Sound absorption properties of sustainable fibrous materials in an enhanced reverberation room Francesco D’Alessandroa, Giulio Pispolab a,b
Department of Industrial Engineering, University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy a
[email protected]; b
[email protected]
Abstract In this paper, the measurement of sound absorption coefficient of novel sustainable fibrous materials is investigated. Nowadays the use of such materials is becoming wider for various applications, being ecological, biodegradable and renewable: they differ from traditional fibrous materials, as rock or glass wool, for their very low toxicity and polluting effects. These materials can be used in many ways: noise mitigation and building acoustic correction are surely among the most important. Sound absorbing layers made of natural fibres and of recycled raw materials have been tested in the reverberation room of the Acoustics Laboratory of the University of Perugia according to ISO 354 standard, in order to quantify their sound absorption properties and to make a comparison with traditional fibrous sound absorbers. An optimization of the reverberation room characteristics has been also carried out. Good sound field diffusivity inside the room is a fundamental requirement for the measurement accuracy. Among the parameters that mainly affect room diffusivity are the room shape and the sample disposition inside the room. In order to obtain accurate values of the sound absorption coefficient, specific actions were adopted. Test specimens were placed on the floor with edges nonparallel to the room walls. A partial closing of the lower room corners with absorbing and reflecting diffusers and suspended plane diffusers were also tested, obtaining a significant improvement of the results. The measured performance of the tested materials seems to be fully comparable with that of mineral wool fibres: because of their low impacts on the environment and the human health they can be seen as a valid alternative to conventional materials. 1. INTRODUCTION Porous materials obtained from synthetic fibres, such as mineral wool or glass wool, are commonly used for thermal insulation and sound absorption, because of their high performance and low cost. Their diffuse-field sound absorption coefficient is very high at mid-high frequencies. On the other hand, they have several cons: they can be harmful for
human health if their fibres are inhaled, since they can lay down in the lung alveoli, and can cause skin irritation (as stated by the European Council Directive on dangerous substances 67/548/EEC [1] and subsequent amendments). Hence such materials must be adequately overlaid if directly exposed to the air. Moreover they can pulverize because of vibrations and are not resistant to water, oil and chemical agents and this makes unwise their application on absorbing noise barriers. In recent years, an increasing attention has been turned to natural fibres as alternatives to synthetic ones, in order to combine high acoustic and thermal performances with a low impact on the environment and on the human health. Natural fibres have very low toxicity and their production processes contribute to protect the environment. Recycled materials, as recycled plastic fibres, can even be regarded as a sustainable alternative. They can be manipulated without any protection device, not releasing fibres or dust even when subjected to an extended mechanical stress. The aim of the paper is to investigate the acoustic performances of such materials through measurements of sound absorption coefficient in reverberation room and comparisons with conventional fibrous sound absorbers.
2. DESCRIPTION OF THE TESTED SAMPLES 2.1 Sustainable Fibrous Sound Absorbers: Kenaf And Recycled Polyester Two different novel fibrous materials were tested in reverberation room. The first ones were sound absorbing blankets of kenaf fibres assembled in semi-rigid panels without using adhesives. Small percentages of polyester (8-10 %) and of a fireproof additive have been added and then the components have been thermobonded. Characteristics of the tested specimens are detailed in Table 1. Kenaf is the name of a hibiscus plant related to cotton and okra, Hibiscus cannabinus L., and is member of the Malvaceae family (Fig. 1a). It has been used in Asia and Africa for thousands of years to obtain raw materials for clothes, paper, oil and medicines. Today it is used especially for its cordage, canvas and sacking. It can easily be cultivated (even with biological methods) and grows quickly. The stems produce two types of fibres, a coarser one in the “bast”, and a finer one in the “core” (Fig. 1b). Bast fibres (about 35 %) are suitable for paper, textiles and rope; core (about 65 %) is usually used as a biomass or it can be reduced to particles and bonded into panels similar to particleboard [2]. The second tested samples were sound absorbing blankets of recycled polyester (PET) fibres. These are made of 100% polyester fibres obtained from a recycling process of PET bottles. Blankets are assembled in a 3D fibre arrangement through a thermobonding process without any adhesive. As the natural fibres, polyester blankets respect both the environment and the human health. Characteristics of the tested samples are detailed in Table 1. Measured sound absorption properties of the described materials are reported below in Sec. 4, following a brief overview of the actions taken to obtain reliable results in the reverberation room of the University of Perugia.
a
b
Figure 1: Kenaf: a) rows; b) bast fibres (left) and core fibres (right).
Table 1: Characteristics of the tested samples.
