M. Connelly, Green Roof Sound Absorption

M. Connelly, Green Roof Sound Absorption
Building and Environment 92 (2015) 335e346
Contents lists available at ScienceDirect
Building and Environment
journal homepage: www.elsevier.com/locate/buildenv
Experimental investigation of the sound absorption characteristics
of vegetated roofs
M. Connelly a, *, M. Hodgson b
Centre for Architectural Ecology, British Columbia Institute of Technology, NE3-3700 Willingdon Ave, Burnaby, British Columbia, Canada, V5G 3H2
Acoustics & Noise Research Group, University of British Columbia, 2206 West Mall, Vancouver, British Columbia, Canada, V6T 1Z3
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 March 2015
Received in revised form
22 April 2015
Accepted 23 April 2015
Available online 1 May 2015
An experimental investigation of the sound absorption characteristics of vegetated roof substrates and
plots has been completed. First, an impedance tube was used to measure the normal-incidence absorption coefficients of substrates and their constituents. Substrates provided significant sound absorption, with coefficients varying from 0.03 at 250 Hz to 0.89 at 2000 Hz. Absorption increased with the
percentage of organic matter and decreased with moisture content and compaction. A multi-variable
regression model was developed for predicting the absorption of substrates. Secondly, the sound absorption of vegetated roof plots was investigated using the spherical-decoupling method. An optimal
method, validated in an anechoic chamber, was used to determine the diffuse-field absorption coefficients of unplanted and planted rooftop test plots. Sound absorption increased with increased substrate depth (without vegetation) and decreased with the addition of vegetation and plant establishment.
The mean noise reduction coefficient of established vegetated roof plots, with distinctly different plant
communities in substrate depths of 50e200 mm, ranged from 0.20 to 0.63 when evaluated over a twoyear period. The results confirm that the sound absorption of vegetated roofs is a function of substrate
depth, plant community establishment, and moisture content in the plants and substrate.
© 2015 Elsevier Ltd. All rights reserved.
Sound absorption coefficient
Noise reduction
Vegetated roofs
1. Introduction
Vegetative roofs are increasingly used in sustainable building
construction. They have the potential to improve the acoustical
environments inside and outside buildings because of beneficial
sound absorption and transmission-loss characteristics. A recent
research project [1] investigated both aspects of the acoustical
performance of vegetated roofs; sound absorption is the topic of
this paper.
Mitigating the impact of noise is critical to sustaining our health
and wellness [2]. Through surface absorption, vegetated roofs have
the potential to reduce noise build-up at the roof level and noise
pollution in urban areas [1,3e5]. The sound-absorptive vegetated
roof materials placed over typically light-weight acoustically
reflective roof membranes will significantly modify the sound path,
as energy is primarily dissipated by the vegetation and substrate.
Additionally, near grazing incidence, even in the absence of high
absorption, phase change of reflected sound further modifies the
* Corresponding author. Tel.: þ1 604 456 8045.
E-mail address: [email protected] (M. Connelly).
0360-1323/© 2015 Elsevier Ltd. All rights reserved.
propagating sound over vegetated roofs e the ground effect [6]. The
boundary conditions of the material layers will play a role in the
acoustical behaviour of sound depending on substrate depth.
Ultimately, the acoustical characteristics of a system are governed by the properties of the multiple layers of fluids (density),
solids (mass and stiffness) and poroelastic (porosity) materials and
the fluid-structure interaction [7,8]. Vegetated roofs as engineered
systems vary widely in terms of design and implementation. As
natural systems, vegetated roofs vary distinctively in terms of the
in-situ ecological succession of the plant species [9]. Vegetated
roofs can be composed of various material layers such as root
barrier, water reservoir/drainage layer, filter fabric, substrates, and
plants. The vegetative growing substrate is complex to characterize,
varying in terms of the substrate constituents and mix, the depth of
the substrate, the vegetation's aerial biomass (foliage above substrate without litter) and root structure, and the in-situ
In order to understand the sound absorption characteristics of
an established vegetated roof system, it is of interest to first
examine the absorption characteristics of the substrate before the
effect of the vegetative substrate layer is investigated. There were
two parts to this investigation: the first part determined the
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
normal-incidence absorption coefficients of six substrates and the
second part assessed the sound absorption of vegetative roof plots.
The investigation on substrates provides insights into the magnitudes and frequencies of sound energy which vegetated roofs can
potentially absorb, and guidance for the specification of the substrate mix to optimize the sound absorption potential of substrates
for noise control. The investigation on vegetated roofs identified
the conditions which significantly affect sound absorption.
2. Background
2.1. Substrates
Vegetated roof substrates were examined to determine which
characteristics contribute most significantly to the absorption of
sound energy. Vegetated roof substrates have a granular structure;
the aggregates are separated from each other in a loosely-packed
arrangement. The substrate must be sufficiently porous to provide internal aeration, and be structurally capable of resisting
excessive compaction beyond the mechanical compaction of the
substrate on the roof during installation. There is a percentage of
sand in most substrates; the void between the sand particles promotes free drainage of water and entry of air into the soil. The
percentage of organic matter provides a balance of drainage and
water retention. The proportion of minerals to organics varies
depending on plant requirements, depth, and the projected maintenance regime. Clay and silts are not as predominant in substrates
as in natural soils [10,11].
