Noise from mechanical ventilation systems in residential buildings

. Modern people spend up to 90% of their time indoors. Therefore, the research aimed to identify noise sources from engineering equipment, and developing recommendations for their elimination was an urgent scientific task. The paper considers the problem of noise pollution from mechanical ventilation systems in residential buildings. One of the most common errors in conducting acoustic calculations is the exclusion of noise generated by regulating devices, which significantly contribute to the final sound pressure levels at the outlet of the ventilation grate. The results of field studies are presented; they prove the need to normalize permissible noise levels in non-residential premises of an apartment and develop a legislative framework for measuring noise in hybrid ventilation systems. Practical recommendations for designing exhaust ventilation systems with a prefabricated vertical duct have been developed. It has been found that the replacement of throttle valves with iris diaphragms and the installation of silencers on prefabricated channels and apartment withdrawals, given the influence of the vortex zone from control devices, reduce the overall noise level from the ventilation system.


Introduction
Human habitation is one of the oldest anthropogenic ecosystems.Since the first huts with a hearth, the technologies that people currently use to create and maintain the parameters of the internal microclimate have turned into complex, branched systems.The design of a modern building implies an integrated approach, including the creation of sustainable architecture: development of optimal design and layout; introduction of modularity, mobility, and flexibility of individual elements of the object; use of environmentally friendly materials and energy-efficient technologies to create a high-quality internal environment [14].The main task of engineers is to find the optimal solution in which the building will provide comfortable indoor microclimate parameters with minimal energy consumption [4,5].
According to [12], mechanical ventilation or hybrid systems should be provided in residential buildings if the parameters and quality of the internal air cannot be provided with ventilation with a natural impulse.Some studies [2; 3] show that the provision of calculated air exchange both in the application of duct ventilation systems and in the organization of through ventilation is difficult to predict value.The duration of effective operation of natural ventilation duct systems in Moscow is determined by the period from late November to mid-February [1].Another limitation of using natural ventilation systems in large cities is noise pollution from highways penetrating through open windows and vents [17].
In connection with the above, in multi-story residential buildings, systems with mechanical motivation are increasingly being used every year.
Mechanical ventilation systems have several advantages: stability of ensuring the parameters of the internal microclimate throughout the year; possibility of pre-cleaning, air treatment, and heat recovery.However, there are some problems associated with the need for maintenance and repair, as well as noise generation outside and inside the building [6].
According to expert estimates [13], up to 70% of the territory of Moscow is affected by excessive anthropogenic noise from various sources.In practice, despite the existence of recommendations for limiting noise from engineering equipment, proper control of acoustic characteristics is practically not carried out during the design and commissioning of the facility.Also, the standard [14] does not contain requirements for conducting acoustic measurements in rooms equipped with exhaust mechanical ventilation systems when organizing natural inflow with open windows and vents.
The problem of noise pollution may limit the use of mechanical ventilation.The results of studies conducted in Europe and North America indicate that residents often turn off ventilation equipment with undesirable noise [9].Without proper ventilation in modern sealed buildings, poor air quality can significantly negatively impact the well-being and health of residents [8].
Currently, the issue of finding optimal solutions to reduce sound pressure levels in ventilation installations of multi-story residential buildings is an urgent scientific task.

Materials and Methods
The research aims to develop recommendations for reducing noise from mechanical ventilation systems in multi-story residential buildings.In accordance with the set goal, the following objectives were formulated: conduct field studies to determine sound pressure levels in the premises of an apartment building from exhaust ventilation systems; carry out a calculation justification of measures to reduce the noise of ventilation installations based on acoustic calculation.

