ArticlePDF Available

Sound reduction by vegetated roof tops (green roofs): a measurement campaign

Authors:

Abstract and Figures

Green roofs (vegetated roofs tops) have many ecological and economic advantages. Noise reduction is one of them. Measurements of sound diffraction over buildings were performed in 5 cases, just before and just after the placement of different types of extensive green roofs. The source and receiver configuration was kept the same in each case, allowing a direct estimate of the sound reducing effect. Measurements showed that green roofs might lead to consistent and significant sound reduction at shielded receivers relative to common non-green roof finishing. Improvements up to 10 dB were found in a wide frequency range, under dry conditions. It was further shown that predicting the green roof noise reducing effect is difficult, caused by the multi-layered build-up of a typical green roof and the shifts in the interference pattern relative to rigid roofs, especially at elevated receivers.
Content may be subject to copyright.
1
Sound reduction by vegetated roof tops (green roofs): a
measurement campaign
Timothy Van Renterghem1 and Dick Botteldooren2
1-2Department of Information Technology, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium
ABSTRACT
Green roofs (vegetated roofs tops) have many ecological and economic advantages. Noise reduction is one of
them. Measurements of sound diffraction over buildings were performed in 5 cases, just before and just after
the placement of different types of extensive green roofs. The source and receiver configuration was kept the
same in each case, allowing a direct estimate of the sound reducing effect. Measurements showed that green
roofs might lead to consistent and significant sound reduction at shielded receivers relative to common
non-green roof finishing. Improvements up to 10 dB were found in a wide frequency range, under dry
conditions. It was further shown that predicting the green roof noise reducing effect is difficult, caused by the
multi-layered build-up of a typical green roof and the shifts in the interference pattern relative to rigid roofs,
especially at elevated receivers.
Keywords: Green roofs, sound propagation, sound diffraction, measurements
1. INTRODUCTION
Green roofs have many ecological and economic advantages, and also its noise reducing
potential has been recently identified. While increased sound insulation of the roof system by the
presence of a green roof has been measured [1], most practical applications related to environmental
noise deal with reducing the intensity of diffracting sound waves over roofs [2][3][4]. Green roofs
have (highly) porous substrates and therefore allow noise reduction. This effect is enhanced given the
fact that sound propagates most often parallel to the roof in practical situations (shearing waves).
Numerical simulations presented in Refs. [2] and [3] show the high noise reducing potential of
green roofs, compared to rigid roofs which are most often encountered. An example of a useful
application of a green roof to achieve noise reduction is a building extension facing a nearby road.
Positive effects were also predicted in a typical street canyon setup, leading to increased quietness in
shielded courtyards or at non-directly exposed façades.
Experimental data of sound propagation over real green-roofed buildings is lacking. In this
study, in-situ measurements of the effect of flat, extensive green roofs are presented. It is intended to
show what can be expected from typical green roof practice (not optimized from the viewpoint of noise
reduction) at various building configurations. Measurements were performed just before and just after
placement of the green roof, with an identical source-receiver setup. In this way, the green roof effect
can be directly estimated. Five cases have been selected where such measurements were possible.
2. MEASUREMENT METHODOLOGY
2.1 Instrumentation and data processing
An alarm gun (with blanks) was used as acoustic source, producing high sound power levels
leading to easily identifiable peaks even at highly shielded locations. Furthermore, the signal-to-noise
ratio was shown to be sufficient over the frequency range between 50 Hz and 10 kHz in the
measurements performed.
1 timothy.van.renterghem@intec.ugent.be
2 dick.botteldooren@intec.ugent.be
2
The reproducibility of successive shots produced by the gun was checked in a full anechoic
chamber. Five shots were released at close distance from the microphone. The standard deviation in
function of sound frequency is shown in Fig. 1. Both at low and high frequencies, the standard
deviations are near 1.5-2 dB. For the intermediate frequencies, this value is between 0.5 dB and 1 dB.
Given the rather good reproducibility, no reference measurements have been included to capture
possible variations in the emitted source power level in the “before” and “after” measurement. Five
repetitions were considered to be sufficient.
Figure 1 – Standard deviation in function of sound frequency in case of 5 successive shots as measured in an
anechoic chamber.
The logging was performed with a Svantek 959 portable device, connected to a pre-amplifier
and a ½” electret microphone capsule (Microtech MK 250 B). The saturation level exceeds 140 dB (at
1 kHz) which was sufficiently high for the envisaged application, keeping in mind the high source
powers emitted. Before each measurement, a calibration was performed with a Bruel & Kjaer 124 dB
pistonphone, producing a pure tone near 250 Hz. Results were logged as 1/3 octave bands integrated
over 10-ms periods. The peaks corresponding to the repeated shots were identified afterwards based on
the time series of the total sound pressure level as illustrated in Fig. 2. After identification of the
correct times, the spectra were energetically averaged over the duration of each shot, and then linearly
averaged over the 5 repetitions.
Figure 2 – Example of measured time series at the microphone.
3
2.2 Description of test cases
For cases 1 to 3, a single diffraction is needed for sound propagating from source to receiver.