In acoustical terms, vegetated roof substrates can be defined as
an unconsolidated granular, sound-absorbing material. The constituent mix of sand, organic matter, and minerals determines the
relative volume of pore space (porosity). Tortuosity and pore
structure cause changes in the flow direction; the expansion and
contraction of the air flow through the irregular pores results in a
loss of momentum of the directional wave propagation of a medium [12]. Several theories of sound propagation within idealized
porous materials have been applied to soils [13e15]. However,
there is little published literature which hypothesizes or reports the
relationship between the properties of vegetated roof substrates
and their acoustical characteristics, or the impact of microclimatic
and site conditions on the sound absorption of substrates. The
following is a summary of the findings of published measurements
on and modelling of sand, natural soils, bare grounds, and manufactured porous materials most relevant to vegetated roof
Laboratory measurements, executed with an impedance tube
with one end of the tube submerged in sand, have shown that the
absorption coefficient increases incrementally with an increase in
the percentage of moisture content up to 6% and then decreases
with a greater percentage of moisture content [16].
Absorption coefficients of grounds, measured by the in-situ use
of an impedance tube, have shown that absorption decreases
immediately after rainfall (relative to pre-rainfall conditions) and
increases as the ground drains over the next few days and then
decreases as the ground dries [17]. Acoustical attenuation and
propagation speed have been determined to be a function of soil
moisture [18,19] and level of compaction [20] although a variety of
sands and soil textures have been evaluated, none of the samples
are in conformance with vegetated roof guidelines for substrates as
required for plant viability on rooftops [21,22].
The characteristic impedances and propagation constants of
manufactured fibrous absorbent materials have been modelled as a
function of frequency and specific flow resistivity, which is predominately a function of the bulk density and the anisotropic fibre
size [12]. The models assume the material is a homogenous,
unbounded, infinitely extended medium. The application of this
model to vegetated roof substrates may not be assumed valid, as
the physical characteristics of substrates do not meet the
The characteristic impedances and propagation constants of
single constituent granular materials have been successfully predicted from porosity, tortuosity, specific density of grain base, and a
characteristic particle dimension [23e26]. Standard soils including
low-density substratum soil and high-density clay-based soils have
been successfully lab-tested and predicted by fluid model [27].
Again, this is not necessarily applicable to the wide variation of
vegetated roof substrates, as they have multiple natural constituent
components with widely varying particle-size distribution and
pore characteristics. A 2013 study showed soil depth of 50 mm
providing significant absorption, and greater soil depths show only
slight changes [28].
2.2. Vegetation
Vegetation is known to impact the physical characteristics of soil
[29]; the vegetation affects porosity and water content through the
mechanisms of soil temperature, organic and inorganic composition, and animal life in soil [17]. The mechanism of accessing water
from the soil is a function of the root structure and soil interface.
Additionally, the aerial biomass affects the microclimate of the soil
properties. Vegetation foliage and root structure are known to
affect the impedance, and thus the absorption, of grounds [30].
Examined plant communities have shown differences in sound
absorption due to differences in height, foliage, and mass [31]. It has
been determined that leaf dimension and mass are important
properties of plants affecting sound reflection and hence the sound
energy incident on soils [30,32,33]. Plant leaf area density and angle
of leaf orientation have recently been shown to affect the absorption coefficient of plants [27]. Vertical greenery systems show
increasing absorption and scattering with increasing area of
greenery coverage in reverberation chamber tests [34,28]. It is not
known if the variability of vegetation foliage and root structure,
within the limit of the plant species suitable for vegetated roofs,
affects the sound absorption of the vegetated roof. Previous work
on various grounds suggested that only the first 90-mm depth of
vegetated grounds affects absorption [17].
3. Objectives
In the first part of the investigation, laboratory experiments
were conducted to determine the relationship of the physical
characteristics of vegetated roof substrates due to the constituent
mix and two in-situ conditions (moisture content and compaction)
to sound absorption. In the second part of the investigation, the
goal was to understand the impact of substrate depth and plant
community and coverage on the absorption potential of vegetated
4. Methods
4.1. Laboratory testing of substrates
Impedance tube measurements of six substrates and their
constituent parts, with test parameters of moisture content and
compaction, were executed to determine which physical properties
and characteristics of vegetated roof substrates contribute most
significantly to the absorption of sound energy. The substrate
characteristics of interest included particle density, bulk density,
total porosity, percentage organic matter, and particle-size distribution. The properties of interest that are a function of the
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
microclimate and site conditions include volumetric water content
and compaction.
The normal-incidence absorption coefficients of six vegetated
roof substrates and their three primary constituentsdsand,
compost, and pumicedwere measured in an impedance tube at
three levels of volumetric water contentdoven-dry (0%), wilting
capacity, and field capacitydand at two states of compaction. The
sample depths were 98 mm, as this was the maximum depth
achievable in the impedance-tube sample holder, allowing for the
analysis of a reasonable range of depths for extensive vegetated
roofs. The permanent wilting capacity and the field capacity define
the minimum and maximum available water contents required for
plant viability and hence defined the limits of volumetric water
content. The substrates were evaluated as either non-compacted or
compacted at a level which approximates in-situ conditions. The
relationship between the absorption coefficient and the properties
of the substrate and constituents was examined using multiple
linear-regression modelling.
