PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Acoustic absorption of a living green wall - parametric transducerand XYZ gantry measurement method
Anna ROMANOVA1; Kirill HOROSHENKOV2
1

University of Greenwich, United Kingdom
University of Sheffield, United Kingdom2

ABSTRACTThis work provides evidence of the importance of considering urban green structures and in particular theLiving Green Walls (LGW) as sound-scaping instruments. An insight to Living Green Walls soundabsorption properties is provided with particular interest of sound absorption coefficient and plantmorphology. The work was carried out in-situ in internal and external environment and makes use ofparametric transducer and XYZ gantry. Novel method of parametric transducer was previously verified by
the use of impedance tube tests benchmarking against porous media material. The proposed XYZ gantry
method eliminates the reflection from the ground and averages the discrepancy of the absorbed/reflectedsound due to plant morphology over a given area of interest. The new method is an alternative to the ISO3542003 and CEN/TS 1793-5:2016 standard methods to measure acoustic absorption of materials.
Keywords: urban noise, Living Green Wall (LGW), living plants, green wall, acoustic absorption, acoustic
measurement, parametric sound.

1.

INTRODUCTION

Soundscape is now recognised as a very important characteristics of the modern, developed civilisation.
Soundscape as a term that appeared in 70’s and denotes an acoustic environment made of cacophony ofsounds as perceived by humans which is different in each given context, urban and non-urban (1). Noise on
the other hand is regarded as a loud and unpleasant/unwanted sound that causes disturbance or stress. A new
term of noise pollution appears alongside soundscape and is now being used to describe unpleasantsoundscapes of urban settings that is dangerous and harmful to both humans and animals (2-3).
In large urban setting as megapolis cities noise pollution is a major problem which is a hindrance to human
day-to-day life and that remaining wildlife. With rapid development of economies and work opportunities
supported by free mobility of workforce, traffic noise is by far the most harmful noise pollution source,
including ground, air and water transportation. America (USA) is the most studied country for noise pollution
and it is reported that in general traffic noise accounts for 70% of overall pollution with instantaneous noise
values being in excess of 80 dB (4), 1-5 m away from the source, which worryingly are increasing year byyear. There are many studies that suggest the harmful effect of noise pollution on human’s health (hearing
loss, stress, restlessness, nervousness, obesity, heart disease, increased blood pressure, abnormalities in brain
reflex, etc.) caused by sharp and short as well as moderate and prolonged noises (5-8). And although cities
like London and Tokyo have opted to reduce traffic noise through electric vehicles, the humanity has still along way to go.
One of the methods to fix the urban soundscapes is to introduce porous materials that can absorb theunwanted noise. Introduction of vegetation is known to help with trapping noise and creating tranquil spaces.
Vegetation or plants are able to attenuate sound waves by reflecting and absorbing the acoustic energy of theviscid thermal boundary layer near the surface of the plant. Alternatively, plant internal branches or stems
can accommodate vibrating sound that helps to reduce sound energy (9).
In our study we have used the Living Green Walls (LGW) with benchmarks to plywood wall and
1 A.Romanova@greenwich.ac.uk
2 K.Horoshenkov@sheffield.ac.uk

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melamine wall to establish how the structures with and without vegetation absorb the sound of different
frequencies. To facilitate the measurement with great precision we have constructed a motorised XYZ gantry
system that also allows to replicate the experiment over time. The system uses the method of parametrictransducer (10) that has feasible benefits over ISO354-2003 and CEN/TS 1793-5:2016 standard methods, as
it eliminates the reflection from the ground and edges of the wall.

2.

EXPERIMENTAL METHODOLOGY

The acoustic method described in (10) was used, where instrumentation consisted of ‘An intensity probe,
Brüel & Kjær, type 4197 with Brüel & Kjær NEXUS conditioning amplifier type 2690 and parametrictransducer (directional loudspeaker HSS-3000 Emitter) with HSS-3000 amplifier.’ The intensity probe wasfirmly attached to a motorised telescopic X-oriented leg and placed at a height of 0.9 m and 1.7 m away fromthe measured surface center (corresponding to 0,0,0 mm reference point or C0). The orientation of theintensity probe with respect to the wall was perpendicular. The directional loudspeaker was also attached toa tripod and it was placed 4 m away from the wall in the same central co-ordinates as per reference point.
The line connecting the center point of the directional loudspeaker and the middle of the intensity probe wasset perpendicular to the wall. See Figure 1 for the set-up.
2.1
XYZ gantry set-upThe novel motorised XYZ gantry to move the intensity probe in 3-dimensions was built using aluminium
profiles with 0.2 mm precision Mclennan motors of type IP57-M1-10 and HepcoMotion moving line
attached. The horizontal X-Y axis, allowing movements left-to-right and forward-to-backward have thecapacity of 1800 mm spread (the internal space is square). The Z axis allows for 80 mm up-to-down
movement. Three of the motors, serving each axis, are connected to the portable SIEMENS TP-Link control
unit that is PCB, DAQ and Webserver enabled. This is synchronised with the SIEMENS online application
to control the position of the intensity probe in the XYZ axis. This can be repeated with the same precision
every time.

