*4.2. Effects of Surface Texture and Porosity on Sound Absorption*

The results of the impedance tube tests revealed significant variances between replicates of the same material (see Table 4). This is hypothesized to be the result of inconsistent mycelia growth, the size of the substrate material, and the random substrate filling technique. The following graphs in Figure 15 present the sound absorption coefficients of SCH replicates cultivated within three different formworks, alongside close-up images of the replicates cultivated within the same formwork. A visual inspection revealed that the replicates with more bumps and pores at the surface have higher sound absorption coefficients; however, further tests are needed to validate this hypothesis.

**Figure 15.** *Cont*.

**Figure 15.** Sound Absorption Coefficient Graphs and the Shredded Cardboard Samples Tested. A visual inspection to compare surface texture and porosity with the respective material performance.

### *4.3. Limitations and Strengths of the Study*

There are two main limitations to this study. The first limitation addresses Section 4.1. The data for the commercially available synthetic sound absorbers were collected from the existing literature [31–33]. While the sound absorption coefficients for these materials are validated numbers provided in their data sheets, to be able to ensure accurate results and have a meaningful comparison, commercially available sound absorbing materials need to be tested for each frequency range using the same testing model, with samples that have the same material thickness, density, and porosity.

The second limitation addresses Section 4.2. As can be seen in Table 4, the sound absorption coefficients for both sample groups in 500 Hz and 1000 Hz frequencies show significant variances. This limitation can be overcome by creating larger subgroups within each sample group through visual inspection of the replicates and testing these subgroups' sound absorption coefficients independently.

The strength of the study was initially performing preliminary tests with multiple waste paper-based samples. This enabled accurately deciding which substrates fit in the testing model and eliminating the ones that did not work.

### **5. Conclusions**

Of the tested samples from the preliminary acoustic tests, the shredded and fine cardboard-based samples show the best acoustic performance. In addition to this, the fine newsprint and shredded paper substrates are not considered to be applicable for paneling purposes due to their (lack of) structural integrity. Due to these findings, the shredded and fine cardboard samples were regrown with larger sample size and tested again. The results show that both shredded and fine cardboard-based mycelium composites do show potential as sound absorbing materials, with shredded cardboard samples slightly performing better in high-frequency sound absorption. However, the inherent nature of bio-fabricated materials causes a variance in performance, even between samples of the same material.

The next steps of this research are to investigate how material thickness, density, and porosity affect sound absorption of shredded and fine cardboard-based mycelium composites. This will be performed by cultivating additional replicates and creating larger subgroups within each sample group by controllably varying their material thickness, density, and porosity. Along with their sound absorption properties, their mechanical properties (compression, bending, torsion, and tension and impact damping) and morphological characteristics (i.e., pore size, porosity, density), as well as the growth mechanisms of mycelium, will be studied. The main objective is to understand how the growth of mycelium at microscopic levels, the morphological characteristics at both mesoscopic and macroscopic levels, and the acoustic absorption performance of the composites interact with one another. Another follow-up study could be to test various commercially available synthetic acoustic absorbers using the same testing model, with samples that have the same material thickness, density, and porosity as the mycelium-based sample groups. This would enable a more thorough comparison of mycelium-based composites' acoustic absorption performance with synthetic absorbers.

Once a holistic understanding and more comprehensive data about the composites' acoustic, mechanical, and morphological characteristics are gathered, the next steps involve the applications of the shredded and fine cardboard-based composites as acoustic paneling. The material itself, though sound absorbing, has physical limitations such as structural integrity and warping when cultivated on larger scales. More experiments must be conducted to ensure the durability of the material. Concurrently with durability assessments, analyses regarding form-to-performance will be conducted. These experiments will be used to determine how the form of the acoustic panels affects the sound absorption performance. Therefore, full-scale prototypes will be built and tested alongside computer simulation models in reverberant chambers. These results will inform parametric iterations of panel systems.

Incorporating mycelium-based composites into architectural systems is significant because of their ability to reduce waste generated and energy consumed during material manufacturing compared to conventional building materials. Mycelium-based composites recycle waste materials for growth, require little energy to manufacture, and completely decompose at the end of their product life. This research is relevant in order to establish protocols for material use and implementation within acoustic systems.

**Author Contributions:** Conceptualization, N.W. and B.G.; methodology, N.W. and B.G.; software, N.W.; formal analysis, N.W. and B.G.; investigation, N.W. and B.G.; writing—original draft preparation, N.W.; writing—review and editing, N.W. and B.G.; visualization, N.W.; supervision, B.G.; project administration, B.G.; funding acquisition, N.W. and B.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Pennsylvania State University's Erickson Discovery Grant and Institute of Energy and Environment Flower Grant.

**Data Availability Statement:** Data are available upon request.

**Acknowledgments:** We thank John Pecchia at the Mushroom Research Center at Penn State University for their expertise and for providing access to their facilities and equipment. We also thank Yun Jing, Hyeonu Heo, and Jun Ji for assisting with the impedance tube tests, as well as Ali Ghazvinian and Alale Mohseni for their help in cultivating the mycelium-based composite samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