4.1.1. Sound absorption measurement
The most direct method for evaluating the absorption potential
of vegetated roof substrates is to measure the complex reflection
coefficient in a normally-incident sound field using an impedance
tube [35,36]. Using broadband noise, plane waves are generated
from one end of the impedance tube normal to the surface of a
sample located in a holder at the opposite end of the tube (two
microphones, B&K ¼” Type 4135 with B&K ½” Type 2669 preamplifiers and SINUS Soundbook). The impedance tube system was
calibrated by the two microphone transfer function method, where
amplitude and phase are calibration quantities measured with
microphones placed in standard and switched configurations, as
described in ASTM E1050-98. In total 105 measurement were taken,
the specific number of measurements for each sample is described
in Section 4.1.2.
On the assumption that the substrate is locally-reacting, the
diffuse-field absorption coefficient and a noise reduction coefficient
(NRC e the average of the sound absorption coefficients in the 250,
500, 1000, and 2000 Hz octave bands) [37] can be calculated from
the complex reflection coefficient (R) derived from the measured
transfer function (H) and the geometry of the impedance tube.
Two impedance-tube diameters were available: one of 98-mm
diameter for frequencies from 177 to 2050 Hz, and one of 29-mm
diameter for frequencies higher than 2050 Hz. This investigation
was limited to the lower-frequency range, as the 29-mm tube
diameter was too small to accommodate a homogenous sample of
the test substrates, due to the granular size of the aggregated
components. The upper-frequency limit is inversely proportional to
the diameter of the tube and equates to 2050 Hz for the 98-mm
tube. The lower-frequency limit is related to the spacing of the
microphones and the accuracy of the sound analyzer [38]. The
lower frequency cut-off of the apparatus was estimated to be the
lower limit of the 250-Hz one-third octave band (i.e., 177 Hz). In this
investigation, the impedance tube was used in a vertical orientation
so that loose granules and water could be retained in the sample
4.1.2. Substrates and constituents
The specifications and proportions of constituent parts of substrates can vary significantly; the mix is engineered based on the
type of vegetated roof (extensive, intensive or rooftop urban agriculture), plant species and micro-climatic conditions. Five of the six
substrate samples were selected randomly from products on-site at
the British Columbia Institute of Technology (BCIT) Green Roof
Research Facility. The samples represented a range of vegetated
roof substrates composed of natural materials and currently used
for extensive and semi-extensive vegetated roofs in the Pacific
Northwest of North America. The sixth substrate had previously
been evaluated for conformance to both British Columbia Landscape and Nursery Association [22] and Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau recommendations [21],
and had been selected for the subsequent tests on 25 experimental
rooftop plots (see Methods, Section B) and subsequent
transmission-loss research [1]. The sand, compost, and pumice,
common to most of the substrates, were also evaluated as samples.
Vegetated roof substrates for extensive and semi-intensive applications have specified ranges of percentage of organic matter
and particle-size distribution of all constituents. The range of
volumetric moisture content between wilting point and field capacity represents the range of moisture for plant viability and
survival. Compaction is a site condition which may occur at the
time of substrate installation and will occur over time due to
loading and rain.
The physical characteristics were measured in a commercial lab
according to standard soil test methods. Particle density [39,40],
available water-storage capacity [39,41], particle-size distribution
[21,42], and total porosity are independent of microclimate conditions. The percentage of organic matter [42] changes over time, but
not within the time duration of this study, and was measured
before acoustic testing. The bulk density and percentage moisture
content were measured within 24e36 h of acoustical testing using
an oven-dry method [42].
Table 1 shows the physical characteristics of the test samples.
The percentage of organic matter (% OM) in the six substrates
ranged from 2% OM to 25% OM. The mean value of 14% OM is
greater than the 8% OM recommended by the FLL. The maximum
water-storage capacity, measured at 10 J/kg, ranged from 24% to
42% by volume and, with one exception, met the FLL recommendation of 35%e45%. Aeration porosity at 10 J/kg ranged from 23% to
34% by volume and, with one exception, met the FLL recommendations of aeration porosity >25%. The particle-size-distribution
curves in Fig. 1 correlate the particle-size distribution of the substrate mix with a recommended percentage of three grades each of
silt, sand, and aggregate. The curves of three of the substrates fit
within the recommended maximum and minimum curves; two
had minor deficiencies with either slightly more-or-less coarse
and/or fine aggregate. Two substrates contained higher than recommended percentages of aggregate particles >2 mm. The constituent as a single component cannot comply with distribution
Large samples of the six substrates and the constituents (sand,
pumice, and compost) were oven-dried to 0% volumetric water
content (%VWC). From a large container, the first random sample
was ladled into the impedance-tube sample holder and levelled to
the rim. In order to maintain a representative and random granular
distribution of the mix, the substrate was ladled into the sample
holder rather than poured. After the first impedance tube measurement was completed, the sample was weighed, and then two
repeat measurements were done with equal masses of substrate
measured from the large container and placed into the sample
The same process was followed to measure three samples of
substrate or constituent at wilting and/or field capacity. The substrate was mixed using a mass approximation of water volume to a
predetermined %VWC representing the substrate's specific wilting
or field capacity. The air, water, and substrate were maintained at
the same temperature (±1 C). After the sample was tested in the
non-compacted condition, the sample was tested in a compacted
state. During installation of vegetated roofs the compaction rate for
the soil component is taken into consideration; the substrate is
installed and compacted to the finished height and then offset with
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Table 1
Test sample characteristics.
% OMa
Particle density
Total porosity
% vol.
Field capacityb
% vol.
Wilting capacityb
% vol.
