Acoustic Comfort in Buildings: Noise Sources & Control
Acoustic Intensity
By definition, there is a difference of 1 Bel between two currents if the relationship is 10. When compared with a reference intensity, the number of dB for each intensity level defines its intensity on a scale where the zero level (0 dB) corresponds to the intensity of reference. This is taken as corresponding to the average threshold of perception at 1000 Hz.
Sound
The magnitude of the sensation of sound depends on the acoustic wave intensity and frequency. Increased sensitivity is observed in the midrange area. Equal loudness curves as a function of acoustic intensity level for each frequency are the basis of physiological acoustics. These curves have served to establish several weighting curves for acoustic intensity versus frequency, including the A-weighted curve (accepted internationally for the assessment of sounds). When measuring a complex sound with an A-weighted filter, the value of the sound intensity in dB(A) is obtained directly. The dB(A) scale is accepted as the single most approximate value to the sensation produced by music, speech, and community noise, including traffic and electrical noise. It provides ease of calculation in relation to material behavior against noise.
Traffic Noise
- Road Traffic: It has a random character because it is composed of contributions from mobile sources of noise. The characterization of road traffic noise requires consideration of its energy aspect and evaluation of its fluctuation over time, requiring statistical treatment to establish a global parameter. Another consideration is the configuration of the road environment, which influences the propagation of noise and the sound field characteristics.
- Aircraft: Requires the use of units of measurement that take into account the specific spectrum and noise level produced, the number of flights at a certain time, and whether they are day or night flights. It can be measured in dB(A), although specific measures exist to assess the nuisance of aircraft noise.
- Railway: Two main factors are considered: noise from traffic vehicles and frequency in a given period. Its main sources are the wheel-rail system and the propulsion system in wheeled vehicles. Inside, ancillary equipment noise must also be considered.
Internal Noise
Community Facilities and Equipment
Heating, hydraulic installations, ventilation and air conditioning, lifts, waste disposal chutes, and lighting systems are all sources of internal noise.
Heating
The boiler and burners produce airborne and structure-borne noise reaching levels between 70 and 90 dB. Central heating systems include a circulation pump that generates airborne noise of 90 dB and structure-borne noise transmitted through pipes and the fluid. The pipes are the preferred path for the transmission of noise generated in the boiler, burner, or circulating pump. They also constitute a source of noise caused by turbulent flow regimes due to high fluid velocity, poor network design, T or X junctions, section variations, etc. The sound radiation from pipes is low, but their connection to walls can lead to significant noise levels. Radiators are minor sources, even when incorrectly purged. Noise emitters originate in boiler and pipe rooms.
Hydraulic Installations
We distinguish between noise due to the circulating pump, piping, water arrival and drainage, taps, and filling or emptying containers. Taps are an important source of noise due to cavitation phenomena, where the sound level increases with pressure and speed. The abrupt closing of a faucet can lead to a phenomenon called water hammer, which can be mitigated by using expansion elements in the plumbing system. The noise from water filling containers can reach levels up to 75 dB.
Ventilation
Ventilation systems are an easy route for the spread of airborne sound between rooms and even for the emission of noise to the outside. In the most common ventilation shaft systems, the proximity of small windows and vents for expulsion in various locations recommends a maze-like arrangement that increases acoustic separation. A right-angle elbow 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 are also a pathway for ambient noise transmission between enclosures. Air driven through vents is an additional source of noise that requires a streamlined design, favored by a decrease in the air impulse velocity. Noise levels can reach 40 dB. The spread of noise through these channels can be reduced by coating the interior surfaces with absorbent materials.
Lifts
The noise is concentrated in the machine room, which needs to be isolated. Other sources are the opening and closing of doors and the sliding of the cabin. The elevator shaft is a resonant cavity whose effect can be reduced by applying a fire-retardant absorbent coating.
Community Waste Disposal Chutes
These are a source of sporadic airborne and structure-borne noise. Inside, noise levels can reach 80 dB. The chute must be isolated from the building structure; it is advisable to place it inside a service shaft. If metal pipes are used, it is essential to treat the exterior with resins or other vibration dampers. The gates must be isolated from the structure with elastic joints and seals.