Such cases are typical for a building extension and a receiver located near a façade/window as
illustrated in Fig. 3. Cases 4 and 5 involve double diffraction towards a completely shielded receiver
(see Fig. 3). These two types represent situations where positive effects of green roofs can be expected
as shown with the numerical simulations reported in Refs. [2] and [3].
Figure 3 – Schematic overview of single diffraction and double diffraction cases (S=source, R=receiver).
In cases 1 to 3, the lengths of the green roofs along the shortest propagation path are 8 m, 4 m
and 4.5 m, respectively. The substrate depths are 20-30 mm, 50-60 mm and 180 mm, respectively. In
cases 4 and 5, the distance propagated over the green roof is near 25 m. In case 4, substrate depth is
only 20-30 mm, while it is near 80 mm in case 5. Measurements were performed mainly just after
placement of the green roof, after dry periods. The layer build-up (drainage layer, water retention layer,
type of substrate, etc.) was quite different in the 5 cases considered. Vegetation cover ranged from 0%
to 100%. A more detailed description of the different cases can be found in Ref. [5].
3. RESULTS AND DISCUSSION
3.1 Single diffraction cases
In Fig. 4, the green roof improvement (i.e. the reduction in sound pressure level by the presence
of the green roof relative to a common non-vegetated roof finishing) is depicted. Rather strong effects
are found over wide frequency ranges, although the short propagation distances. At some frequency
bands, negative effects are observed. The latter is more pronounced for higher receiver positions (see
Ref. [5]), caused by shifts in interference pattern due to changes in roof cover. For the longer
propagation paths interacting with the green roof (compare case 1 to case 2 or 3), a more consistent
green roof improvement in function of frequency is found. Case 3 is characterized by a thick substrate
layer of 180 mm, yielding noise reductions exceeding 10 dB in the frequency range between 300 Hz
and 1 kHz.
Below 100 Hz, no significant effects are measured in any case. Also above 5 kHz, there is no net
effect by the presence of the green roof in the single diffraction cases considered.
The results show rather complex behavior and prediction does not seem straightforward. The
exact layer build-up is quite different among the tested cases and is expected to play an important role.
Figure 4 – Green roof improvement for the single diffraction cases.
4
3.2 Double diffraction cases
For the double diffraction cases (cases 4 and 5), receivers are now fully in the acoustic shadow
zone of the building, and the interaction path between the green roof and the shearing waves is much
longer than in the single diffraction cases. Results are shown in Fig. 5. In case 4, consequent and rather
uniform effects are observed from 300 Hz till 10 kHz, with green roof improvements up to 5 dB. In this
case, the substrate thickness was very limited (prefabricated green roof tiles). In case 5, strong effects
are observed between 100 Hz and 800 Hz, but at higher frequencies effects become very limited. The
substrate depth in case 5 is much larger than in case 4.
The continued positive effect above 3-4 kHz in case 4 could be caused by the presence of
vegetation, leading to scattering of sound waves diffracting over the roof. Strong scattering can be
expected from these frequencies on. In case 5, vegetation was absent at the time of the measurement.
The amount of data presented here is however insufficient to sort out the effect of the presence of
vegetation on green roof substrates. The influence of receiver height (see Ref. [5]) is less pronounced
in the double diffraction cases compared to the single diffraction cases.
Figure 5 – Green roof improvement for the double diffraction cases
4. CONCLUSIONS
The noise reducing effect of green roofs has been confirmed by means of in-situ diffraction
measurements. Important noise reductions were measured compared to common (non-greened) roof
finishing, both for single and double diffraction cases. Consistent positive effects over wide frequency
ranges are mainly present in case of double diffraction cases. The multi-layered build-up of green
roofs leads to complex acoustic behavior. It has to be noted that all measurements were performed at
dry green roofs. The green roof improvements are expected to be significantly lower after rainfall
events.
REFERENCES
[1] J. Kang, H. Huang and J. Sorrill, “Experimental study of the sound insulation of semi-extensive green
roofs,” Proc. INTER-NOISE 2009 (2009).
[2] T. Van Renterghem and D. Botteldooren, “Numerical evaluation of sound propagating over green
roofs,” J. Sound Vib. 317 (3-5) 781-799 (2008).
[3] T. Van Renterghem and D. Botteldooren, “Reducing the acoustical facade load from road traffic with
green roofs,” Build. Env. 44 (5) 1081-1087 (2009).
[4] H. Yang, M. Choi and J. Kang, “Laboratory study of the effects of green roof systems on noise
reduction at street levels for diffracted sound,” Proc. INTER-NOISE 2010 (2010).
[5] T. Van Renterghem and D. Botteldooren, “In-situ measurements of sound propagating over extensive
green roofs,” Build. Env. 46 (3) 729-738 (2011).
... The growth medium of the green roofs helps in arresting the lower frequencies of noise and plants arrests the higher frequencies there by creating an insulated system for sound. Timothy and Dick (2011) conducted an experiment to observe the impact of green roofs on sound reduction in a building. The results showed that green roofs helps in reducing the sound significantly in when compare to non-green roof building. ...
