PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The influence of organic matter on acoustical properties of soil
Kamrun Nahar SURAVI1,2, Ho-Chul SHIN2; Keith ATTENBOROUGH1; Shahram TAHERZADEH1;
William Richard WHALLEY2
1

Open University, United Kingdom

2

Rothamsted Research, United Kingdom

ABSTRACT
Organic matter influences physical and mechanical properties of soils. Samples collected from a long-term
agriculture experiment involving different treatments, have been subjected to differential thermal analysis(DTA) to deduce the organic matter content and its composition. Water release characteristics and soil
strength have been measured with the suction plate method and indirect tension method, respectively.
Impedance tube measurements made on sand samples with various water content show that soil surface
absorption decreases with increasing water content. Changes in absorption coefficient spectra deduced from
impedance tube measurements on soil samples extracted from the long-term agriculture experiment involving
different organic matter content but similar water content, are consistent with predictions of the effects ofchanges in porosity and permeability in rigid-porous air-filled media.
Keywords: Soils, Porosity, Permeability

1. INTRODUCTION
Soil is a complex porous elastic medium. Structural properties of soil (porosity, pore size
distribution, tortuosity, air permeability etc.) can be regulated by different factors (both biotic andabiotic). It has been observed that organic matter had a strong interaction with physical properties ofsoil such as porosity and resistance, mechanical stresses (1). Poor structural condition of soil
(compaction, degradation) can restrict plant growth and affect crop yield. So, it is very important inagricultural research to have reliable information and knowledge regarding physical properties andstructural stability of soil (2). In this study, we are observing the physical behavior of long-term
agricultural soil including pore-related properties, water retention, mechanical properties etc. andtrying to identify the impact of organic matter and nitrogen applications on soil structure-related
properties. To achieve our research goal, we adopted different measurement techniques, commonlyused in soil physics and engineering. Acoustical excitation provides a non-invasive alternative to theclassical methods used in soil physics and can be utilized to de termine properties of soil (2, 3, 4, 5,
6). Sound wave propagation through soils is a mechanical phenomenon that causes a small
perturbation without altering the fabric of soils (7). Attenborough and colleagues (4) found that soundcan penetrate through soil due to surface porosity and associated air permeability and thus to beabsorbed and undergo phase change through friction and thermal exchanges between the pore fluid
and the surrounding solid. Sound propagation in soil or any porous media is influenced by its texture,
structure, roughness, degree of compaction and the moisture content (3, 5, 7, 8, 9). Acousticaltechniques have been used to study the formation of surface crusts (10).
Classical Biot theory for a porous and elastic medium (11, 12) predicts the existence of three wave
types in the porous medium: two types of dilatational or compressional waves (1 st and 2 nd kind) and
one rotational or shear wave. In a soil consisting of a dense solid frame with a low-density fluid, suchas air saturating the pores, the first kind of dilatational wave, often called the ‘fast’ wave which ischaracterized by particle motion in phase with fluid motion, has a velocity very similar to the 2 nd
dilatational wave (or "P" wave in geophysics) travelling in the drained frame. However, theattenuation of the first dilatational wave type is higher than that of the P wave in the drained frame.
The extra attenuation comes from the viscous forces in the pore fluid acting on the pore walls. This
first dilatational wave has negligible dispersion and its attenuation is proportional to the square of thefrequency, as is the case also for the rotational wave. If fast waves are excited predominantly, viscous
1 kamrun.suravi@open.ac.uk
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kamrun.suravi@rothamsted.ac.uk

coupling at pore walls means that some of their energy is carried in the pore fluid as the second typeof dilatational wave or ‘slow’ wave. In air-saturated soils, the second dilatational wave, often called
the ‘slow’ wave, has a much lower velocity than the first. The attenuation of the ‘slow’ wave stems
not only from viscous forces at the pore walls but from thermal exchange with the pore walls. In therigid-frame limit, theory predicts a single ‘slow’ wave which governs the acoustical properties ofrigid-framed porous solids and is the wave type responsible for the acoustical properties of soundabsorbing materials (13). Models for the acoustical properties of rigid-porous media introduce several
parameters such as the tortuosity of the pore space, porosity, viscous characteristic length, thermal
characteristic length, flow resistivity and dynamic shape factors (14, 15).

