Proceedings of the Institute of Acoustics

 

 

Prediction of the noise from mechanical ventilation systems in dwellings: case studies

 

Arne Dijckmans¹, Belgian Building Research Institute, Brussels, Belgium

 

ABSTRACT

 

Noise caused by mechanical ventilation systems in dwellings remains a common problem in practice. To help designers and installers, a simplified acoustic calculation tool has been developed which can be used without detailed acoustic knowledge. The tool is based on the prediction models in EN 12354- 5 and VDI 2081 and takes into account the airborne duct sound. The model has been validated by multiple case studies in dwellings. For each case study, the noise levels and airflow rates of the mechanical ventilation system have been measured in each room. Generally, the global A-weighted noise level is predicted with an accuracy of ± 3 dB, on the condition that the airborne duct sound is dominant and sufficiently accurate acoustical input data are available for key components like fans, silencers and air openings. However, some measurement results cannot be predicted by the simplified calculation framework, e.g. when turbulence in two closely placed fittings leads to additional flow noise. The uncertainty also increases when the fan sound is dominated by tonal components.

 

1. INTRODUCTION

 

Noise is one of the key elements that determines the occupants satisfaction of mechanical ventilation systems in dwellings. Too noisy ventilation systems are often turned off by the users, which has a direct impact on the indoor air quality. Therefore, national standards and regulations impose maximum noise levels on ventilation systems in buildings. For small projects, the acoustical requirements are typically only accounted for by means of simple design guide lines and rules of thumbs (e.g. placing a silencer and limiting the air velocity). However, this does not necessarily guarantee that the ventilation system satisfies the acoustic requirements. To help designers and installers, we want to develop an acoustic calculation tool which can predict the acoustical performance with sufficient accuracy, but doesn’t require detailed acoustic knowledge from the user.

 

Different acoustical design methods, software tools and guidelines exist. EN 12354-5 gives a general framework to calculate installation noise. Guidelines are given in various handbooks (often complementary to the standard and each other), like the German VDI [1], the American ASHRAE [2] and the Dutch ISSO [3].

 

Several questions arise when applying these calculation schemes [4]. The guidelines use simplified tables or empirical relations but their field and limits of application are not well known. Specific acoustic input data are often lacking for components used in ventilation systems (e.g. networks with manifolds). There is also a lack of confidence in the calculation process, as e.g. interaction between components is not considered. Previous and current research mainly focusses on improving the acoustic calculation methods for ventilation systems by addressing these issues. The ACOUREVE project [4] focused on improving the quality of the acoustical input data for components like junctions, bends, dampers and manifolds. Similarly, numerical analysis and laboratory measurements are being used to investigate the interaction effects between components. On system level, more advanced models have been developed based on a transfer matrix method and a two-port [5] or multi-port [6] characterization of individual components. While this approach can give valuable insight, it is not readily applicable for real-life situations as input data covering the entire frequency range of interest are still lacking for many ductwork elements.

 

The aim of this study was therefore to investigate the accuracy and limitations of the standardized prediction framework of EN 12354-5 when applied to mechanical ventilation systems typically encountered in dwellings in Belgium.

 

2. PREDICTION MODEL

 

2.1. General Description

 

The acoustic design tool for mechanical ventilation systems, developed at BBRI, is based on the prediction models in EN 12354-5 and VDI 2081 [1] and takes into account the airborne duct sound. This includes both the inlet or outlet fan sound power radiated into the ducts and the flow noise generated in each component. The calculation is performed in octave bands from 63 Hz to 8000 Hz.

 

The sound power level radiated into a room by the air terminal device (ATD) is determined by a sequential calculation, starting at the fan. For each element, the outgoing sound power level is calculated from the incoming sound power level, the sound power level reduction of the element (ΔLw), and the flow noise generated in the element.

 

The standardized sound pressure level L_Aeq,nT in the room is calculated at 1.5 m from the ATD (i.e. the minimum distance to the noise source in the measurement procedure according to ISO 10052), accounting for both the direct sound and the reverberant field. When more than one ATD is present in a room, the L_Aeq,nT is determined at 1.5 m from the noisiest ATD, taking into account only the reverberant field contribution of the other ATD(s).

 

2.2. Input Data for Elements

 

The following type of elements are implemented in the acoustic calculation model: fans, straight ducts, bends, area changes, junctions, valves, manifolds, silencers and air terminal devices. For all the elements, the generic formulae of VDI 2081 [1] are implemented for flow noise and sound level reduction. Some of the generic formulae are simplified to minimize the required input from the user.