Experimental sudy
The object of the research was a 32-story residential building located in Moscow.Mechanical exhaust ventilation systems with prefabricated vertical channels are designed in the building, serving kitchens and sanitary units in each apartment.Apartment-by-apartment taps are equipped with throttle valves for the implementation of aerodynamic coupling of the system.Ventilation units are made with 100% redundancy and are located on the roof of the building.Sound pressure levels were measured using the Svan 945 spectrum analyzer noise meter.The permissible absolute error of the device is ±0.7 dB.Measurements were made at one design point in kitchens and sanitary units in four apartments on the top floor according to [16].Measurements were made during the warm season.In order to assess noise pollution from mechanical ventilation systems, measurements were made with the windows closed.Fragment of the scheme of the exhaust system of the sanitary unit is shown in Fig. 1.In the Russian Federation, the noise level is regulated exclusively for residential rooms of apartments.Therefore, the assessment of the results of field studies and calculations is somewhat conditional [15].

Acoustic calculations
Based on the methodology presented in [7; 11], acoustic calculations of mechanical ventilation exhaust systems were carried out to identify the causes of noise formation and develop recommendations for its reduction.The attenuation of the sound power from the fan was determined sequentially for each element of the network, and the noise generated in the control devices, , dB, was also taken into account: (1) Where: v -the speed of the airflow in the duct, m/s; -the local resistance coefficient in regulating device; F -the cross-sectional area of the duct, m 2 ; -the similarity criterion; -the correction for the influence of the acoustic duct connections to the power control unit, dB;

Results
The graph (Fig. 2) shows the results of experimental studies and normalized sound pressure levels [15].It was revealed that there is an excess of low-frequency and medium-frequency noise (up to 500 Hz) on the premises.The acoustic calculations made it possible to determine the sound pressure levels at the outlet of the ventilation grate.It has been found that significant noise generation occurs from the throttle valve installed on the apartment outlet (Fig. 3, 4).Replacing the throttle valve with an iris diaphragm allows somewhat reducing the noise in the exhaust ventilation system.However, this measure is not sufficient (Fig. 5).As the research results show, the presence of regulating devices has a significant impact on the amount of noise generated by the mechanical ventilation system.This noise depends on the speed of the incoming flow, the design and the coefficient of local resistance of the regulating device, and the degree of turbulence of the incoming flow.

Discussion
When carrying out acoustic calculations of mechanical ventilation systems, design organizations often take into account exclusively the noise generated by the fan, which leads to underestimated values of the final sound pressure levels (Fig. 4).A closed vortex zone is formed behind the control devices placed in the ventilation network, the size of which depends on the flow rate and airflow velocity.The higher the turbulence and the length of this zone, the more powerful the noise.When the control devices are placed close to the outlets, a sharp increase in the generated noise can also occur.Design errors can also be associated with an incorrect choice of local resistance coefficients of regulating devices [10].

Conclusion
Currently, the number of complaints from residents of new buildings about the noise generated by ventilation systems located in the kitchen and the sanitary unit has significantly increased [18].This factor shows the urgent need to conduct theoretical and field studies to identify acceptable noise levels in non-residential apartments [19,20].The necessity of carrying out acoustic calculations at the design stage of mechanical ventilation systems in residential buildings, given the generation of noise from regulating devices, is proved.It is recommended to use iris diaphragms instead of throttle valves for the implementation of aerodynamic coupling of ventilation systems and the installation of silencers on the prefabricated channels.When placing silencers on the apartment outlet after the regulating device, it is necessary to carry out a verification calculation to determine the length of the vortex zone.

Fig. 1 .
Fig. 1.Fragment of the exhaust ventilation scheme of the sanitary unit of the apartment.Source: Compiled by the authors.

Fig. 2 .
Fig. 2. The results of field measurements.Source: Compiled by the authors.

Fig. 3 .
Fig. 3.The results of the acoustic calculation of the ventilation system (regulation is carried out using throttle valves).Source: Compiled by the authors.

Fig. 4 .
Fig. 4. Noise from the throttle valve and fan generated in the ventilation system.Source: Compiled by the authors.

Fig. 5 .
Fig. 5.The results of the acoustic calculation of the ventilation system (regulation is carried out using iris diaphragms).Source: Compiled by the authors.