Parameter
Unit
Kenaf layers
PET layers
Structure
-
Thermobonded panels with no added adhesives
Thermobonded panels with no added adhesives
Raw material
-
Natural hemp fibres, polyester backing fibres
Recycled polyester fibres
Thickness
mm
50
50
Density
Kg/m3
30
30
Surface area of the test specimens
m2
7,56
7,56
3. REVERBERATION ROOM IMPROVEMENT When measuring sound absorption coefficient α s in reverberation room, several causes of errors can occur. Indeed it is often difficult to obtain high repeatability and reproducibility, even though the ISO 354 standard [3] specifies the essential requirements to increase measurement accuracy. The factors that mainly affect the measured absorption coefficients are the shape of the reverberation room, the placement of absorbing surfaces, the position of the source, of the microphones and of the samples under test, the control of the microclimatic conditions (temperature and relative humidity) and above all the diffuseness of the sound field [4]. The latter subject, being fundamental assumption of this kind of measurements, has been thoroughly studied by several Authors (for a brief exhaustive review, refer to [5]). Cook et al. [6] and Balachandran [7] had already suggested in the 1950s to increase the diffuseness of the sound field through the use of devices as diffusers and rotating vanes. Especially in a rectangular reverberation room, the sound field is likely to be far from a perfectly diffuse one. When absorbers are laid on the floor of such a reverberation room, the sound field characteristics (e.g. sound pressure standard deviation, spatial cross-correlation and coherence functions) are deeply altered. Considering the geometrical features of the
reverberation room available at the Acoustics Laboratory of the University of Perugia [8] (see Fig. 2), it was necessary to work mainly on its shape and on the diffuseness of the sound field inside the room. Several measurements were performed to investigate the aforementioned factors employing the interrupted stationary noise method. A two-channel data acquisition system (01dBMetravib model Symphonie) was used to record every 20 ms the sound pressure decays (through ½’’ microphones G.R.A.S. model 40 AR) and to drive an omnidirectional (dodecahedral) source with random noise. The reverberation time in each third octave band was evaluated by linear interpolation of the averaged decay curve (starting 5 dB below the beginning of the decay and over a decay range of 30 dB).
Figure 2: Plan of the coupled reverberation rooms of the Acoustics Laboratory of the University of Perugia. The aperture between the two chambers was left open during sound absorption measurements.
The influence of the sample position inside the room was firstly examined. ISO 354 standard suggests to place the test specimens with edges nonparallel to the room walls in order to reduce the influence of the horizontal axial modes: data for absorbing noise barrier panels can be seen in Figure 3, where the effect of a different positioning on the absorption coefficient is clearly shown. A partial closing of the lower corners of the room (Fig. 4 on the left) was then performed by means of plane absorbing and reflecting diffusers, in order to reduce the influence of tangential modes [9]. Samples of an absorbing noise barrier were tested in three different ways. The first configuration (standard) is a bare room one; then rock wool blankets were placed in each lower corner, while in the third configuration an assembly of four reflecting gypsum-board panels was employed. The results are shown in Figure 4 on the right. The use of corner wall reflecting diffusers seems to give the best results, at least for the tested samples.
1.00
parallel edges (a)
0.90
oblique edges (b)
0.80
a ALPHA
0.70 0.60 0.50 0.40 0.30 0.20
b
0.10
80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00
50 0
63 0
40 0
31 5
25 0
20 0
16 0
12 5
10 0
0.00
frequency (Hz)
Figure 3: Effect of the positioning of the sample (sound absorbing panels of a noise barrier) on the absorption coefficient: edges parallel (a) and not (b) to the room walls.
1.00 0.90 0.80
a
ALPHA
0.70 0.60 0.50 0.40 0.30 0.20 0.10
absorbing panels
reflecting panels
80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00
10 0 12 5 16 0 20 0 25 0 31 5 40 0 50 0 63 0
b
standard
0.00
frequency [Hz]
Figure 4: Effect of corner diffusers on the absorption coefficient of sound absorbing panels of a noise barrier: bare room (“standard”), absorbing panels (a) and reflecting ones (b).
As stated in the annex A of the ISO 354 standard, diffuseness of the sound field can be enhanced by means of fixed or rotating diffusers. They are especially important in rectangular rooms; in those cases, the standard suggests that the area of diffusers (both sides) should be in the range from 15 % to 25 % of the total surface area of the room. Different numbers of suspended plane gypsum-board diffusers were hung from the ceiling to evaluate their effect on sound absorption measurements (Fig. 5a); because of the limited available room, a smaller percentage of diffuser area was used (at maximum 5 % just with suspended diffusers). The tested samples were layers of polyester fibres (thickness 50 mm, density 50 kg/m3) and were laid on the floor of the reverberation room. Two source positions and four microphone positions were employed for each single reverberation time measurement, and the decay recordings were repeated three times at each point (24 acquisitions). Results are illustrated in Figure 6: it can be seen how the absorption coefficient raises towards reasonable values increasing the diffuser number, particularly at mid-high frequencies. Similar behaviours were reported by other Authors for measurements in rooms of regular shape [10].
