Façade insulation at low frequencies influence of room acoustic properties

Transcription

1 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century PROCEEDINGS of the nd International Congress on Acoustics Challenges and Solutions in Acoustics Measurement and Design: Paper ICA06-8 Façade insulation at low frequencies influence of room acoustic properties Dag Glebe (a), Krister Larsson (b) (a) SP Technical Research Institute of Sweden, Sweden, dag.glebe@ sp.se (b) SP Technical Research Institute of Sweden, Sweden, krister.larsson@ sp.se Abstract Exposure to environmental noise in the neighbourhood has negative effects on the wellbeing and the quality of life of residents. Protection from environmental noise and keeping indoor noise at acceptable levels are therefore essential properties of building façades. The sound insulation of a façade depends not only on the design of the wall elements, but on the combination of all components and their assembly, such as windows, air terminals, seals etc. However, windows are often the weakest component and determine the sound insulation. Energy demands, as well as building cost and sustainability demands, lead to the development of new building elements and constructions, often using lightweight solutions. In many cases the low frequency (<00 Hz) sound insulation is a challenge for lightweight constructions, and the resulting indoor levels also depend on the source spectrum. Additionally, the low frequency sound insulation is not only a characteristic of the separating element itself, but depends on room design and modal behaviour. Although A-weighted indoor levels meet requirements, residents may be annoyed by low frequency noise and it might be difficult to meet additional low frequency demands. The influence of window designs on façade insulation is evaluated in a corresponding paper, and the scope is here broadened to include room designs. In this paper, the low frequency interaction between the source spectrum, the façade insulation and various room acoustic properties is evaluated numerically and experimentally. In particular, the influence of various absorber configurations on the modal behaviour is discussed, in relation to corresponding measurement challenges in the low frequency region. Keywords: Room acoustics, low frequency, absorbers

2 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century Façade insulation at low frequencies influence of room acoustic properties Introduction In January 05, the previous recommended maximum external noise level at façades of new buildings in Sweden was removed, under the condition that at least half of the inhabitable rooms were facing a quiet side (i.e. a side where the noise level at the façade is lower than L Aeq = 55 dba). This opened up for new residential areas in zones with traffic noise exposure levels that previously were considered unacceptable. At the same time, it is known that long term exposure to road traffic noise increases the risk of negative health effects [], and there are also reported risks associated with low frequency noise [], [3] and [4]. Therefore, protection from environmental noise and keeping indoor noise at acceptable levels are essential properties of building façades. But, the contributions of the individual elements are hard to evaluate and measure in the low frequency regions with few room modes. The challenges of low frequency measurements have been addressed with various approaches in e.g. [7], [8], [9], [0], [] and []. However, the indoor sound pressure level from external noise sources at low frequencies is system dependent, and involves a chain of elements starting from the source spectrum through the outdoor propagation path, the façade construction, to the room acoustic properties. But the chain also includes the modal behaviour and the coupling of building elements to the room modes. Various aspects of this relationship have been discussed or studied in e.g. [7], [4], [5], [6] and [6]. Here, we are focussing on how room acoustic properties influence the modal behaviour, and in particular how absorbing patches interact with the room. In a corresponding article [5] the role of windows are evaluated, but both papers are considering the problem from a system perspective. The papers are intended to be considered together, as a broad approach on low frequency insulation, and will be a base for future work including the source spectrum, the façade design and room properties. Measurement of low frequency façade performance In order to investigate the acoustic behaviour of a pre-fabricated low energy wooden framed villa, measurements were performed in a replica of a modern single family house which was built as a demonstrator for the research project NEED4B at the premises of SP Technical Research Institute of Sweden, in Borås. The façade design of the house is shown in figure. Frequency response function measurements were performed using MLS technique. One microphone was mounted on the outer side of the exposed window and one microphone was used to measure indoor levels in 3 evenly distributed positions. Four Genelec subwoofers were used to generate the external low frequency sound. The reverberation time was measured in the same positions and also in 4 additional positions. The difference in sound pressure level between the microphone mounted on the outer surface of the window and 6 indoor

3 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century microphones forming a plane at.0 m above the floor in the receiving room was compared with calculated room modes in the low modal density region and the reverberation time measurements. More information on the measurements is given in [5] and [6]. Figure The facade construction of the measurement object is well documented. Difference in sound pressure microphone height. m,f= Hz Difference in sound microphone height. m, f= Hz Measured reverberation field at the 3.5 Hz third octave band Figure Measured frequency response function at 33.6 Hz (top left) and at 35 Hz (top right), measured reverberation time in the 3.5 Hz third-octave band (bottom left) and calculated lowest room mode, at 3 Hz (bottom right). Only the measured frequency response function at 35 Hz is similar to the calculated mode. 3

