Noise Pollution Essentials

Cheatsheet Content

What is Noise? Unwanted sound judged to be unpleasant, loud, or disruptive to hearing. Measured by amplitude and frequency of a sound wave. Sound Characteristics & Measurement Sound is produced by alternating pressure changes (compressions and rarefactions) in the air due to vibrations. These phenomena produce sound waves, often represented graphically as a sine wave or sinusoidal signal . Sine Waves Frequency: Number of cycles per second, measured in Hertz (Hz). Humans hear frequencies between 20 Hz and 20,000 Hz (20 kHz). Amplitude: Variations in air pressure, measured in decibels (dB), perceived as loudness. Frequency and amplitude are independent. Wavelength: Physical distance between two comparable points in neighboring cycles (e.g., pressure peaks). Period: Time between successive peaks or troughs of oscillation. Inverse of frequency. Levels of Noise (Decibels) Sound pressure is often expressed on a logarithmic scale (Bels or decibels) due to the wide range of human hearing. Sound Level (L): $L = \log_{10} (Q/Q_0)$ $Q$: Measured quantity (sound pressure or intensity) $Q_0$: Reference standard quantity Unit: Bels (B) Decibel (dB): One-tenth of a Bel. Generally used as it's a smaller, more practical unit. Sound Pressure Level (Lp) in dB: $L_p = 10 \cdot \log_{10} (P_{rms}^2 / P_0^2)$ $P_0$: Reference pressure, typically 20 micro Pa. Sound measuring instruments measure RMS pressure. Sound Intensity Level (Li) in dB: $L_i = 10 \cdot \log_{10} (I / I_0)$ $I_0$: Reference intensity, typically $10^{-12} W/m^2$. dB(A): A-weighted decibels. Measures overall noise level with a frequency weighting to compensate for the human ear's varying sensitivity to different frequencies. Types of Noise Continuous Noise: Produced by machinery running without interruption (e.g., factory equipment, HVAC). Intermittent Noise: Noise level that increases and decreases rapidly (e.g., freight train, cycling factory equipment). Impulsive Noise: Sudden burst of noise, often associated with construction (e.g., explosions, pile drivers). Low Frequency Noise: Low-pitched sounds (e.g., power plants, diesel engines). Hardest to reduce at source. Sources of Noise Pollution Traffic: Road, air, rail. Domestic: Household appliances, music systems. Industrial: Factories, machinery. Incompatible Land Use: Residential areas near industrial zones. Construction: Building sites, demolition. Outdoor Sources: Industries, traffic (road, rail, air), construction, public works. Indoor Sources: Air conditioners, coolers, fans, radio, TV, home/office appliances. Other: Firecrackers, loudspeakers. Noise Propagation Factors affecting noise level at receiver: distance, barriers, atmospheric conditions. Point Source: Sound energy spreads spherically. Sound pressure level decreases by 6 dB per doubling of distance. Line Source: Sound energy spreads cylindrically. Sound pressure level decreases by 3 dB per doubling of distance. Area Sources: Large dimensions compared to propagation distance (e.g., factory roof/facade). Factors Affecting Noise Propagation Type of source (point or line) Distance from source Atmospheric absorption Wind Temperature and temperature gradient Obstacles (barriers, buildings) Ground absorption Reflections Humidity Precipitation Noise Measurement Instruments Sound Level Meter: Main instrument to measure sound in the air. Provides instantaneous noise measurements. Noise Dosimeter: Specialized sound level meter to measure a person's noise exposure over time (e.g., for OSHA compliance). Sound Level Meter Block Diagram Pre-amplifier $\rightarrow$ Weighting network or filters $\rightarrow$ Amplifier $\rightarrow$ Rectifier $\rightarrow$ Averaging system $\rightarrow$ Display Characteristics: Frequency Response: Deviation between measured and true value as a function of frequency. Should be good (variations $\le$ 1 dB) over 20 Hz to 20 kHz. Dynamic Range: Range in dB over which measured value is proportional to true value. Limited by electrical background noise (low levels) and signal distortion (high levels). IEC 60651 Standard: Specifies specifications for four types of meters (0, 1, 2, 3) based on measurement precision. Type 0: Laboratory reference standard. Type 1: Laboratory and specified field use. Type 2: General field applications. Type 3: Field noise survey applications. Frequency response for all types: 10 Hz to 20000 Hz (higher accuracy 100 Hz to 8000 Hz). Microphone Types Interface between acoustic field and