Particles >6.3 mm
% by mass
Percentage of organic matter.
Field capacity measured at 33 J/kg, Wilting Capacity at 1500 J/kg.
Particle size distribution (See Fig. 1).
the addition of substrate based on a specified substrate-specific
compaction rate. The 18% compaction rate by volume is representative of on-site compaction rates [43]. At wilting and field capacities, a compaction rate of 18% was feasible within the impedancetube sample holder. The sample was compacted in layers while
being placed in the sample holder, to avoid excessive surface
If the percentage difference between wilting and field capacities
was less than 5%, then only the oven-dried substrate was tested, at
wilting capacity. The bulk density, volumetric water content, air
porosity, and total compaction are parameters which are functions
of the volumetric water content, and were determined within
24e36 h of the acoustical measurement using an oven-drying
4.2. Field testing of small-dimensioned vegetated roof plots
This section discusses field measurement of the absorption coefficients of small-dimensioned (1.68 m 1.68 m) vegetated roof
test plots. It first investigates the implementation and application
of the spherical-decoupling method for optimal in-situ absorption
measurement. It then discusses the use of the optimal method to
perform in-situ measurements of the absorption coefficients of 25
vegetated and non-vegetated test plots representing a range of
substrate depths and plant communities commonly used for
extensive vegetated roofs.
The first investigation on one test plot was completed inside the
controlled environment of an anechoic chamber. Within the
chamber, a dimensionally-representative test plot was built on a
plywood floor, creating a hemi-anechoic condition to approximate
a rooftop context.
Next, the in-situ measurement of 24 of the 25 rooftop test plots
was completed on a 1400-m2 roof top. These test plots were constructed with material layers representative of the common vegetated roof systems of the Pacific Northwest of North America. The
test plots ranged in substrate depth in 25-mm increments from 50
to 200 mm; seven test plots contained the substrate only. The
varying atmospheric conditions were accommodated at the field
site. Based on real-time data from the on-site weather station,
acoustical measurements were taken only when wind was less than
5 m/sec; and measurements were not taken during measurable
precipitation. The impact of variable pre-test rain fall was eliminated by the use of an irrigation system to provide a common
condition of volumetric water content.
4.2.1. Experimental method background
The spherical-decoupling method is a two-microphone technique which measures the reflection coefficient at any incident
angle, from which the diffuse-field absorption coefficient can be
derived [44]. A more general version of this method has been used
to measure sound reflection properties and deduce the sound absorption of locally-reactive surfaces and grounds, such as playing
Fig. 1. Particle-size distributions of the sample substrates.
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
fields and forest floors [45,46]. However, the substrate surface
particles and the plants of the vegetated roof do not provide a
homogeneous and specularly-reflecting surface, assumed by the
theory supporting the spherical decoupling method. Less than ideal
surface conditions have previously been accommodated in measurement methods through the determination of an appropriate
geometric configuration of the sound source, the microphones, and
the surface plane, and through repeated measurements at multiple
surface locations.
The experimental setup and the geometry of the measurement
setup for the spherical-decoupling method are illustrated in Fig. 2
[47]. For locally-reacting surfaces, as we assume grounds and
vegetated roofs to be, the impedance does not change with the
angle of incidence. The diffuse-field absorption coefficient can be
calculated from the impedance [48].
Considerable work has been done to define the optimal geometry to use in the measurement of the frequency response, to
calculate the impedance of grounds. The frequency limits of validity
of the spherical-decoupling method are functions of the distance
between the two microphones and the angle of incidence of the
sound source [46].
4.2.2. Experimental method optimization and validation
The experimental set-up addressed the effect on sound absorption of substrate depth and of the differences in plant community. In order to investigate these factors, small-dimensioned
(1.67 m 1.67 m) vegetated test plots (see Fig. 3) bounded by
plastic wood composite borders were constructed on the rooftop.
In preparation for using the spherical-decoupling method to measure absorption coefficients of small-dimensioned vegetated roof
test plots in the rooftop environment, measurements were made in
controlled laboratory conditions to optimize and validate the
experimental methods. Given that the application of the test
Fig. 2. Geometric configuration of spherical decoupling method experimental setup
Fig. 3. Rooftop test plots of substrates with P3 community (cultivated grasses) in
foreground and P1 community (sedums) in mid-ground.
method was on small-dimension plots, not large expanses of
ground or vegetated roofs, the objectives of the investigation were
as follows: a) to determine the optimal geometric configuration of
the sound source and microphones relative to the surface; b) to
investigate potential compromises on a rooftop to the ideal hemifree-field test environment and how they affect measurement accuracy; and c) to determine the required number of repetitions of
To achieve these objectives, measurements in various test configurations were made in a fully anechoic chamber with usable
dimensions 4.7 m 4.1 m 2.6 m high. A 1.68-m 1.68-m test plot
was constructed using the same materials and details as the rooftop
plots; the perimeter frame, which separated plots from each other
on the roof, was wooden and the subsurface was the plywood floor
(Fig. 4). The substrate was from the same batch mix as in the
rooftop test plots e a blend of white pumice, sand, 15% organic
matter, and a non-significant percentage of proprietary amendments. The substrate was evaluated in previous lab tests; the
percent organic matter, percent porosity, particle size distribution,
and available water content met the regional guidelines [22] and
standards, and the international guidelines [21] for extensive
vegetated roof substrates.