• Fans: because the fan speed n and optimum efficiency are generally unknown, tonal components and efficiency terms are neglected. A standard relative octave sound power spectrum is used (based on n = 2100 / 3000 / 250 min^-1for fan type AM / RR / T, respectively).
• Flow noise in bends and junctions: a relative rounding radius of r/da = 0.15 is assumed.
• Bends: the sound power reduction of a bend of 90° with a bend radius and without a lining is used (which is the most conservative and generally most relevant for bends used in dwellings). No sound power reduction is taken into account for bends smaller than 90°.

 

For junctions, the general formula based on the area ratio is adjusted as follows [3,4].

• The sound level reduction in the straight branch is reduced by 1 dB above the limit (or cut off) frequency fG of the straight branch.
• For the additional level reduction in a bend, the spectrum of a 90° sharp-edged bend without lining is used. Only half the bend reduction is accounted for when the section of the straight branch changes.

 

Manifolds are modeled as a junction, but only accounting for the area ratio level reduction, thus neglecting any additional reduction due to bend or plenum effects. Finally, for fans, silencers, valves and ATDs, measured acoustic data from technical sheets can be given as input.

 

3. CASE STUDIES

 

3.1. Description

 

In total, 12 dwellings with recently installed mechanical ventilation systems were selected for the study. Table 1 gives an overview of the main characteristics of each case study. The nominal flow rates varied between 200 m³/h for the smallest dwelling and 588 m³/h for the largest dwelling. In three dwellings, a mechanical ventilation system C was installed with natural supply through ventilation grilles above the windows in the living room and bedrooms and mechanical extraction in the wet rooms (kitchen, bathroom, toilet) and bedrooms. For the other eight dwellings, a system D with balanced mechanical supply (in living room, bedrooms, playrooms and/or studies) and mechanical extraction (in kitchen, bathroom, toilets, storage rooms and/or technical rooms) was installed. The ventilation system of case 3 was optimized in four steps [7]. The dwelling of case 5 was divided in two sections with two separate ventilation systems.

 

Three different types of network lay-out and ductwork were encountered. For the dwellings with system C, the fans have multiple connections (Figure 1a). The airflow rate in each connected branch is determined by a single-leaf damper valve which is controlled by one or more sensors in the corresponding room(s) (CO2, humidity and/or VOC). For the dwellings with system D, five have a branched network of rigid metal ducts (Figure 1b) and three make use of a manifold system to which multiple, smaller round plastic ducts are connected (Figure 1c). In dwelling 9, a combination of both systems has been used (manifold for ground floor and branched network for upper floor).

 

In most dwellings, primary silencers (either rigid or flexible circular silencers with an absorptive material of 25 mm or 50 mm thickness) are installed in the technical room close to the fan. Only for case 1 and case 9, no silencers were present. Here, non-acoustic flexible ducts were used to connect the ducts to the fan. In three cases, additional secondary silencers were installed in the branched network (Figure 1b).

 

 

Figure 1: (a) Fan of case 8, (b) ductwork and secondary silencer of case 7, (c) manifold of case 5.

 

3.2. Measurements

 

Measurements have been done for the fan setting corresponding to the nominal flow rate (which is generally the highest setting of the fan). The flow rates in the rooms have been measured at the air terminal devices (ATD) using a zero pressure compensation measurement device. The electrical power absorbed by the ventilation unit has been measured using a power meter placed directly into the electrical plug. The standardized service equipment sound pressure levels L_Aeq,nT in the rooms have been measured following ISO 10052. The equivalent sound pressure levels were measured over a time interval of 30 s in three fixed positions (two in the reverberant field of the room and one in the corner with the most reflective surfaces) using a handheld sound level meter. The reverberation time in each room was measured in octave bands using an impulsive noise source (hand clap). The octave band service equipment sound pressure levels were standardized with the measured reverberation time in each octave band and corrected for background noise following the procedure of ISO 16032.

 

Table 1: Main characteristics of the mechanical ventilation systems of the case studies

 

 

3.3. Results and Comparison with Prediction

 

For the prediction of the case studies, first the fan sound power was estimated at the working point (airflow rate – pressure drop) of the fan. The total airflow rate was determined from the measured airflow rates in each room. The pressure drop was estimated from a detailed aeraulic model of the ventilation system and/or the measured electrical power consumption of the fan. If detailed acoustic data were available at or around the working point of the fan, the measured sound power spectrum was used as input. Sometimes, only global values were available in the technical sheets, in which case a standard relative spectrum was applied. Generally, an interpolation was needed between different working points stated in the technical sheets.

 

Afterwards, an acoustic calculation was performed for each branch. The flow noise of the ATDs was determined at the working point of the ATD (measured airflow rate and estimated pressure loss after balancing of the system). For most ATDs, no detailed spectrum of the flow noise is available and thus the relative frequency spectrum of VDI was used. The global A-weighted sound power level was determined by an interpolation of the available acoustic data in the technical sheets.