4 Level difference (db) Leq (db) nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century The first room mode was calculated to around 3 Hz. The expected modal shape corresponded to the measured frequency response at around 34 Hz. However, the measured frequency response in a plane at.0 m height in the room varied significantly in the adjacent frequency range, and the modal pattern was not exposed in the reverberation time measurement, in the 3.5 third-octave band (cf. Figure ). Some overlap can be expected to the 40 Hz third-octave band, but was not found here. A more rigorous analysis of the reverberation measurement is planned to be presented in future work. The room modes found in the calculations were generally not clearly discernible in the frequency response function measurements. This may, however, be expected since the boundary conditions of e.g. the inner walls are hard to assess in the low frequency region. The most distinct higher mode was found around 74 Hz and bared no clear resemblance to the near calculated mode at 76.6 Hz (Figure 3). Difference in sound pressure microphone height. m,f= Hz Figure 3 The most prominent higher mode found in the measurements was found at approximately 74 Hz (left), with the closest calculated mode at 76.6 Hz (right) Frequency (Hz) 0 Figure 4 Level difference of the outdoor window microphone and the indoor microphones (left). Traffic noise spectra, typical for mixed traffic in Sweden (right, in red: Measured DAPC, in blue: SMA 0/6, calculated with Nord000 model Measured and calculated traffic noise spectra 0 5 3, Frequency (Hz) 4

5 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century The level difference of the outdoor microphone at the window and the indoor microphones indicate poor façade insulation properties below 00 Hz. This is a frequency region where e.g. typical traffic noise contains energy which has small impact when the sound is A-weighted (Figure 4), but can be problematic if the noise level is high. 3 Influence of absorber configurations in a room model 3. Eigenmodes The principal modal behaviour was simulated with a finite element (FEM) model for a simple room shape (shoebox), with the dimensions 4 m x.5 m x.5 m. The walls are here assumed to be rigid (i.e. the impedance is infinity, or the particle velocity zero). The interaction between the lowest room modes and a set of three absorbing patches with an area of 0.6 m x. m was evaluated for three configurations: a) all three patches juxtaposed and horizontally centred at one wall, forming a coherent absorbing area of.8 m x. m, b) the middle patch as in a), but the two off-centred patches positioned at the horizontal centre of the two adjacent (opposite) walls, and c) as in b) with the exception that the two patches on the opposite walls are positioned off-centre (cf. Figure 5). The absorbing patches were modelled as 0.5 m thick porous layers with a resistivity of 50 k rayls m -, an arrangement which gives quite small absorbing effect in the low frequency region (the first room mode comes at 3 Hz, equalling a wave length of almost m). But, since the absorbing properties are well defined, and it still results in observable effects on the modal behaviour, it was considered an acceptable set-up for this evaluation, with dimensions in an order of magnitude that are realistic for room treatments. Figure 5 The simulated room, with the three valuated configurations of absorbing patches. The designations are from left to right: a), b) and c). There are seven eigenmodes of the simulated room in the frequency range up to 00 Hz, which are shown for configuration a) in Figure 6. 5

6 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century Figure 6 The simulated room modes below 00 Hz arranged from the first, top left, to the seventh, bottom. The double eigenmodes around 68 Hz and 80 Hz, respectively, do not coincide at the exact same frequency, since the absorbing patch is neither symmetric nor centred in the plane. 3. Forced excitation with various absorber configurations The modal behaviour was simulated by a applying a point source in the low near corner in Figure 5 and study the response at the opposite top far corner. The response for configuration a) is shown in Figure 7. As can be seen, the two pairs of symmetric eigenmodes around 68 Hz and 80 Hz, respectively, appear as single peaks in this representation. 6

7 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century Figure 7 The frequency response in the top far corner of configuration a) in Figure 5 generated by a point source in the near low corner. In the cases of configurations b) and c), all seven eigenmodes are visible (cf. Figure 8 and Figure 9). But, the rearranged absorbing patches will influence the two pairs of double eigenmodes in different ways. At 68 Hz: In the case of the second eigenmode in Figure 6, the absorbing patches in configuration b) and c) are all crossed by the node line. In the case of the third eigenmode in Figure 6, the two opposite absorbing patches in configuration b) and c) are situated at walls which coincide with the maximum and minimum pressure planes, respectively. This applies both for configuration b) and c), and thus the split double peak at 68 Hz will appear similarly in Figure 8 and Figure 9. On the other hand, the single united patch of configuration a) is crossed by the node line in a similar way for both of the eigenmodes, and therefore the corresponding 68 Hz peaks are so close that they appear as one peak in Figure 7. 7

8 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century Figure 8 The frequency response in the top far corner of configuration b) in Figure 5. The sound field is generated by a point source in the near low corner in Figure 5. Figure 9 The frequency response in the top far corner of configuration c) shown in Figure 5. The sound field is generated by a point source in the near low corner in Figure 5. 8

