## 3 Experimental Setups  
Two physical experiments were carried out using, firstly, a long exposed MDPE pipe, and secondly a shorter buried MDPE pipe. This allowed the evaluation of the acoustic communication over a longer distance as well as the assessment of the impact of soil bedding. A 40 m long exposed MDPE pipe, to accommodate only single-path acoustic propagation along the pipe (i.e., without acoustic signal reflection at the pipe discontinuities), was set up in an approximately straight configuration above the ground surface as shown in Figures 2 and 3. Table 1 provides the geometric and material properties of the MDPE pipe which were obtained from [24] using a similar pipe. For acoustic excitation, an Eccentric Rotating Mass (ERM) vibration motor [28] was installed along the pipe with its rotating shaft parallel to the pipe axis (see [29] for more details on the acoustic transmitter selection). This arrangement enabled acoustic excitation of the pipe in the radial and circumferential directions, thus allowing direct comparison between the laboratory experiments and the numerical model [29]. A temperature sensor was also installed to measure the ambient temperature to determine the elastic modulus of the pipe (see Section 4.1). As shown in Figures 2 and 3, the pipe was supported by a combination of two wooden blocks and support jacks to elevate the pipe above the ground (thus preventing any contact with the ground). The support mechanisms were also placed more than 6 m apart, allowing the section of the pipe along which the experiment was conducted to be elevated above the ground, without the need for extra wooden supports within this section,  
Figure 1: Buried MDPE pipe showing the acoustic excitation directions and surrounding soil. to avoid introducing an extra path for acoustic signal radiation away from the pipe, thus compromising the experimental results. Preliminary tests had shown that this was critical as each wooden block resulted in acoustic signal leakage into the wood and the ground [29]. Figure 3: (**a**) Pipe cross-section indicating the acoustic transmitter location and (**b**) schematic of the exposed MDPE pipe (not to scale) in the laboratory. Figure 2: Experimental setup of the exposed MDPE pipe. Since acoustic wave speed along an MDPE pipe material is 1455 m/s [23], the ERM motor was installed mid-way along the pipe (and at 90 degrees according to the circumferential configuration of Figure 3a) to generate an acoustic signal along the pipe wall. The pipe was subsequently excited with an acoustic pulse of 0.01 s (since this was short enough to allow only single path acoustic propagation along the pipe). Since the ERM vibration motor attains maximum speed (and resonant frequency between 100 and 200 Hz) in approximately 0.08 s [28], the detected acoustic signals along the pipe were expected to be at less than 100 Hz (as will be shown by the corresponding results in Section 4.1) since acoustic excitation was less than 0.08 s.  
For acoustic excitation, the ERM vibration motor (which was driven at 12 V to maximise the acoustic power input into the pipe) was programmed to repeatedly transmit a single pulse of 0.01 s along the pipe. To capture the axial motion of the longitudinal acoustic wave, small (3 mm length, 3 mm width and 1.5 mm thickness), low-power (0.9 mW) and low-cost (GBP 8) triaxial accelerometers (ADXL 337 from Analog Devices) were installed at 0, 90, 180 and 270 degrees around the pipe (according to the configuration in Figure 3a) by adhering them to a plastic casing before placing the sensors in direct contact with the pipe. Four sets of these accelerometers were installed at 0, 2, 4 and 6 m from the vibration motor. The accelerometers were subsequently connected to a digital acquisition device (DAQ) (NI USB 6211 from National Instruments), which was in turn connected to a Personal Computer (PC) to store the acoustic signals acquired during the experiments.