## 2 Materials and Methods
### Measurements  
We decided to calculate the CEB samples' transmission coefficients. This was due to the direct relationship between the transmission coefficient and airflow, a mechanism that influences the loss of sound energy. The prepared CEBs samples, which were further characterised, were designed to perfectly fit into a segment of an 18.0 m long pipe (acoustic waveguide) with an internal diameter of 76.2 mm (3.00 inch), which was made up of three polyurethane water pipes connected together. The frequency range of the pipe (18.8 to 2621.6 Hz) was determined from the Formula (1) [39]:  
\[f_{L}=\frac{v}{l+1.6r};\,f_{H}=\frac{1.8412v}{2\pi r} \tag{1}\]  
where \(v\) is the sound velocity of the air used as the fluid in this work, \(l\) is the total length of the pipe, \(r\) is the radius of the waveguide, \(f_{L}\) is the lower threshold frequency of the pipe, and \(f_{H}\) is the higher threshold frequency. By extending the pipe, it was possible to reduce the lower frequency limit even more. However, this was dependent on how much empty space there was in the testing area. A speaker-driven ribbon tweeter was fixed to the waveguide through a metallic cylindrical tube, with one end flattened and holes made, which were later used to fix the loudspeaker with bolts and nuts, while the other end had an external diameter of 76.2 mm (same as the internal diameter of the waveguide), such that it fits exactly into the waveguide. Rubber was then wrapped around the joint so as to minimise acoustic leakage (Figure 4a-c). Before feeding the signal into the loudspeaker, it was amplified using an amplifier removed from a sub-woofer. An omnidirectional microphone (ABM-716-RC Pro-Signal, Farnell, Leeds, England) was used to record the incident and transmit acoustic waves in order to record the loudspeaker's signals. It was positioned downstream of the pipe. Figure 4c shows the microphone being fixed onto the waveguide using black tape. This microphone was inserted into the waveguide at a distance of 9 m from the loudspeaker. The microphone was operated by a 9 V heavy-duty battery. The loudspeaker then produced sound waves in the pipe, which was joined to a signal generator (a Thurlby Thandar TG 210 generator made in the UK). The intended output of the generator was a Heaviside signal (step function). Putty (brand and address) was used to stop the leakage of waves that occurred between the margins of the samples and the walls of the waveguide as a result of the samples' imperfect circularity. Finally, a digital oscilloscope (picoscope 4262, Cambridgeshire, UK) was attached to the microphone. This oscilloscope has a frequency limit of 5 MHz, with a 16 bits-bit rate. The signal from the microphone was fed to channel 1, while the signal from the signal generator was fed to channel 2 of the picoscope. Figure 5 shows the scheme of the experimental setup. Figure 3: (**a**) The cylindrical metallic mould for making the compressed earth block samples; (**b**) The mould placed inside the normal brick mould; (**c**) The wet compressed earth block that was still inside the cylindrical mould; (**d**) Some of the prepared compressed earth block samples. The signal with and without the sample were both recorded. The acoustic waveguide's overall length was set to be sufficient to prevent a time overlap between the transmitted acoustic waves moving downstream and those reflected from the open end. The microphone's placement was carefully considered to ensure that the reflected waves were placed at a distance from the transmitted ones. A personal computer (PC) was then used to visualise these response signals. The duration of the temporal window in which the recorded transmitted pressure wave signals were digitalised was determined in order to eliminate the reflected waves from the open end of the waveguide. The transmission coefficient was calculated from this incident (without the sample) and transmitted (with the sample) signals using the transfer function. According to [PERSON] et al.