Lighting Systems
Noise sources include fluorescent tubes and ballasts (which do not exceed 60 dB but can be bothersome due to discrete frequency noise), switches, relays, timers, and switched lighting in staircases and hallways (impulsive in character, exceeding 75 dB). The reduction of these noises requires mounting them on absorbent plastic supports lined with fire-resistant material.
Community Facilities and Equipment (Non-essential)
These include electrical service and sound reproduction equipment.
Appliance Service
Washing machines, blenders, vacuum cleaners (70 dB), refrigerators (35 dB), and dishwashers (90 dB) are all sources of noise. In the case of washing machines and dishwashers, the connection to water supply and drainage pipes can transmit noise.
Electric Radiators
The mechanical system in electric radiators can form resonances that cause noise. These are especially troublesome because discrete frequencies predominate. Similar situations can occur if the radiators are connected to walls or pipes.
Unitary Air Conditioners
These are installed on windows or walls. Direct transmission of power to the building structure should be avoided as much as possible. They should be installed on their own isolated supports, preventing direct contact with the building structure.
Sound Reproduction Equipment
Musical Instruments
Noise reduction measures need to consider instruments like the piano, which can transmit a significant part of their energy to the building structure through their supports.
For their impact on community relations in a building, proper installation, maintenance, and adherence to regular work schedules are very important during renovation and construction works.
Activities of People
Noise produced by people occupying and using the building includes footsteps (mainly structure-borne and rich in low-frequency noise, 55 dB), children’s games, people moving through common areas (structure-borne), dragging furniture (65 dB), operating shutters (70 dB), and conversations.
Acoustic Absorption
Acoustic absorption involves a process of extracting energy from the acoustic field. Two phenomena of a mechanical nature are involved: resonance and conversion into heat by friction. Most absorbent materials have properties with a predominance of one of these phenomena. We distinguish between resonator materials and friction-type materials (porous materials).
Absorption coefficient: The ratio of absorbed energy to incident energy. This coefficient varies with frequency.
Absorption for Noise Reduction
Absorption is significant enough that an average coefficient deduced from the absorption characteristic can be used. This is measured in Sabins. Absorption has a logical complementary action to acoustic insulation in reducing noise levels within the same space by absorbing the energy of the emission. The maximum noise level reduction achieved by the absorption effect is 10 dB; in most cases, it does not reach 5 dB, with normal reductions of 2-4 dB. This reduction is important as a complement and, in particular, by introducing a reduction in the reverberation time of the enclosure. The reverberation time is the time it takes from when the sound source ceases until the intensity level decreases by 60 dB.
Resonator-Type Materials
Resonator-type absorbers have a predominant effect in a particular band of frequencies located around the resonant frequency. There are two basic types: membrane resonators (made of light planks of wood, plastic, or metal, designed to leave an intermediate air chamber between the rigid walls that support them) 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 if it is desired to widen the frequency band.
Porous-Type Materials
These are composed of particles or fibers bound together, leaving gaps that confer their porous nature. Expanded materials can be assimilated to such materials. Absorption occurs by the degradation of mechanical energy into heat by air friction on the surfaces of contact with the material.
Structural Noise Reduction
A structure can be considered as a spring-mass oscillating system. The behavior of the system is essentially defined by its resonance frequency, a function of which we distinguish two areas of different behavior. For frequencies above twice the resonance frequency, the system attenuates the transmission of vibrations; if the frequency is less than twice the resonance frequency, not only does it not attenuate the transmission, but it increases it. This transmission is reduced by increasing the resonant frequency and the damping factor. The damping factor can be assessed by the static deflection of the insulating material corresponding to the load of the source. The elastic treatment is chosen so that it works in the mitigation area. As a guideline, you should choose a system whose resonant frequency is equal to or less than one-third of the lowest excitation frequency. The damping factor should be kept as low as possible (e.g., metal springs). In the case of intermittent sources with a marked operating regime (e.g., engines), a damping factor as high as possible should be chosen.