measuring system. Transforms sound pressure into an electric signal. Characteristics: Frequency response, dynamic range, directivity, stability. Piezoelectric: Membrane attached to crystal; generates current under mechanical tension. Condenser: Membrane parallel to fixed plate, forming a capacitor. DC voltage applied creates potential difference. Sound waves cause variations in capacitance and electric current. Electret: Variation of condenser microphone with permanent electrostatic charge, no external polarizing voltage needed. Effects of Noise Physical Manifestations (Noise Stress): Hearing loss, tinnitus. Stress, anxiety, increased heart rate, high blood pressure. Gastrointestinal problems. Chronic fatigue. Delayed cognitive development in children. Psychological triggers for PTSD. Lower threshold for noise leading to sleep disturbance. Elevation of cortisol production. Hypertension, myocardial infarction, vasoconstriction. Elevated adrenaline levels. Noise Induced Hearing Loss (NIHL): Irreversible subtle change in sensory cells (hair cells) in the Cochlea. Leads to permanent threshold shift (PTS). Classification of Hearing Effects: Temporary Threshold Shift (TTS) Permanent Threshold Shift (PTS) Acoustic Trauma Noise Control Source Control (Design) Reduction of mechanical shock between parts. Reduction of noise from out-of-balance components. Reduction of noise from friction between metal parts. Reduction of noise from vibration of large structures (plates, beams). Reduction of noise from fluid flow. Control of Noise Propagation Use of barriers (single walls), partial or full enclosures. Use of local enclosures for noisy components. Use of reactive (low frequency) or dissipative (high frequency) mufflers. Use of lined ducts for air handling systems. Reverberation control (sound absorbing materials). Use of silencers and lubricants. Sound Absorption and Reflection Sound Absorbing Materials: Fibrous, lightweight, porous with interconnected cells. Convert acoustic energy into thermal energy. Loss mechanisms: Viscous flow losses and internal frictional losses. Absorption characteristics depend on thickness, density, porosity, flow resistance, fiber orientation. Examples: Vegetable, mineral, ceramic fibers, elastomeric foams (glass blankets, fiberboards). Control at the Receptor End Use of earmuffs and ear plugs. Construction of green belts. Public awareness. Banning of loud horns. Minimum use of loudspeakers. Locating airports, industrial zones, highways outside city limits or residential areas. Proper road planning, flyovers, efficient traffic management. Noise Pollution Control Rules, 2000 (India) Restrictions on Loud Speakers/Public Address Systems: Requires written permission from authority. Cannot be used at night except in closed premises (auditoria, conference rooms, etc.) or during public emergencies. Restrictions on Horns, Construction Equipment, Firecrackers: No horns in silence zones or at night in residential areas (except emergency). No firecrackers in silence zones or at night. No sound-emitting construction equipment at night in residential areas and silence zones. Ambient Air Quality Standards (Noise) - Limits in dB(A) $L_{eq}$ Area Code Category of Area / Zone Day Time Night Time (A) Industrial area 75 70 (B) Commercial area 65 55 (C) Residential area 55 45 (D) Silence Zone 50 40 Consequences of Violation Penalty under the Act for: Playing music or using sound amplifiers. Beating drums, blowing horns (musical or pressure), or sounding instruments. Exhibiting mimetic, musical, or other performances to attract crowds. Bursting sound-emitting firecrackers. Using a loudspeaker or public address system. Complaints: Can be made to the authority if noise level exceeds ambient standards by 10 dB(A) or more, or for violation of night-time restrictions. Authority will act on complaint. OSHA's Noise Standard (29 CFR 1910.95) Requires hearing conservation program if workers are exposed to a time-weighted average (TWA) noise level of 85 dBA or higher over an 8-hour work shift. Permissible Exposure Limit (PEL) for noise exposure is 90 dBA for an 8-hour TWA. Uses a 5 dBA exchange rate: A 5 dBA increase halves the permissible exposure time. E.g., 95 dBA (5 dBA above PEL) allows for only 4 hours of exposure. OSHA's Permissible Noise Exposure Limits Noise Level Exposure Limit 90 dBA 8.0 hours 92 dBA 6.0 hours 95 dBA 4.0 hours 97 dBA 3.0 hours 100 dBA 2.0 hours 102 dBA 1.5 hours 105 dBA 1.0 hours 110 dBA 30 minutes 115 dBA 15 minutes