 

The noise levels in the technical rooms (where the fans are installed) were calculated based on the sound power levels of the housing radiation of the fan at its working point.

 

Figure 2 compares the global measured and predicted L_Aeq,nT - values in each room of the 12 dwellings. For the measured values (in red), an error of ± 1 dB is indicated on the figure, corresponding to the measurement margin in the new acoustic Belgian standard for dwellings (NBN S 01-400-1:2022) which is based on the precision indicated in ISO 16032. For the predicted values (in blue), an error of ± 3 dB is indicated.

 

For more than 95% of the rooms, the measured L_Aeq,nT was determined by the airborne duct sound. For the other cases, either the background noise was higher than the ventilation noise (bedroom 1 of case 3 step 4, studio case 4) or the ventilation noise was caused by other transmission paths, like airborne transmission from the technical room via the building structure or duct breakout (atelier and bedroom 1 case 7, laundry case 10). For the cases where the airborne duct sound was dominant, the prediction error is smaller than 2 dB in 54% of the cases, smaller than 3 dB in 65% of the cases and smaller than 4 dB in 75% of the cases. Larger prediction errors can e.g. be caused by an inaccurate model of the actual ventilation network (which often differs from the design), inaccurate input data for the ductwork elements, the presence of tonal components or the interaction between components creating additional turbulence and flow noise (see section 4).

 

4. DISCUSSION OF RESULTS

 

Generally, the L_Aeq,nT at low frequencies (63 – 250 Hz) is determined by the fan sound, while at high frequencies (2 – 8 kHz), the flow noise of the ATD is the most important noise source. As a result, the accuracy at low frequencies depends on the prediction of the fan sound power level and the sound power level reduction of the network, which is mainly determined by the silencers, junctions, mani folds and the end reflection of the ATD at low frequencies. The accuracy at high frequencies depends on the prediction of the ATD flow noise and thus on the prediction of the pressure losses.

 

In dwellings, the airflow rate in each room is generally controlled by turning the ATD in the correct setting point. When the system is well balanced and the ATD of the branch with the highest pressure loss is put completely open, all the ATDs will be relatively open and produce little flow noise. In this case, the measurements at high frequencies were often determined by the background noise. However, when the network is not well balanced or the ATDs are not well adjusted during installation, the flow noise is often clearly noticeable and even determining the global L_Aeq,nT.

 

 

Figure 2: Comparison between measured L_Aeq,nT (  ) and predicted L_Aeq,nT (  )

 

4.1. Accuracy of Network Model and Input Data

 

In some cases, it was clear that the actual ventilation network differed from the design, e.g. because the location of the ATDs was different. When pictures were taken during installation, the exact layout of the duct network could be modeled. Otherwise, it was assumed that the actual network corresponds to the design, with estimated adjustments to account for the actual location of the ATDs. Generally, small differences between the actual and modeled geometry of the network don’t have a large influence on the accuracy, because straight ducts and bends do not considerably contribute to the overall sound power level reduction of the network at low frequencies.

 

The presence and type of secondary silencers was not always indicated in the design and verifiable in situ. For case 11 and 12 (situated in the same building), for example, the measured ventilation noise levels in the bedrooms are much lower than in the other rooms and also much lower than the predicted values. Possibly, additional silencers were installed before the ATDs of the bedrooms.

 

Sometimes, critical input data are missing for the model, for example for bedroom 4 of case 2 where the noise was clearly dominated by the ATD flow noise. As no laboratory measurements were available for the ATD, the general formula of VDI was used, leading to an overestimation. Generally, VDI is on the safe side by overestimating the flow noise of ATDs used in dwellings. When the L_Aeq,nT is dominated by the flow noise, the use of laboratory data of the ATD flow noise is therefore essential.

 

Another problem encountered, is the estimation of the ΔL_w of non-acoustic flexible ducts. While the high-frequent insertion loss is negligible, there is a significant amount of damping at low frequencies due to duct breakout. As no measurement data are available, this was estimated based on the average difference between the insertion loss of flexible and rigid acoustic silencers. This gave good results for case 1 and case 9, but for the bathroom and toilet of case 8, the damping is strongly under estimated.

 

As a rule of thumb, the laboratory insertion loss values of silencers at the lowest frequency bands seem also less reliable and less relevant for the actual insertion loss of the silencers as installed.

