Acoustic Noise Control: A Comprehensive Guide for Buildings
Acoustic Intensity
By definition, the difference in intensity between two sound waves is measured in Bels. If the ratio is 10, the difference is 1 Bel. When compared to a reference intensity, the number of decibels (dB) for each intensity level defines its intensity on a scale where the zero level (0 dB) corresponds to the reference intensity. This reference intensity is typically taken as the average threshold of human perception at 1000 Hz.
Sound Perception
The magnitude of the sensation of sound depends on both the acoustic wave intensity and frequency. Humans have increased sensitivity in the midrange frequency area. Equal loudness curves, which depict the relationship between acoustic intensity level and perceived loudness for each frequency, are fundamental in physiological acoustics. These curves have been used to establish several weighting curves for acoustic intensity versus frequency, including the A-weighting curve (internationally accepted for assessing sounds). When measuring a complex sound with an A-weighting filter, the resulting value directly represents the sound intensity in dB. The dB value is widely accepted as the most approximate measure of the sensation produced by music, speech, and community noise, including traffic and electrical noise. This provides ease of calculation in relation to the material behavior against noise.
Traffic Noise
- Road Traffic: Road traffic noise has a random character because it is composed of contributions from mobile noise sources. Characterizing road traffic noise requires considering its energy aspect and evaluating its fluctuation over time, necessitating statistical treatment to establish a global parameter. Another important factor is the configuration of the road environment, which influences noise propagation and sound field characteristics.
- Aircraft: Aircraft noise requires the use of specific measurement units to account for the unique spectrum and noise level produced, the number of flights at a given time, and whether they occur during the day or night. While measured in dB, specific measures exist to assess the nuisance caused by aircraft.
- Railway: Railway noise is primarily influenced by two factors: noise from traffic vehicles and the frequency of traffic in a given period. The main sources are the wheel-rail system and the propulsion system in wheeled vehicles. Additionally, ancillary equipment inside the train also contributes to noise.
Internal Noise
Community Facilities and Equipment
Common sources of internal noise include heating, hydraulic installations, ventilation and air conditioning, lifts, waste disposal systems, and lighting.
Heating
Boilers and burners produce airborne noise and structural noise, reaching levels between 70 and 90 dB. Central heating systems include circulation pumps that generate airborne noise of 90 dB and structural noise transmitted through pipes and the fluid. Pipes are a primary pathway for noise generated in the boiler, burner, or circulating pump. They also act as a noise source due to turbulent flow regimes caused by high fluid velocity, poor network design, and junctions or variations in pipe sections. While pipe sound radiation is low, their connection to walls can lead to significant noise levels. Radiators are minor noise sources, even when purged. Noise emitters originate in boilers, pipes, and the room itself.
Hydraulic Installations
Noise from hydraulic installations can be attributed to circulating pumps, piping, water arrival and drainage, taps, and filling or emptying containers. Taps are a significant noise source due to cavitation phenomena, where the sound level increases with pressure and speed. Abrupt faucet closure can lead to water hammer, a phenomenon that can be mitigated by using expansion elements in plumbing. Noise from water filling containers on the premises can reach levels up to 75 dB.
Ventilation
Ventilation systems can easily spread airborne sound between rooms and even transmit external noise. The most common ventilation system, using shafts, can be problematic due to the proximity of small windows and vents for expulsion. A maze-like arrangement of ventilation shafts is recommended to increase acoustic separation. A right elbow in a ventilation shaft provides an average attenuation of about 3 dB.
Air Conditioning
Air conditioning ducts facilitate the spread of noise and vibration from compressors and fan drives. They also act as a pathway for ambient noise transmission between enclosures. Air driven through vents is an additional noise source that requires a streamlined design to reduce air velocity. Noise levels can reach 40 dB. The spread through these channels can be reduced by coating the interior surfaces with absorbent materials.
Lifts
Noise from lifts is primarily centered in the machine room, which should be isolated. Other sources include the opening and closing of doors and the sliding of the lift car. The elevator shaft acts as a resonant cavity, whose effect can be reduced by applying fire-retardant absorbent coating.