Fig. 4. Planted vegetated roof test plot with surrounding baffles in the anechoic
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Three primary setups were used: the plywood subfloor covered
with 50-mm cotton baffles; a non-vegetated roof test plot; and a
planted roof test plot. To determine the optimal geometric configurations and investigate disruptions to the theoretical hemi-freefield condition, variations were made to the first two primary
In the final of the three primary setups, the vegetated roof test
plot was planted with Sedum album (“Coral Carpet”); the coverage
of the aerial biomass was 70% (Fig. 5). The sedums were planted for
the duration of the measurements only; as such, the root growth
was not established. The water content was approximated at
wilting capacity. . Optimal geometric configuration. To investigate the
optimal configuration of the equipment setup, measurements were
repeated with dimensional variations to the geometric configuration (the distance between the two microphones; the distance from
the test surface to the closest microphone) and the location of the
sound source with respect to angle and distance to the microphone
probe. Measurements were made on baffles on plywood, substrate
only, a vegetated roof test plot, and a sedum-planted vegetated roof
test plot.
The optimal geometric configuration derived from the investigation involved the following: loudspeaker 175 cm above and
normal to the test surface; microphone probe directly below the
loudspeaker; height of the base microphone above the surface (hb)
equal to 55 mm; and 25-mm microphone spacing. . Non-ideal field test environment. To investigate the significance of sound-field perturbations by rooftop architectural
features, measurements were made in the approximate hemi-freefield condition of the anechoic chamber with and without representative perturbations. On the rooftop, known perturbations
included the perimeter edge frame of the vegetated roof test plots,
a non-continuous surface with variable acoustical characteristics
outside the frame (absorptive and/or non-absorptive surface), posts
which supported the loudspeaker and architectural apertures.
In the anechoic chamber, measurements were repeated with
and without the perimeter edge, at q ¼ 0 , 45 , and 75 . Only at
q ¼ 0 was it found that the perimeter frame did not interfere
significantly with the planar wave reflecting from the sound source
at q ¼ 0 .
Fig. 5. Rooftop test plots of planted community.
Depending on the location of the plot in the test area, the surrounding surfaces could involve adjacent test plots and/or the
highly-reflective roof membrane in varying geometry. To investigate the conditions of the surrounding surface, in the anechoic
chamber the cotton baffles were laid on the plywood subfloor to
simulate the various surface conditions beyond the perimeter
frame. The results indicated that there was no interference by the
highly reflective surfaces surrounding the test plot when the sound
energy from the source was normally incident; i.e., q ¼ 0 . As the
angle of incidence increased, the agreement between the measurements in the three surface conditions decreased. . Measurement repeatability. Standards recommend averaging four repeat measurements to accommodate the variations in
natural grounds [45]. At the location of wave reflection for each
measurement the vegetated roof may have various surface properties, variations in foliage (biomass and root structure) and substrate condition (exposed constituents). Measurements were taken
at ten different locations on the substrate test plot using a fixed
geometric configuration to determine the minimum number of
repetitions required. It was concluded that four repeat measurements were sufficient to represent the surface conditions.
For detailed results of the experimental method optimization
and validation work, refer to Connelly, 2011.
4.2.3. Vegetated and non-vegetated test plots e methodology and
plant selection
The site for the in-situ measurements was on a 1400-m2 rooftop.
The roof is 10 m above grade and has full sun exposure. The annual
precipitation is 1885 mm and the average daily temperature is
10.5 C. Allowable load capacity determined the locations and
limits of the test areas on the rooftop. The roof slope is 2% and the
test areas are free-draining without any residual rainwater pond
below the test plots. The rooftop has a weather station which
provides temperature, relative-humidity, wind direction and speed,
rainfall, and solar-radiation data.
Two 25-m2 test areas comprised nine 1.68-m 1.68-m test
plots. One 25-m2 test area had seven 1.68-m 1.68-m plots with
substrate only. Fig. 6 illustrates the material layers. The range of
substrate depth in the Pacific Northwest region of North America is
most typically from 75 to 150 mm. The wider range of 25e200 mm
depth was selected to confirm the minimum limit of substrate
depth for plant viability and to evaluate the extended range of
depths acoustically. It was not known if the plants would in fact
thrive in the shallowest depth allocated, as viability is a function of
the site-specific context and seasonal climatic conditions. The goal
was to measure the vegetated plots at two levels of coverage, in the
fall and the spring. The compacted substrate was installed adhering
to the same mass measurement techniques as the laboratory
samples (see Section 4.1.2).
The plant species were selected based on the diversity of the
structural characteristics of aerial biomass and root systems. Three
communities were selected: P1, a Sedum album ‘Coral Carpet’,
which has a dense and evenly-distributed foliage with an
extremely shallow root structure of only a few millimetres depth;
P2, a coastal community of plants with structural foliage and deep
or massive root systems, including Eriophyllum lanatum, Allium
cernuum, Armeria maritima, and Festuca rubra (in the P2 community
the foliage above and the root system below the substrate surface
were not homogenous across the test plots); and P3, a mix of
cultivated grasses sowed and maintained as required for a playing
field. The plants were grown in 100-mm pots in the nursery with
the same substrate as in the test plots rather than in a typical peatbased potting soil, to ensure consistency through the substrate
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Fig. 6. Rooftop test plot section detail and material layers.
parameter. The test plots were planted in May 2009 with an
average of 150-mm plant spacing on-centre.