 

4.2. Tonal Components

 

For case 10, the measured L_Aeq,nT - values at the nominal flow rate were determined by a tonal component in the one-third octave band of 315 Hz. Measurements were also made for a higher fan airflow rate of 400 m³/h in some of the rooms (living room, 3 bedrooms and bathroom 1). In this case, the tonal component was less pronounced in the measured L_Aeq,nT - spectra and the prediction errors on the overall L_Aeq,nT are significantly lower (average prediction error of 2.4 dB for 400 m³/h compared to 4.5 dB for nominal flow rate).

 

It can be expected that the prediction is less accurate for tonal components. First of all, the tonal component of the fan is strongly sensitive to fan speed. Because no fan sound power level data were available at the nominal flow rate, an interpolation was needed. While such an interpolation generally gives a good estimation of the broadband fan noise, the shift of tonal components in the fan power spectrum is impossible to account for. Secondly, the acoustic properties of the elements are only available in octave bands. These average values are appropriate for noise levels which are relatively constant in that frequency band. However, this is not the case for tonal components. Furthermore, the insertion loss of specific elements like silencers and manifolds can vary significantly with frequency, especially at lower frequencies where modal effects can be important (e.g. the wavelength at 315 Hz corresponds to the length of the silencers and the dimensions of the manifold used in the dwelling).

 

4.3. Turbulence

 

When two fittings (like bends, junctions, valves or ATDs) are placed close to each other, the airflow will become more turbulent and noise generated in the fitting can be substantially higher. For example, when an ATD is placed immediately after a bend or a junction, the flow noise can increase by as much as 12 dB according to ASHRAE [2]. Furthermore, there can be a strong interaction between the elements, influencing the ΔLw-value [5]. This additional flow noise and interaction is not taken into account in the acoustic model and may explain some of the larger prediction errors. For example, the high airflow rate in the junction placed immediately before the ATD of the study room of case 4 probably leads to strong turbulence and a significant increase of the ATD flow noise (estimated increase of 12 dB). The foam sound attenuators initially placed behind the ATDs in the living room of dwelling 3 (step 0) also led to a significant increase of the flow noise generated by the ATDs [7].

 

5. CONCLUSIONS

 

The accuracy of a simplified prediction tool for mechanical ventilation noise based on EN 12354-5 and VDI 2081 has been investigated by comparing measurements and predictions for 12 dwellings. While more complex modeling techniques exist, the simplified model is generally sufficient to check the acoustic requirements of a mechanical ventilation system to be installed in a dwelling. One of the problems is that the exact lay-out of the ductwork is often not known in the design phase. Furthermore, acoustic input data are not always available for every element in the ductwork. Therefore, a simplified prediction with generic prediction formula is usually the best option. For 75% of the rooms where the airborne duct sound was dominant, the global A-weighted noise level in the room can be estimated with an accuracy of ± 3 dB, incorporating the fact that the measurement error is ± 1 dB. Sometimes (< 5%), the noise is determined by alternative transmission paths (e.g. airborne transmission from technical room, duct breakout or structure-borne transmission). While these contributions cannot be easily estimated without detailed knowledge of the building design, they can be avoided by following the general rules of thumb given in design guides like VDI, ASHRAE and ISSO.

 

6. ACKNOWLEDGEMENTS

 

This work has been supported by the Flemish Agency for Innovation and Entrepreneurship (VLAIO) in the Flux50 project Smart Ventilation (HBC.2020.2520). I would like to thank C. Delmez for the help with the aeraulic measurements.

 

7. REFERENCES

 

1. VDI 2081 Noise generation and noise reduction in air-conditioning systems (2001)
2. ASHRAE Handbook – HVAC Applications – Chapter 48: Noise and Vibration Control (2015)
3. ISSO-publicatie 24. Installatiegeluid – Ontwerpaanbevelingen en theoretische grondslagen t.b.v. kleine utiliteit en onderwijsgebouwen (2019)
4. Bessac, F., Guigou-Carter, C., Bailhache, S. & Lefebvre, C. Ductwork noise calculations: main outputs of AcouReVe project. Proceedings of 39^th AIVC conference. Juan-les-Pins, France, 18- 19 September 2018.
5. De Roeck, W. & Desmet, W. Indirect acoustic impedance determination in flow ducts using a two-port formulation. Proceedings of 15^th AIAA/CEAS Aeroacoustics Conference, pp. 2831-2840, Miami, Florida, USA, 11-13 May 2009
6. Denayer, H. Flow-acoustic characterization of duct components using multi-port techniques. PhD Thesis, KU Leuven (2017)
7. Caillou, S. & Dijckmans, A. Improvement of the acoustical performance of mechanical ventilation systems in dwellings: a case study. Proceedings of 39^th AIVC conference. Juan-les-Pins, France, 18-19 September 2018.

 

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¹arne.dijckmans@bbri.be