Such results may be regarded as an indirect proof of the poor diffuseness of the sound field, as it may be expected considering the shape of the room.
a
b
Figure 5: Configuration of the reverberation room with suspended plane diffusers (a) and with a combination of suspended and corner plane diffusers (b).
Then a combination of 11 suspended and 16 corner wall diffusers was employed (Fig. 5 b) for an overall area equal to 8 % of the room surface area. Results are shown in Figure 6, as well. This latter configuration seems to increase the absorption coefficient at the medium frequencies, while its effect is poorer on the high frequencies range (the peak at 5 kHz has been regarded as due to measurement errors). Annex A of ISO 354 prescribes to calculate the mean value of the sound absorption coefficients in the range from 500 Hz to 5000 Hz for different numbers of diffusers: as reported in Table 2, the mean value calculated in the mentioned range is next to unity for the configuration with 11 hanging diffusers and 16 wall diffusers placed at the corners. 1,20 1,00 0,80 αS 0,60 0,40
bare room 6 hanging diffusers 11 hanging diffusers 11 hanging + 16 corner wall diffusers
0,20
80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00
63 0
50 0
40 0
31 5
25 0
20 0
16 0
12 5
10 0
0,00
frequency [Hz]
Figure 6: Effect of the installation of different numbers of hanging diffusers and of wall diffusers at the room corners.
Table 2: Mean sound absorption coefficients of polyester fiber blankets for different room configurations.
Frequency range [Hz] 100-5000 500-5000
bare room 0.69 0.84
6 hanging diffusers 0.70 0.85
11 hanging diffusers 0.77 0.96
11 hanging + 16 corner wall diffusers 0.84 1.05
It has to be duly noticed that introducing diffusers in a reverberation room has inevitably the consequence of increasing its equivalent absorption area or, that is equivalent, of lowering the reverberation time T, although reflecting are the diffusers. Figure 7 on the left shows this phenomenon for the empty room in the abovementioned configurations. Estimating the relative standard deviations S = σ T / T of the reverberation time (here it has been used the formula provided in [3]), it is possible to calculate the sound absorption coefficient standard deviation by simply regarding the reverberation times as the only independents variables: 2 2 2 2 2 ∂α ∂α 55.3V S1 S 2 σα = σ T1 + σ T2 = + cS T1 T2 ∂T1 ∂T2
(1)
where V is the volume of the reverberation room (120 m3), c is the sound speed in air, S is the test specimen area and the suffixes 1 and 2 denote respectively measurements without and with the test specimen. It is evident from Figure 7 on the right that the accuracy of sound absorption measurements becomes poorer as the number of diffusers increases. This means that a trade-off is needed among the sound field diffuseness and the measurement accuracy, simply due to the modified room absorption area. 14,00
0,2
12,00 10,00 8,00
bare room
bare room
0,18
6 hanging diffusers
0,16
6 hanging diffusers
11 hanging diffusers
0,14
11 hanging diffusers
11 hanging + 16 corner wall diffusers
0,12
11 hanging diffusers + 16 corner wall diffusers
0,1
6,00
σα [-]
0,08
T1 [s]
0,06
4,00
0,04
2,00
0,02
0,00
frequency [Hz]
63 0 80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00
0
0
50
5
40
0
31
25
0
0
20
5
16
12
10 0 0 12 0 5 16 0 0 20 0 0 25 0 00 31 5 40 0 00 50 00
0
80
63
0
0
50
5
40
31
0
0
25
0
20
16
0
12
10
5
0
frequency [Hz]
Figure 7: Diffuser effect on the measurement accuracy. Reverberation times of the empty room (on the left) and estimated standard deviations of the absorption coefficient (on the right) for different room configurations.
Climatic conditions inside the test room should be always kept under control: temperature and relative humidity have to be kept as similar as possible for the measurements with and without the sample. In this way, the effect of the correction introduced by the ISO 354
standard (by means of a power attenuation coefficient defined by ISO 9613-1 [11]) can be minimized, so eliminating one possible cause of uncertainty. Such correction is applied on the calculation of the equivalent absorption area A through the second term of the following [3]:
A=
55.3*V − 4*V * m c *T
(2)
where m is the power attenuation coefficient related to the air sound absorption. Its effect is greater at high frequencies and for low values of temperature (t