The method to determine plant coverage was visual assessment
using a 25-square matrix array over each 1.65-m 1.65-m plot. The
percent coverage of total plants, the percent coverage of original
planted species, and the spontaneous plant coverage were determined. For each matrix, values were assigned on a scale of 1e10,
where 1 indicated 0%e10% coverage, 2 indicated 11%e20%
coverage, and so on up to 10 which indicated 100% coverage. The
values assigned were averaged over the 25-square arrays to
determine the percentage coverage. The total plant coverage is a
value which describes the surface of the plots in terms of vegetation
relative to exposed substrate, inclusive of the original species
planted and the spontaneous growth. The spontaneous coverage is
a value which describes the surface of the plots in terms of spontaneous plant establishment relative to exposed substrate [49]. The
three plant communities selected have significant structural differences in terms of the aerial biomass and root development.
The plant root structure of the test plots was so well-developed
that removal of substrate to evaluate the volumetric water content
would have been destructive. Nominally, all plots were evaluated at
field capacity. The plots were irrigated to full saturation to
approximate field capacity 36e48 h after water saturation.
5. Results
5.1. Laboratory testing of substrates
The average normal-incidence absorption coefficients of the
substrates are presented in Table 2 in octave bands from 250 to
2000 Hz for each test condition. There was a high level of consistency in the test repetitions of each substrate in each of the five
evaluated states: oven-dried, wilting capacity, wilting capacity
compacted, field capacity, and field capacity compacted. The significant variation in the substrate characteristics resulted in a
wide range of absorption coefficient values. In the oven-dried
state the range of the average absorption coefficient was 0.71
across the frequency bands. The average absorption coefficients
for wilting capacity, wilting capacity compacted, field capacity,
and field capacity compacted were 0.58, 0.48, 0.35, and 0.22,
respectively. These results indicate that, in the octave bands from
250 to 2000 Hz, the percentage of organic matter and volumetric
water content have the most significant effect on the sound absorption of the substrates. The absorption coefficients of the
evaluated substrates correlated positively with the percentage of
organic matter and negatively with moisture content and
increased compaction, except that the state of compaction had a
negligible impact on sound absorption in the lowest (250 Hz)
octave band.
Table 2
Average normal incidence absorption coefficients of test substrates.
Octave band (Hz)
Oven dried
Wilting capacity
Wilting capacity e compacted
Field capacity
Field capacity e compacted
Fig. 7 shows the absorption coefficients of the compost (organic
matter), sand, and pumice (which together formed the majority of
the substrate mix), and the average of all test substrates, at 0%
moisture content and in a non-compacted state. With the conditions of moisture and compaction removed, the sand (with less
than measureable organic content and low pore volume) had the
lowest absorption coefficient (0.26e0.40). Absorption increased
with frequency in the range of 200e2050 Hz. The pumice, with less
than measureable organic content but with a high pore volume,
had a higher absorption-coefficient range. The compost, with a high
Fig. 7. Measured octave-band absorption coefficients of the average substrate and
constituents, at 0% moisture content, at wilting capacity.
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
percentage of organic matter, had the highest absorption range. The
average of the absorption coefficients of the substrate mixes was
between those of sand and compost. An increased absorption at
around 500 Hz is observed in the higher porosity materials (this
excludes sand); similar results have been observed in other
impedance tube studies on granular material [50]. The apparent
high absorption at about 500 Hz is explained as the first resonance
maximum attributed to high level of interference between the
direct wave and the reflected wave from the base of the standing
wave impedance tube. The pumice with high percentage of coarse
particles (see Table 1) exhibits the most noticeable second
Fig. 8 shows the relationship between the percentage of organic
matter and the absorption coefficient of the compacted substrate at
wilting capacity. One substrate had only 2% organic matter, and the
absorption coefficient was closer to that of sand than any other
substrate. Four of the substrates had 14% or 15% organic matter,
which increased the absorption coefficient by 0.15 (1000 Hz); the
absorption coefficient of the substrate with the highest organic
content (25%) was an additional 0.15 higher (1000 Hz). As organic
matter decreased, the nearly non-organic samples exhibited
increased erratic spectra behaviour attributed to the increase and
volume deviation of inter-granular pore spaces and the deviation
from the ideal boundary condition of the sample substrate in the
sample holder.
Fig. 9 shows the impact of increased volumetric water content
and of compaction on the absorption of the six substrates. In general, absorption decreased with both factors. The moisture content
required for plant viability of the substrates evaluated has a range
of 12.5%e33% volumetric water content. This range of water content translates to a dynamic range in absorption coefficient of 0.36
(1000 Hz) averaged over all of the evaluated substrates.
5.1.1. Frequency-dependant multi-variable regression models
The relationships between the absorption and the physical
properties of the test substrates and constituents (in Table 1) were
examined using multi-variable linear-regression modelling. The
measured absorption coefficients of the substratesdthe dependent
variable to be predicteddwere normally distributed and did not
require transformation prior to analysis. The soil properties and
characteristicsdthe independent variables that are candidate
predictorsdincluded percentage organic matter, bulk density,
particle density, porosity, available water content, air-filled
porosity, volumetric water content, compaction, particle-size distribution, and percentage of particles >2 mm. On bivariate analysis,
independent variables that were statistically significant (p < 0.05)
were considered for inclusion in multiple linear regression models.
Fig. 8. Variation of measured octave-band normal-incidence absorption coefficients
with percentage organic matter and compaction.
