a sound wave
• Inverse square law states that SPL decreases by 6 dB for every doubling of distance from a point source in free field conditions

Wave Propagation and Behavior

• Sound waves exhibit behaviors such as reflection (bouncing off surfaces), refraction (bending due to changes in medium properties), diffraction (bending around obstacles), and interference (combining of waves)
• Reflection occurs when sound waves encounter a boundary between two media with different acoustic impedances
  • Angle of incidence equals angle of reflection
  • Reflected waves can interfere constructively (in phase) or destructively (out of phase) with incident waves
• Refraction happens when sound waves pass through media with varying densities or temperatures causing the wave front to bend
• Diffraction allows sound waves to bend around obstacles and spread out after passing through openings
  • Extent of diffraction depends on the size of the obstacle or opening relative to the wavelength
• Interference can be constructive (doubling amplitude) or destructive (cancellation) depending on the phase difference between waves
• Standing waves form when incident and reflected waves interfere creating nodes (minimal displacement) and antinodes (maximum displacement)
• Absorption occurs when sound energy is converted into heat as waves propagate through a medium or interact with surfaces
  • Porous materials (fiberglass, foam) are effective absorbers due to their high surface area and air pockets

Sound Transmission Paths

• Sound can transmit through various paths including airborne (through air), structure-borne (through solid structures), and flanking (indirect paths)
• Airborne transmission occurs when sound waves propagate through air and interact with partitions (walls, floors, ceilings)
  • Transmission loss (TL) quantifies the reduction in sound power as waves pass through a partition measured in decibels (dB)
• Structure-borne transmission involves vibrations propagating through solid structures (beams, pipes, ducts) and radiating sound on the other side
  • Coupling between airborne and structure-borne paths can occur at junctions and connections
• Flanking transmission refers to indirect paths that bypass the main partition such as gaps, cracks, and openings
  • Common flanking paths include ceiling plenums, ductwork, and electrical conduits
• Sound bridges are rigid connections between two isolated structures that allow vibrations to transfer bypassing insulation
• Resonance occurs when the frequency of a sound wave matches the natural frequency of a structure amplifying vibrations and sound transmission
• Mass law states that TL increases by 6 dB for every doubling of mass per unit area or frequency assuming a limp, homogeneous, and isotropic partition

Materials and Their Acoustic Properties

• Acoustic properties of materials determine their ability to absorb, reflect, or transmit sound
• Absorption coefficient (α) measures the fraction of incident sound energy absorbed by a material ranging from 0 (perfect reflection) to 1 (perfect absorption)
  • Varies with frequency and angle of incidence
  • Porous materials (fiberglass, mineral wool) have high α due to their open-cell structure and air pockets
• Sound transmission class (STC) is a single-number rating that quantifies the airborne sound insulation of a partition
  • Higher STC values indicate better insulation performance
  • Calculated from TL values measured in one-third octave bands from 125 Hz to 4000 Hz
• Noise reduction coefficient (NRC) is the arithmetic average of α at 250, 500, 1000, and 2000 Hz
  • Provides a simplified measure of a material's overall absorption performance
• Density and stiffness affect a material's acoustic behavior
  • Dense materials (concrete, steel) generally have high TL but low α
  • Stiff materials (wood, gypsum board) can resonate at certain frequencies reducing their insulation effectiveness
• Damping refers to a material's ability to dissipate vibrational energy and reduce resonance
  • Viscoelastic materials (rubber, polymers) provide high damping due to their internal friction and energy dissipation
• Anisotropy and inhomogeneity can cause variations in acoustic properties depending on the direction and location of sound incidence

Insulation Techniques and Strategies

• Sound insulation aims to reduce the transmission of sound energy through partitions and structures
• Mass-air-mass (MAM) systems consist of two dense layers separated by an air gap
  • Effective at reducing low-frequency transmission due to the decoupling of the layers and the resonance frequency of the system
  • Increasing the air gap thickness improves low-frequency performance but may compromise high-frequency insulation
• Resilient channels are metal furring strips that decouple gypsum board from studs reducing structure-borne transmission
• Staggered studs involve alternating the placement of studs on either side of a partition to minimize direct coupling
• Acoustic sealants (caulk, gaskets) are used to seal gaps and cracks around the perimeter of partitions and penetrations
• Floating floors incorporate a resilient layer (rubber, foam) between the subfloor and the finished flooring to isolate impact noise
• Suspended ceilings with absorptive tiles can reduce reverberant noise in a room and improve speech intelligibility
• Vibration isolation mounts (springs, neoprene pads) decouple equipment and machinery from the supporting structure reducing structure-borne noise