9 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century At 80 Hz: In the case of the forth eigenmode in Figure 6, the absorbing patches are all crossed by a node line, and the two opposite patches in configuration b) by two node lines. In the case of the fifth eigenmode in Figure 6, the two opposite absorbing patches in configuration b) are all crossed by a node line, whereas the two asymmetric placed patches in configuration c) are placed at the side of the node line (which is also the case in configuration a) ) Thus there is a distinct difference between the appearance of the split double peak at 80 Hz in Figure 8 and Figure 9, and less between configuration a) and configuration c) (cf. Figure 7). 4 Summary and future work The indoor sound pressure level from external noise sources at low frequencies is system dependent, and involves a chain of elements starting from the source spectrum, the outdoor propagation path, the façade construction and its elements and their coupling to the room modes, and the room acoustic properties. In the façade transmission measurements, single room modes were discernible, which could not be detected in the corresponding third-octave band reverberation measurements. The relationship between the reverberation field and the modal behaviour will be further explored in future work. Modelling results show that identical room modes could be moved in frequency depending on where sound absorbers were positioned in the room in relation to the modal patterns. Although the differences were apparent, they were also quite small, since the absorbing properties of absorbing material in general are modest in the low frequency region, which is reflected in the calculations. In real rooms, the walls, the ceilings and the floors cannot be considered rigid, and the responses will be smoothed out, making it difficult to detect subtle effects experimentally. However, other choices of absorbers (e.g. Helmholtz absorbers) may have a much stronger impact on individual modes. The relationship between the placement of absorbing and diffusing elements will be further studied both experimentally and with simulations. This work is a part of a larger framework to develop models for predicting indoor noise levels in environments where, in particular, low frequency noise may be problematic. The intention is to use a holistic approach and also address e.g. heterogeneous noise sources which are often handled in a general way in prediction models. One example is the traffic noise levels from vehicles, which show great variations, particularly in the low frequency region, depending on both the individual vehicles and other factors as driving behaviour, speed bumps et cetera. Acknowledgments The work is financed by SP Technical research Institute of Sweden and carried out as a part of the Urban Tranquility InterReg project. 9

10 nd International Congress on Acoustics, ICA 06 Buenos Aires 5 to 9 September, 06 Acoustics for the st Century References [] World Health Organisation. Burden of disease from environmental noise. Quantification of healthy life years lost in Europe, 04. [] Kerstin Persson Waye. On the effects of environmental low frequency noise. PhD Thesis. Göteborg University, 995. [3] Colin H. Hansen. The effects of low-frequency noise and vibration on people. Multi-Science Publishing Co. Ltd., Brentwood, 007 [4] M.v.d. Berg. Influence of low frequency noise on health and well-being. Informal document No. GRB-4-8. Netherlands, February 005. [5] Larsson, K.; Glebe, D. Façade insulation at low frequencies Influence of room acoustic properties. Proceedings of ICA06, Buenos Aires, Argentina, 5-9 Sept. 06. [6] Glebe, D.; Larsson, K.; Persson, K. Comparisons of various approaches to low frequency insitu measurements and corresponding models, Proceedings of InterNoise 06, -4 Aug. 06, Hamburg, Germany. [7] Simmons, C.; Measurements of sound pressure levels at low frequencies in rooms. Comparison of available methods and standards with respect to microphone positions. Proposal for new procedures. NORDTEST Project No , Sweden, 997. [8] Pedersen, S.; Møller, H.; Persson Waye, K.; Indoor measurements of noise at low frequencies - Problems and solutions. J. Low Freq. Noise, Vib. Active Contr, Vol 6(4), 007 [9] Ministry of the Environment Government in Japan. Handbook to deal with low frequency noise, Japan, 004. [0] Moorhouse, A.; Waddington, D.; Adams, M.; Procedure for the assessment of low frequency noise complaints. University of Salford. February 005, Contract no NANR45 [] Ostendorf, C.; How to find the source of low frequency noise: three case studies. J. Low Freq. Noise, Vibration and Active control. Vol. 8 No [] Oliva, D.; Koskinen, V.; Keränen, J.; Hongisto, V.; New measurement method of low frequency noise in rooms. 4th International Meeting on Low Frequency Noise and Vibration and its Control, Aalborg, Denmark, 9- June 00. [3] Nilsson, E.; Decay Processes in Rooms with Non-Diffuse Sound Fields Part I: Ceiling Treatment with Absorbing Material, Building Acoustics, Vol (), 004, pp [4] Nilsson, E.; Decay Processes in Rooms with Non-Diffuse Sound Fields Part II: Effect of irregularities Building Acoustics, Vol (), 004, pp [5] Kropp, W.; Pietrzyk, A.; Kihlman, T. On the meaning of the sound reduction index at low frequencies. Acta Acustica,, 994, pp [6] Larsson, K.; Amiryarahmadi, N. Influence of excitation position and floor-room modal coupling on low frequency impact noise, Proceedings of InterNoise 05, 9- Aug. 05, San Fransisco, USA. [7] Hopkins, C., Turner, P. Field measurement of airborne sound insulation between rooms with non-diffuse sound fields at low frequencies, Applied Acoustics, 66 (), 005, pp