Fig. 9. Variation of measured octave-band absorption coefficients with percentage
moisture content.
Co-linearity of the independent variables was evaluated using the
Pearson correlation for continuous variable pairs, the Spearman
correlation for categorical pairs, and a Chi-squared test for the association of the categorical pair. Highly correlated independent
variables (rp > 0.4, rs > 0.4, p < 0.05 significance) were evaluated
and the variables with the strongest association with the dependent variable on bi-variant analysis were retained in the model.
Frequency-dependent models were developed separately to predict the absorption coefficients in octave bands from 250 to
2000 Hz. The final frequency-dependent models were regressed
against the determinants by a backward, stepwise linear-regression
process, retaining only statistically significant variables (p < 0.05).
There were 87 observations for each model. Table 3 reports the
coefficients and R2 values for the final regression models. The R2
values indicate that the models explain a very satisfactory 66%e78%
of the variability in the absorption coefficient. The models for
prediction of the absorption coefficient are:
a ¼ b0 þ b1 ð%OMÞ þ b2 ðPÞ þ b3 ðVWC ¼ WiltingÞ
þ b4 ðVWC ¼ FieldÞ þ b5 ðCOMPÞ
The multi-variable analysis supports the observations made
during the measurements. The normal-incidence absorption coefficient is positively associated with the percentage of organic
matter and negatively associated with compaction and water
content. An increase in water content from wilting to field capacity
decreases the absorption coefficient by 0.16; progressing from a
state of non-compacted to compact decreases absorption by 0.10 at
frequencies above 250 Hz. The effects of these changes in state are
of the same magnitude for substrates with 2%e25% organic matter.
Although the unit increase in sound absorption coefficient is
similar for the percentage of organic matter and the percentage of
porosity, in reality the constitute part of the substrates which is
most significantly altered/engineered is the percentage of organic
matter which can vary from as low as 2% to as high as 25%, whereas
porosity varies within a smaller range (57%e72% in the samples
evaluated). Therefore, the greater range in the variability of the
percentage of organic matter can affect absorption to a greater
extent than porosity. The trends of the four frequency-dependent
models are similar, with the exception of the percentage of
organic matter and porosity. This is difficult to explain physically.
The multi-variable models indicate that there is no effect of
compaction in the lowest frequency band of 250 Hz. Fig. 1 illustrates the conformity level of the substrates to FLL guidelines for
particle-size distribution (PSD). PSD and Particles >6.3 mm, as independent variables, were candidate predictors of absorption.
However, it was determined by modelling that there was no direct
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Table 3
Coefficients for multi-variable octave band regression models.
Multiple R- squared
Frequency band Models
Intercept b0
% OM b1
Porosity b2
VWC¼ wilting b3
VWC¼Field b4
Compaction b5
250 Hz
500 Hz
1000 Hz
2000 Hz
relationship between these variables and octave band absorption
The models are applicable to a wide range of green roof substrate designs which meet the physical characteristics specified by
the FLL design guidelines to support green roof plant viability,
however, they would not be applicable to substrate designs which
deviate significantly from the design guidelines, substrates with
manufactured amendments to alter water holding capacity nor
novel hydroponic alternates. A limit of the experimental set-up was
the selection of pre-engineered industry products as the test
samples; a fabrication of substrate samples, with an incremental
gradient of the percentages of constituents, would have benefited
the analysis.
5.2. Field testing of vegetated roof plots
The spherical decoupling technique that was used was viable to
evaluate the absorption capacity of vegetated roofs in the architectural range from 200 to 5000 Hz. Although frequencies less than
200 Hz are also important in sound propagation over roofs, they
were not measured in this experimental set-up. In the controlled
environment of an anechoic chamber, the optimal geometric
configuration of the spherical decoupling setup for the measurement of vegetated roofs was achieved when the distances of the
base microphone above the surface plane and between the two
microphones were minimized; however this defined the lowfrequency cut-off.
Absorption of the sedum-planted test plot in the anechoic
chamber, as quantified by the NRC, was 0.63; the NRC of the in-situ
sedum-planted plot of the same depth was 0.37. The results are not
directly comparable, as the surface properties of the test plots
varied considerably. In addition to variation in plant species
coverage and establishment, the significant differences between
the two plots included the level of compaction, the level of
n ¼ 87
p- value
moisture content, and the surface level particle size distribution. In
the anechoic chamber, the plot was planted with an even distribution of 4-inch pots. On the rooftop the coverage was not evenly
distributed; the plant viability and mortality varied as the plot
established over time. These findings suggest that short-term
experimental setups and controlled lab conditions, which do not
accurately represent in-situ rooftop properties of a vegetated roof
substrate and complex plant ecology, may exaggerate the absorption potential of the vegetated plots. The method developed for this
investigation addresses the lack of the generalizability of experimental setups with simulated plantings in controlled
Measurements in third-octave bands from 200 to 5000 Hz were
completed on seven substrate plots and seventeen of the eighteen
planted rooftop test plots over the course of two seasons. Measurements were completed within a time span of 36e48 h after
complete saturation, to represent field capacity. Air temperatures at
the time of testing ranged between 15 C and 23 C.