Measurement and Testing Methods

• Sound pressure level (SPL) measurements are conducted using a sound level meter (SLM) which consists of a microphone, preamplifier, and processing unit
  • A-weighting is commonly used to account for the frequency-dependent sensitivity of human hearing
  • Time-weighting (fast, slow) determines the averaging time constant for fluctuating sounds
• Reverberation time (RT) is the time required for the SPL to decrease by 60 dB after a sound source is abruptly stopped
  • Measured using the interrupted noise method or the integrated impulse response method
  • RT is affected by the room volume, surface area, and absorption coefficients of materials
• Transmission loss (TL) measurements involve generating a known sound field on one side of a partition and measuring the transmitted sound power on the other side
  • Requires an anechoic or reverberant receiving room to minimize the influence of reflected sound
  • TL is calculated as the difference in SPL between the source and receiving rooms plus a correction factor for the receiving room absorption
• Impact insulation class (IIC) is a single-number rating that quantifies the impact noise insulation of floor-ceiling assemblies
  • Measured using a standardized tapping machine and a microphone in the receiving room below
  • Higher IIC values indicate better impact noise insulation performance
• Flanking transmission can be assessed using intensity scanning or vibration measurements to identify dominant transmission paths
• In-situ measurements are conducted in actual buildings to evaluate the acoustic performance of partitions and room acoustics
  • Normalized noise isolation class (NNIC) and normalized impact sound rating (NISR) are single-number ratings derived from in-situ measurements

Practical Applications and Case Studies

• Office buildings require a balance between speech privacy and communication
  • Open-plan offices benefit from absorptive ceilings, partitions, and furnishings to reduce noise propagation
  • Private offices and meeting rooms need high STC partitions and door seals to minimize sound transmission
• Residential buildings aim to provide a quiet and comfortable living environment
  • Party walls and floor-ceiling assemblies should have high STC and IIC ratings to minimize neighbor noise
  • Mechanical equipment (HVAC, elevators) should be vibration-isolated and located away from sensitive spaces
• Healthcare facilities prioritize patient comfort and confidentiality
  • Nurse stations, waiting areas, and corridors benefit from absorptive treatments to reduce noise levels
  • Patient rooms and exam rooms require high STC partitions and sound-absorbing finishes to ensure speech privacy
• Performing arts spaces (theaters, concert halls) require precise control over reverberation, clarity, and sound distribution
  • Room shape, volume, and surface treatments are designed to provide optimal acoustics for the intended use
  • Sound-isolating construction (box-in-box) is used to minimize external noise intrusion and sound transmission between spaces
• Industrial facilities generate high levels of noise from machinery, processes, and equipment
  • Noise control measures include enclosures, barriers, silencers, and vibration isolation
  • Personal protective equipment (earmuffs, earplugs) is used to protect workers from excessive noise exposure
• Transportation noise (aircraft, trains, vehicles) can impact communities near airports, railways, and highways
  • Noise barriers, berms, and sound-absorbing pavements are used to mitigate outdoor noise propagation
  • Building façades and windows are designed to provide sufficient outdoor-indoor transmission class (OITC) ratings

Regulations and Standards

• Local building codes and noise ordinances set minimum requirements for sound insulation and noise levels in different types of buildings and zones
• ASTM International develops test methods and classification standards for acoustic materials and assemblies
  • ASTM E90 for laboratory measurement of airborne sound transmission loss of building partitions
  • ASTM E492 for laboratory measurement of impact sound transmission through floor-ceiling assemblies
  • ASTM E413 for determination of sound transmission class (STC)
  • ASTM E1332 for determination of outdoor-indoor transmission class (OITC)
• International Organization for Standardization (ISO) provides international standards for acoustics and noise control
  • ISO 140 series for field and laboratory measurements of sound insulation in buildings
  • ISO 717 series for rating of sound insulation in buildings and of building elements
  • ISO 10140 series for laboratory measurement of sound insulation of building elements
• Occupational Safety and Health Administration (OSHA) sets permissible exposure limits (PELs) for noise in the workplace
  • 90 dBA for an 8-hour time-weighted average (TWA)
  • 5 dBA exchange rate for halving the exposure duration with each increase in noise level
• World Health Organization (WHO) provides guidelines for community noise to protect public health and well-being
  • 45 dB LAeq (night) for bedrooms to prevent sleep disturbance
  • 35 dB LAeq (day) for classrooms to ensure speech intelligibility and learning
• Green building rating systems (LEED, BREEAM) include credits for acoustic performance and noise control to promote occupant comfort and well-being