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Figure 1 - Laboratory side view schematics (top) and XYZ gantry picture (bottom)
In these experiment sinusoidal chirp sound of 20 Hz - 20 kHz was emitted. As per (10) ‘The primary
frequency of the parametric transducer used in this work was 44 kHz. The peak sound pressure of this primarywave was 440 Pa at 0.3 m from the transducer’s center. This was sufficient to develop strong non-lineareffects causing the emission of the differential wave. The sound pressure in the primary wave reduced toapproximately 35 Pa at 4 m away from the transducer.’
2.2LGW and benchmarks
The LGW can be constructed from different materials and in our case the 2 m wide and 1.8 m tall wall
donated by ANS Global Ltd (see Figure 2 and 3) is assembled from HDPE modules, growth medium (soil &fertilisers) and pipeline with pump based irrigation system that is hidden behind the modules together withwater membrane and wood hinges.

Figure 2 - Living Green Wall schematics (11), left-to-right: single empty module, module with soil, modulewith plants, attached module to the LGW construction wall568

Figure 3 - Plywood and melamine foam (left), green wall vegetation (right)
In this study 3 sets of experiments have been performed: (i) 100mm melamine foam, (ii) 18 mm plywood
wall, (iii) LGW with selection of four random plants and 10% moisture content. In each cases the test area
was 2 m by 1.8 m, where the intensity probe was moved in the following XYZ positions:Figure 4 - Intensity probe XYZ positions, for center (C0) and 7 cm or 15 cm shift from high (H) to low (L)and left to right
Although the plant species selection was mostly random, it was ensured that the chosen morphology ofeach of the group of plants would differ. The morphology parameters included: leaf and stem length andwidth to give leaf area, leaf thickness, number of leafs / flowers per 10 cm2 to calculate density and plantheight as distance from the module until the plant edge to give volume.

3.

DATA ANALYSIS

3.1
Acoustic Analysis
The signal used with the described experimental setup was a 10 sec sinusoidal sweep in the frequency
range of 20 Hz - 20 kHz, but only100 - 5000 Hz used on reflection. Below 500 Hz the sensitivity of theparametric loudspeaker was too low to overcome the background noise. Above 5000 Hz the directivity
pattern of the parametric loudspeaker was found too complex and the sensitivity of the intensity probe too
low to apply the proposed method. The signals recorded on the microphone pair in the intensity probe weresampled using a National Instrument USB-4431 card at the sampling rate of 22.05 kHz (10). The recorded
signals were processed with Matlab® to obtain the acoustic instantaneous intensity using the samedeconvolution method as detailed in Chapter 5 of (12). The application to deconvolution enabled to achievea very high signal to noise ratio which is important in the presence of high levels of ambient noise while
taking measurements in-situ.
The instantaneous acoustic intensity was calculated as

I (t )p(t )u(t ) ,(1)where p(t) is the time-dependent mean sound pressure recorded on the two microphones in the intensity
probe and u(t) is the acoustic particle velocity estimated from the sound pressure data, p1,2(t), recorded onmicrophones 1 and 2569

tu (t )1³ ( p2 (W )  p1 (W ))dW'U0 f,(2)where ∆ = 12 mm and is the microphone separation in the intensity probe and ρ0 is the equilibrium densityof air. Intensity spectra of the sound waves were used to calculate the absorption coefficient which wasdetermined as the following ratio

I (Z )C (Z )I r (Z ),D (Z ) 1  a(3)where Ia(ω) is the intensity spectrum reflected from an absorbing layer (e.g. melamine), Ir(ω) is theintensity spectrum reflected from the wall which was assumed rigid and C(ω) is the correction which takes
into account the peculiarities in the propagation and attenuation of the sound wave radiated by the parametrictransducer. This coefficient was calculated based on the assumption that the wall is a perfectly reflecting
surface, i.e.