Article
Full-text available
This paper presents the advantages, disadvantages, planning consideration, components of green roofs with its holistic benefits.
Preprint
Full-text available
Green roof is an environment-friendly structure developed globally as a result of increasing urbanization. In this review paper we first tried to collect classification, structure and materials of green roofs. Increasing water availability of substrate using water retention additives is also collected and studied in this paper. Accordingly, different materials are applied in green roofs, among which polymers have attracted a lot of attention. Polymer materials are widely used in different layers of green roofs due to their characteristics like light weight, which is an important concern about green roofs. So, in the next step, we gathered different polymeric materials that are used in different layers of green roof or can be used in this structure. For example, low density polyethylene (LDPE) or polyethylene (PP) material use as physical barriers. Therefore, this article provides an opportunity to review and compare different polymeric materials that have been studied in different articles in various layers.
Article
Full-text available
Along with the strong movement towards sustainable urban environments, there is an increasing use of green roof systems, including the semi-extensive green roofs, which are composed of lightweight layers of free-draining material that support low-growing, tough, and drought-resistant vegetation. Various kinds of green roof systems are also used at street levels, on the top of underground car parks, for example. To predict the sound fields in street canyons, it is important to demonstrate the acoustic effect of such green roof systems, especially considering diffracted sound waves. In this study, therefore, a series of measurements of sound pressure level were carried out in a semi-anechoic chamber using 20 green roof trays which consist of the Zinco substrate with a depth of 100mm and low-growing vegetation. To consider a low-profiled structure in urban areas, a box with a height of 1200mm was located on the ground, and the trays were installed on the top of the box. As experimental parameters to verify the effects of the green roof system on noise abatement for diffracted sound waves, the structure, area and position of the green roof system, and the type of vegetation were considered. The experimental results demonstrate that the green roof system at street levels can be effectively used to mitigate noise in urban spaces for diffracted sound waves.
Article
In this study in-situ measurements of sound propagating over flat extensive green roofs are presented for 5 cases Measurements were performed Just before and just after the placement of the green roof (under dry conditions) with an identical source-receiver configuration in both situations allowing a direct estimate of the acoustical effect Situations involving a single and double diffraction over the green roof were considered for substrate thicknesses ranging from 20-30 mm to 180 mm and for vegetation cover ranging from absence to 100% The green roof acoustic effect was analyzed for propagation path lengths interacting with the roofs ranging from 2 5 m to 25 m Measurements show that green roofs might lead to consistent and significant sound reduction at locations where only diffracted sound waves arrive relative to common non-vegetated roofs A single diffraction case with an acoustic green roof improvement exceeding 10 dB was found for sound frequencies between 400 Hz and 1250 Hz although the green roof interaction path length was only 45 m For less shielded receivers a change in interference pattern might be observed leading to positive or negative effects relative to a non-vegetated roof top For the double diffraction cases the green roof improvement is less frequency-dependent and a case with positive effects up to 10 dB was found (C) 2010 Elsevier Ltd All rights reserved
Article
Noise annoyance by road traffic is a major issue in urbanized regions. In this study, the influence of a green roof on the façade noise load was investigated numerically for road traffic at close distance. Consistent positive effects of the presence of a green roof are observed at non-directly exposed (parts of) façades. A sufficient green roof area is needed to obtain significant reductions in total A-weighted road traffic noise level. With increasing traffic speed, the green roof effect increases for light vehicles. In case of heavy vehicles, this dependence is less strong. In a street canyon situation, the façade load in the non-exposed canyon is largely influenced by both the roof slope and the presence of a green roof. A flat roof generally results in the best average shielding. A green roof is especially interesting in case of a saddle-backed roof. With a good choice of green roof parameters, the shielding of a flat green roof can be approached.
Article
Sound propagation over intensive and extensive green roofs was numerically studied by using the finite-difference time-domain method. The Zwikker and Kosten model was used to simulate sound propagation in the substrate layer itself. The presence of a green roof is mainly interesting in it street canyon configuration, and fits well in the concept of quiet sides. Positive effects of green roofs, relative to fully rigid roofs, are mainly observed at the octave bands with centre frequencies 500 and 1000 Hz. The source type was shown to be unimportant when considering this particular parameter. There is a linear relationship between the fraction of the roof covered with green and the decrease in sound pressure level at the nonexposed canyon. The slopes increase with octave band centre frequency. The width-height ratio of the street canyon configuration has only a limited effect. For extensive green roofs, a pronounced attenuation peak is found when varying the layer thickness, leading to a maximum reduction of up to 10dB, relative to an acoustically rigid roof, for the octave band of 1000 Hz. A good overall efficiency is observed near the maximum layer thickness (15-20 cm) for this type of green roof. For an intensive green roof with it substrate layer thickness exceeding 20cm, which is common, positive effects are not influenced anymore by substrate thickness. (c) 2008 Elsevier Ltd. All rights reserved.
Experimental study of the sound insulation of semi-extensive green roofs
  • J Kang
  • H Huang
  • J Sorrill
J. Kang, H. Huang and J. Sorrill, "Experimental study of the sound insulation of semi-extensive green roofs," Proc. INTER-NOISE 2009 (2009).