Community Waste Landfill
Waste landfills are a source of sporadic aerial and structural noise. Noise levels inside can reach 80 dB. The waste disposal system should be isolated from the building structure. It is advisable to place it inside a dedicated work area. If metal pipes are used, it is essential to treat the exterior with resins or other vibration dampers. Gates should be isolated from the structure with elastic joints and snaps.
Lighting Systems
Noise sources in lighting systems include fluorescent tubes and ballasts (which can produce discrete frequency noise, not exceeding 60 dB), switches, relays, timers, and lighting controls for staircases and hallways (which produce impulsive noise exceeding 75 dB). Reducing these noises requires assembly using absorbent plastic lined fire-resistant supports.
Community Facilities and Equipment Not Covered
This section includes electrical service and sound reproduction equipment.
Appliance Service
Appliances such as washing machines, blenders, vacuum cleaners (70 dB), refrigerators (35 dB), and dishwashers (90 dB) can generate noise. In the case of washing machines and dishwashers, noise can be transmitted through water pipes and drainage systems.
Electric Radiators
Electric radiators can form resonant systems that cause noise. These are particularly troublesome because they produce discrete frequencies. If walls are linked to similar situations, noise can also be transmitted through pipes.
Unitary Air Conditioners
Unitary air conditioners installed on windows or walls should be avoided to minimize direct transmission of power to the building structure. They should be isolated from the building with elastic supports.
Sound Reproduction Equipment
Musical Instruments
Noise reduction for musical instruments, such as pianos, is essential because they can transmit significant energy to the building structure through their supports.
For its impact on community relations in a building, proper conditioning, installation, and maintenance of equipment are crucial, including regular work schedules.
Activities of People
Noise produced by people through the occupation and use of a building includes footsteps (structure-borne, mainly rich in low-frequency noise, 55 dB), children’s games, people transiting areas (structure-borne), dragging furniture (65 dB), operating shutters (70 dB), and conversation.
Acoustic Absorption
Acoustic absorption involves extracting energy from the acoustic field. This occurs through two mechanical phenomena: resonance and conversion into heat by friction. Most absorbent materials have properties that favor one of these phenomena. We distinguish between resonator materials and friction-type (porous) materials. The absorption coefficient is the ratio of absorbed energy to incident energy. It varies with frequency.
Absorption for Noise Reduction
For practical applications, an average absorption coefficient is often used, deduced from the absorption characteristics of the material. The Sabine unit is used to measure absorption. Absorption complements acoustic insulation in reducing noise levels inside a space. While absorption reduces the energy of emission, the maximum noise level reduction achievable through absorption is 10 dB. In most cases, it does not reach 5 dB, with typical reductions ranging from 2 to 4 dB. This reduction is important as a complement and, in particular, for reducing the reverberation time of an enclosure. Reverberation time is the time it takes for the sound intensity level to decrease by 60 dB after the sound source ceases.
Resonator-Type Materials
Resonator-type absorbers have a predominant effect in a specific band of frequencies centered around the resonant frequency. There are two basic types: membrane resonators (made of light planks of wood, plastic, metal, etc., designed to leave an intermediate air chamber between the rigid walls that support it) and Helmholtz resonators (consisting of a cavity connected to the exterior through holes or slits). The chambers can be filled wholly or partially with porous material to broaden the frequency band.
Porous Materials
Porous materials are composed of particles or fibers bound together, leaving gaps that confer their porous nature. Expanded materials can be assimilated to porous materials. Absorption occurs through the degradation of mechanical energy into heat by air friction on the surfaces of contact with the material.
Structural Noise Reduction
The set is a spring-mass oscillating system. The behavior of the system is essentially defined by its resonance frequency, a function to which ditinguimos two areas of different behavior. For frequencies above 2 source system attenuates transmission of vibrations, if less than 2 not only failed but attenuates the transmission is an increase of it. This transmission is reduced by increasing the resonant frequency disipativo.La factor can be assessed by the static deflection of the insulating material corresponding to the load of the source. The elastic treatment is chosen for you to work on mitigation area. As a guideline you should choose a system whose resonant frequency is = or less than one third of the lowest frequency of excitation. Dissipative factor as to be kept as low as possible (metal spring). sources in the case of intermittent regime marked operation (engines) should elgirse dissipative factor as high as possible.