2. METHODS AND MATERIALS
2.1 Soil sampling and preparation
In this study, we collected top-soils (0-23 cm) from Broadbalk wheat experiment (51° 48´ 32.14´´N, 0° 22´ 15.21´´ W) – the world’s oldest long-term experiment (since 1843) in Rothamsted Research,
Harpenden, UK in late October 2018. Four plots on section 9 of Broadbalk experiment were sampled
to give a range of Soil Organic Matter (SOM) contents including: control plot (Nil), farm yard manure(FYM) plot, inorganic fertilizers plots with two levels of nitrogen (N) e.g., Mid Nitrogen plot N3(NPKMg)- 48 Kg N per ha and highest nitrogen plot N6 (NPKMg)- 288 Kg N per ha. Soil Organic
Carbon (SOC) content for Nil, FYM, N3 and N6 are 8.4, 31.75, 11.48 and 12.82 t /ha respectively.
Prior to acoustic characterization, the soils were partially saturate d by adding water by weight andsuction plate data follow the water retention curves shown in Figure 1. The water retention
characteristic is an important physical property controlled by pore geometry (16). Figure 1 shows thatsoil samples with higher organic matter content have higher water retention.

Figure 1 – Water retention characteristics fitted with van Genuchten model (17), of soil samples from theBroadbalk experiment (BK)The soil samples were air-dried and passed through 2 mm mesh sieve. Initial water content for airdry soils were very low and quite similar. The water content for Nil, FYM, N3 and N6 air-dry samples
are 1.8%, 2.6%, 1.9% and 1.9% respectively. All partially saturated soils were allowed to equilibrate
for at least 24 h in air-tight plastic bags at a constant temperature to prevent evaporation before
impedance tube measurements. By using pneumatic pressure chamber immediately before themeasurements, the dry and moist samples were packed to different densities into a sample holder witha hard base specially designed to fit with the impedance tube. The samples were about 40 mm thick.
2.2 Impedance tube measurements
In the impedance tube measurement, a loudspeaker is used to excite the sample with sound. And a
microphone is used to measure the total pressure above the soil surface (10). In this study, we used athree microphone Bruel and Kjaer 100 mm impedance tube (Type BK 4206), operated in the wide -559

spacing mode (100 mm microphone spacing) in the frequency range between 50 and 1600 Hz,
connected to a National Instruments data acquisition system. The impedance tube was set up verticallyby screwing on to a laboratory wall shown in Figure 2.

Figure 2 – Impedance tube set up
The 100 mm inner diameter sample holder was clamped to the impedance tube. In this study weused a single microphone method and a white noise signal and measured the transfer function of theacoustic pressure between two different positions in the measurement tube with the same microphone.
The reflection coefficient of the reference plane is estimated from the transfer function. Finally, theimpedance and normal incidence absorption coefficient are calculated from the reflection coefficient.
The use of the same microphone for measuring the pressure at the two different positions does not
require a relative calibration between two microphones. The data acquisition and processing werecarried out with MatLab® codes.
Initially, measurements were made on 50 mm thick fine sand samples with different moisture
content. The results (Figure 3) show that as the percentage of water in sand increases the absorption
gradually decreases in agreement with Horoshenkov and Mohamed (18).

Figure 3 – Absorption coefficient spectra of fine sand samples with different water content

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3. RESULTS WITH VARYING ORGANIC MATTER
3.1 Repeatability
First, we have considered the repeatability of the measurements. The example absorption spectrain Figure 4 show good repeatability between 200 Hz and 1400 Hz.

Figure 4 – Three repetitions of absorption coefficient deduced from measurements on FYM soil
3.2 Air-dry soils
Results of impedance tube measurements show that soils with minimal water content (air-dry soil)
packed to same density (1.3 g/cc) but different organic matter content give rise to different absorption
spectra. For the air-dry soil samples all the nitrogen treatments (even the control sample with nonitrogen) result in higher absorption than FYM (Figure 5).

