The noise problems associated with air conditioning systems are generated by the fans, pumps and compressors that make up these systems. Noise can be generated and transmitted in aerial or structural form. In this series of articles we will see the phenomena of noise generation, noise regeneration problems that can be produced by a bad selection or installation of noise reduction systems and their solutions.
EVALUATION PARAMETERS
The evaluation parameters for ambient noise present in a room must consider the spectral behavior (by frequency) of the perceived noise. For this there are several evaluation curves, within which the most used and that allow to better evaluate the discomfort caused by a noise in a closed environment, are the NC curves and the RC Mark II curves. Nc (Noise Criteria) curves correspond to a single number classification that weights the spectral content of the noise evaluated for its loudness and interference with the word. The method consists of a family of curves comprising a range of frequencies between the 1/1 Oct bands. From 63 Hz to 8000 Hz. The noise evaluated is associated with the largest NC curve where it intersects.
This curve is usually used for places where sound quality is not especially important. The RC Mark II curves correspond to a 1997 revision of the RC (Room Criteria) curves, created in 1981. The evaluation procedure for this method, based on the calculation of the arithmetic average rounded to the nearest integer of the sound pressure levels in the octave bands of 500, 1000 and 2000 Hz (the main frequency region of the voice), the resulting number corresponds to the RC criterion. Subsequently, an analysis of the differences between the RC curve and the noise evaluated in the regions of low frequency (LF), medium frequencies (MF) and high frequencies (HF) is carried out, with the aim of determining the noise nuisances in these regions. When the noise evaluated has few differences with the associated RC curve, the noise is said to be neutral (N). To evaluate if the noise is of the type LF, MF HF or N, the QAI (Quality Assessment Index) must be calculated, for each frequency region, if it is greater than 5 dB, the noise evaluated with the associated RC criterion followed by the acronym (LF), (MF) or (HF), as the case may be, is described. If the QAI is less than or equal to 5 dB, the acronym (N) is used.
VENTILATORS
The noise emitted by a fan is basically due to the cut of the air by the fan blades. The noise level, then, depends on the flow rate handled by the fan, the static pressure with which it is working and the type of fan. Since the generation of noise from a fan is produced by the blow of air through the blades, the greatest emission of noise is through the discharges and intake of air from the fan, and secondarily, the housing of the fan - if it is of the centrifugal or tuboaxial type - and the motor. While it is true that fans are the largest sources of noise, they are not the only sources, producing regenerated noise in the elbows of ducts, dampers, forks and other elements even in noise attenuators. Noises related to aerodynamic problems in the ducts can be avoided with:
Notes:
1.- The speeds in the branches must have speeds not exceeding 80% of the value shown.
2.- The speeds in the final routes towards the downloads must have speeds not exceeding 50% of the value shown.
3.- Elbows and other elements can substantially increase the level of flow noise, depending on the type.
Speeds should be reduced by up to 50% depending on the type of element.
- Providing smooth transitions.
- Allowing a uniform flow in the discharge of the fans before any elbow, attenuator or damper
NOISE ATTENUATORS
Noise attenuators are sections of prefabricated ducts that contain internal cells filled with sound absorption materials (usually glass wool or mineral wool). Its acoustic and aerodynamic performance depends on the size, shape and length of the internal cells. The longer and thicker the internal cell, the more noise attenuation it will produce, but usually at the expense of greater pressure loss. The noise that is regenerated by the noise attenuators, which is directly related to the speed of air passage between the cells, must also be taken into account.
Another way to produce noise attenuation in ducts is by internally coating the duct with fiberglass wool palmettes specially designed for this purpose, however, the acoustic performance of this coating compared to a noise attenuator is very low. For example, to produce the same acoustic performance at the 250 Hz frequency of a 1.5m long commercial attenuator, it is necessary to coat 25 m of duct internally with to produce the noise attenuations and aerodynamic performance shown by the manufacturers' catalogs, the noise attenuators must be installed according to the drawings in Fig. 8 (a-h).
If the attenuators are installed very close to the fan discharges or the singularities of the ducts (elbows, forks), a turbulence noise regeneration can occur that will affect the final acoustic and aerodynamic performance.
EVALUATION OF NOISE EMISSIONS FROM ENGINE ROOMS
Equipment installed on the outside of the building, whether on the top floor, at ground level or on intermediate floors, can cause nuisance due to noise or even violate existing regulations regarding maximum noise emission limits (D.S. No. 146/97 of MINSEGPRES) to nearby homes, apartments or offices. Each situation must be studied in detail considering the location of the equipment with respect to the potentially affected neighbors, the noise emission levels of the equipment and the ventilation and maintenance requirements of the equipment. Currently there is technology in computer programs, such as SoundPlan, that help us predict noise levels, three-dimensionally modeling the real situation. These programs allow us to visualize noise maps (Fig.2) and know the contributions of each of the noise sources studied to the noise perceived by the neighbor, which allows us to determine and design the appropriate mitigation measures for each case.
ACOUSTIC LATTICE SOLUTIONS
Noise from the mechanical rooms can be transmitted to the outside through the lattices or openings that the room has for ventilation or air intake. To solve this problem it is possible to use acoustic lattices, which can reduce noise by up to 13 dB(A) and allow ventilation of the room. Since these lattices are made of cells filled with sound-absorbing materials, the free area can be 30 to 50% of the total area of the lattice. This should be taken into account for the selection of the area required for the room.
NOISE FROM MECHANICAL FLOORS
Often mechanical rooms are located on the top floor above an executive office or room. The noise and vibrations emitted by the equipment is transmitted to the lower floor causing discomfort and impossibility of commercially occupying the floor under the mechanical room. It is very important for these cases to have an adequate vibration control system in equipment and pipes so as not to transmit vibrations and the noise generated by them to the floors below, but it is also necessary to isolate from the transmission of aerial noise. It is possible to minimize the transmission of noise by running a floating floor, tuned to a frequency not exceeding 15 Hz, The most modern system that exists to execute floating floors for mechanical rooms is the one made by Jack up type insulators (fig. 5), this system allows to build a floating slab on 100mm directly on the structural slab and then lift it by screws incorporated into the insulator.
Another alternative is to install a hanging sky of plasterboard elastically insulated by neoprene or spring hangers. Figure 7 shows a floating sky under construction where you can see the hanger and the support structure of the sky before installing the cardboard plasterboard. Fig. 7. Floating sky hanger
ACOUSTIC BARRIERS
When the receiver is at the same level or at a level below the noise source to be treated, and when the required noise reductions are less than 12 dB(A), it is possible to implement a solution based on acoustic barriers. These barriers are made up of panels with the absorbent face on the side facing the noise source and acoustic lattices can be incorporated when ventilation is required. When the ventilation that is required is a lot or when the spaces cannot be left for an adequate intake or discharge of air to the equipment, the barriers can be completely acoustic lattices. Fig. 8. Barrier Acústica.Es necessary to clarify that the performance of acoustic barriers depends not only on the panels on which it is built, but also on the dimensions related to the noise source and the receiver. The design of the barrier must be studied very well to obtain the expected results. Computer programs to study noise emission also help us determine the dimensions of barriers.