I a (Z )C (Z ) 1
I r (Z ),(4)in the case of the wall. In this calculation the intensity spectrum reflected from the wall was effectively
used as a reference. It was also assumed that the ambient conditions for the generation and propagation of
the ultrasonic career resulting in the audible parametric sound were identical in all of the reportedexperiments.
3.2
Initial Experimental Results
The instantaneous intensity (Equation 1) recorded in the presence of the wall and in the presence of the100 mm layer of melamine is presented in Figure 5. The presented wave samples are numerous repetitionsof the same experiments before averaging. The reflective wave for the wall is 10 fold higher than for themelamine sample as expected.
Figure 6 demonstrates the reflected intensity spectrum (Equation 3) over 0 - 2000 Hz for the wall sample,
hedera helix (ivy) and 100 mm melamine sample. The performance of ivy plant with 10% of moisture content
is placed between the wall and melamine results which yield promising results as sound absorbent medium.
Figure 7 shows absorbtion coefficient (Equation 4) for melamine sample and four different plant typebased LGW’s. To remind, each plant type exhibited unique values in the morphological analysis. Whenexamining these parameters all together and separately it was established that the leaf area density has a direct
correlation with the obtained acoustic coefficient. The leaf area density for each of the plant types is also
identified in Figure 7 and shows that the absorbtion coefficient improves as the leaf are density increases. In
the case of bergennia crassifolia the acoustic absorbtion coefficient in the range of 1100 - 1600 Hz is very
similar to the performance of the melamine foam.

4.

DISCUSSION AND IMPROVEMENTS

The current XYZ method proved to be reliable at performing high quality repetitive measurements andcould be set up at any location to perform the in-house or in-situ experiments under the conditions that would
be comparable. It should be noted that the equipment should be placed as indicated in geometricalarrangement in Figure 1 (top). To further advance the set-up either laser or GPS system can be used toestablish on-site repetitive co-ordinates in correlation to the wall of interest.
The presented experiments demonstrate the data from 15 intensity probe locations and in the future moreco-ordinates will be examined. It is proposed to look at 50 - 150 mm steps in X and Y directions expanding
to 1 m and 600 mm in Z direction to provide an understanding of the sound field that can be modelled.
To further prove the validity of the method and provide a range of data to inform the modelling exercise
it is proposed to carry out systematic experiments with a large variety of plants and LGW systems.

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Figure 5 - The time histories for the instantaneous intensity recorded in the presence of the wall (top)and in the presence of the 100 mm layer of melamine (bottom)

Figure 6 - Reflected intensity spectrum for the wall sample, ivy and melamine sample

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Figure 7 - Absorbtion coefficient for melamine sample and four LGW based on different plant types
with indication of leaf area density

5.

CONCLUSIONSThe newly proposed method is less prone to the unwanted ground and edge effects compared to standard
methods (ISO354-2003 and CEN/TS 1793-5:2016) because the adopted loudspeaker and intensity probes arehighly directional and enable us to focus the radiated sound on the green wall area primarily. This method isrelatively easy to implement and replicate. Automated XYZ instrumentation allows to easily perform a large
number of measurements to capture the complexity of the acoustic field in space radiated by the parametrictransducer and scattered by the green wall or other walls of interest.
The results confirm that the presence of plants with a relatively high leave area density can significantly
enhance the absorption properties of a Living Green Wall system, particularly in the medium and highfrequency range, i.e. above 1000 Hz.
The results also show that a compartmentalised Living Green Wall system can support acoustic
resonances at frequencies which are controlled by the cell dimension and wall thickness. Some of theseresonances are reduced or disappear when the wall is treated with a plant with a relatively high leaf area
density. These resonances deserve a refined numerical modelling to understand better the in-situ acoustic
performance of a complete Living Green Wall system.
There is also evidence that in some cases plants can scatter sound coherently resulting in an apparent
decrease in the absorption coefficient. Refined numerical modelling of such phenomenon would be highly
beneficial for future development of the theory.

ACKNOWLEDGEMENTS
Authors are willing to acknowledge: Mr Marc Van-de-Peer, Mr Andy Reed and Mr Tony Dodson forextended support provided in laboratory space allocation and set-up, Mr Andy Hale for his complementary
consultancy, ANS Global Ltd for providing LGW specimen, UKAN (EPSRC) for networking opportunities
and further in-kind assistance in the project, as well as Prof. Keith Attenborough for inviting us as a key
speaker at Natural means for noise control session in ICA2019.

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