Figure 5 – Absorption coefficient spectra of 4 cm thick Air-dry soils with same packing density of 1.3 g/cc

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On the other hand, when the dry soil samples were packed at 10 kPa pneumatic pressure, which
implies lower densities, soils with high organic matter (FYM) along with higher nitrogen (N6) show
higher absorption (Figure 6).

Figure 6 – Absorption coefficient spectra for air-dry soil samples packed at 10 kPa pressure
3.3 Effects of partial saturation
Also, results of measurements on partially saturated soils show that the absorption of soils treatedwith FYM has a strong dependence on water content. FYM gives higher absorption than N6 whenwater content is high and at same matric potential (Figure 7). But the same samples with reduced
moisture content, i.e. in drier conditions, give higher absorption (Figure 8).

Figure 7 – Absorption coefficient spectra of soil samples with moisture content at 300 kPa matric potential
and packed to 1.3 g/cc

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Figure 8 – Absorption coefficient spectra of FYM soil samples with different packing and moisture
conditions

4. DISCUSSION
Velea and coworkers (19) investigated the effect of different saturation levels on the speed ofcompressional and shear waves in porous material. Also, Horoshenkov and Mohamed (18) found thatchanges in the acoustic surface admittance result from a relatively small change (from 0 to 15%) of
the water content in sand as supported by the data in Figure 3.
It has been observed that organic matter particles fill in the coarse pore fraction to produce a gr eaternumber of fine pores and increased total porosity (20).
We have observed that soil samples with high organic matter show relatively high absorption ofsound. The increase in measured absorption with organic matter is consistent with the predicted effects
of increasing the porosity and permeability on the acoustical properties of a 4 cm thick rigid-porous
hard-backed layer. Figures 9 and 10 show some representative predictions of the effects of changes inporosity and permeability using the Johnson-Champoux-Allard model (14,21).

flow resistivity 682.5 kPa s m-2,
tortuosity 2,
viscous characteristic length 0.025 mm
thermal characteristic length 1 mm

0.6

0.4
0.2

Frequency Hz
Figure 9 – Predicted effect of changes in porosity (assuming constant permeability etc.) on the normal
incidence absorption coefficient of a 4 cm thick rigid-porous hard-backed layer.

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300 kPa s m-2
500 kPa s m-2
700 kPa s m-2

Frequency Hz
Figure 8 – Predicted effect of changes in flow resistivity (viscosity coefficient/permeability) assuming
constant porosity 0.4 etc., on the normal incidence absorption coefficient of a 4 cm thick hard-backed layer.
In making these predictions, no attempt has been made to find best fit parameters for the measured
absorption spectra other than to ensure that the predicted absorption coefficients have similar
magnitudes to those measured. Furthermore, in the predictions, the characteristic lengths have been
kept constant whereas, strictly, they should have been changed also. Nevertheless, these predictions
suggest that the changes in absorption coefficient spectra observed in Fig ures 5 to 8 are consistent
with the predicted effects of changes in porosity and permeability in a rigid-porous air-filled hardbacked layer.

5. CONCLUSION
It has been confirmed that different levels of moisture content cause variations in the acousticalcharacteristics of both sand and soil. Moreover, the acoustical results of increasing organic matter
content in soil are consistent with increasing porosity and permeability. While this has implications
for the potential variations of ground effect over cultivated soils, it is also of interest to the use of
non-invasive sound reflection for monitoring soil condition. Further work will involve deducing
physical parameters from the acoustical data for various soil states and comparing the acoustically deduced values with those obtained by other means.

ACKNOWLEDGEMENTS
William Richard Whalley and Ho-Chul Shin are supported by the BBSRC Designing Future Wheat
project (BB/P016855/1) and the BBSRC/NERC ASSIST project (NE/N018117/1). Kamrun Suravi isfunded jointly by Rothamsted Research and the Open University.

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