Fig. 10 shows the diffuse-field absorption coefficients of the
substrates. They increased with frequency from 200 to 1250 Hz and
then remained constant to 4000 Hz. The absorption coefficients
tended to increase with depth. The mean NRC was 0.62 for depths
from 50 to 200 mm (see Table 4). There was no significant
Table 4
10 NRC of substrates e 50e200 mm depths.
Depth (mm)
Fig. 10. Measured third-octave diffuse-field absorption coefficients of reference roof and substrates of 50- to 200-mm depth in rooftop test plots.
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Table 5
NRC of test plots planted with sedums (P1).
Plant species
Depth (mm)
Coverage (%)
association between the gravimetric water content and the NRC of
the substrates. The absorption of the substrate plots, without
planting, was significantly greater than that of the exposed roof
membrane; e.g., the NRC was 0.62 averaged over depths of
50e200 mm compared to 0.06 on the exposed roof.
The P1 plots, planted with Sedum album, were colonized with
mosses over the time frame of plant establishment. The total plant
coverage in the plots was distributed between the planted species
of sedums and the spontaneous establishment of mosses. The plot
with a 25-mm substrate depth was not considered sufficiently
viable in terms of plant coverage to evaluate. The mosses accounted
for 37%e80% of total coverage (Table 5). The trend of increased
absorption relative to depth, which was observed in the nonplanted plots, was not observed in the plots with the established
P1 community (see Fig. 11). However, there was a negative trend
between the percentage of spontaneous coverage and the NRC with
the range of plots from 50 to 200 mm. In this community, the range
of the NRC was 0.39 across substrate depths of 50e200 mm.
Overall, the absorption of the plots planted with community P1
was lower than that of the substrate plots. The variation with frequency was similar for both; however, the mean difference in the
absorption coefficient ranged from 0.14 at 200 Hz to 0.35 at 800 Hz
in the 1/3 octave band analysis. The mean NRC decreased by 0.24
(see Fig. 12).
The plots were compared between two seasons to illustrate
seasonal climatic impacts (Fig. 13). The plots were saturated
36e48 h before evaluation; however, it is reasonable to assume that
there was greater evapotranspiration in July when the maximum
daily temperature before acoustical evaluation was 27 C, versus
16 C in November. The variability in plant coverage, and the climatic difference, affected the absorption at lower frequencies; a
greater impact on absorption was observed above 500 Hz. In the P2
(coastal meadow) community, the range of the NRC across substrate depths of 125e200 mm was 0.60 as measured in the fall and
0.13 as measured in the summer. In the P3 (grass) community, the
range of the NRC was 0.12 across depths of 125e200 mm in both
seasons. The lowest absorption (NRC 0.20) was in the P2
Fig. 11. Measured third-octave-band diffuse-field absorption coefficients of rooftop test plots planted with sedums (P1).
Fig. 12. Measured third-octave-band diffuse-field coefficients of substrate and sedums (P1) in rooftop test plots.
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Fig. 13. Measured third-octave-band diffuse-field absorption coefficients of P3 community (cultivated grasses) in rooftop test plots (substrate depths 125e200 mm) in fall and
summer seasons.
Fig. 14. Measured third-octave-band diffuse-field absorption coefficients of 3 plant communities (substrate depths 125e200 mm) after 2 seasons of establishment.
community planted in a 200-mm substrate depth. This measurement took place in the late fall in conditions of cool temperatures
and high levels of moisture in the plants and substrate. The same
community measured in summer climatic conditions in a 100-mm
substrate depth had the highest absorption (NRC 0.62).
After two years of establishment, a common trend of absorption
across the plant communities over the range of depth was
observed. However, the ranges of absorption performance between
communities overlapped and, therefore, did not allow for differentiation between the three communities (see Fig. 14).
6. Conclusion
In this study, the impedance tube and the spherical decoupling
methods have been used to discover the normal- and randomincidence (or diffuse-field) absorption coefficients of a variety of
green roof substrates, substrate constituents, and green roof plots.
Results showed normal-incidence absorption coefficients of six
types of substrates varying from around 0.03 at 250 Hz to 0.89 at
2000 Hz, and that absorption increased with the percentage of
organic matter and decreased with moisture content and
compaction. Random-incidence or diffuse-field absorption
coefficients of green roof plots increased with substrate depth and
decreased with the addition of vegetation and an increase in plant
establishment. The NRC of seventeen different vegetated roof plots
ranged from 0.20 to 0.63.
The investigation confirmed several findings. One, the sound
absorption of vegetated roofs is a function of substrate depth,
plant community establishment, and moisture content of substrate. Two, vegetated roofs as a building-envelope system has
highly sound-absorptive characteristics and some values and
conditions for optimal absorption have been quantified. From the
observations and results of this work, a multi-variable regression
model was developed for predicting the normal-incidence absorption coefficients of substrates, which can be used to assess the
sound absorption of substrates in the terms of design specifications, such as proportions of the constituent parts. This is particularly useful as the latest predictive models require input of valid
absorption coefficients [51,52]. The use of vegetated roofs as
absorptive surfaces to reduce noise build-up and reverberation on
rooftop spaces, and the acoustical performance of multilayer
building partitions containing green roof systems, can now be
investigated and modelled using sound absorption data provided
through this work.
M. Connelly, M. Hodgson / Building and Environment 92 (2015) 335e346
Funding support for this research has been provided by the
British Columbia Institute of Technology, the University of British
Columbia, and Soprema Canada. Agriculture and Agri-Food Canada
and the BC Landscape & Nursery Association provided additional
funding for the field testing of vegetated roof plots.
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