3.2.1. Results of the Preliminary Acoustic Absorption Testing

The results of the impedance tube for each sample were recorded and graphed. For each material mixture, two replicates grown in a single formwork were tested, and their results were averaged. It is important to note that the surface of the samples varied depending on the growth of the mycelium. These tests were useful in determining which mixtures performed better than others and informed the second stage of acoustical testing with additional replicates.

Of the low to mid-frequency samples, the fine cardboard samples (FCL) showed the best absorption, as can be seen in Figure 7, though none of the samples showed very high absorption in the low-frequency range (50 Hz to 500 Hz). It was noted that the sound absorption results shown do not include the effect of an air gap behind the material. The introduction of an air cavity between the material and the rigid backing surface can increase the sound absorption performance at low frequencies [30]. Of the mid-range frequencies (500 Hz to 2 kHz), the fine cardboard samples FCL had the best acoustic absorption performance.

**Figure 7.** Sound Absorption Coefficient of low-frequency samples (100 mm).

Of the high-frequency samples, the shredded cardboard samples (SCH) had the highest sound absorption from the 2 kHz to 6.4 kHz frequency range, as can be seen in Figure 8. Samples SCH is followed by the fine cardboard samples (FCH), then the fine paper samples (FPH). The lowest absorption is from the shredded newsprint samples (SNH). EMH does not show to be a successful absorber.

respectively. The first graph (Figure 7) represents the materials' absorption from the 50 Hz to 1.6 kHz frequency range. The second (Figure 8) is from 500 Hz to 6.4 kHz.

The graphs in Figures 7 and 8 display the results for the 100 mm and 29 mm samples,

**Figure 8.** Sound Absorption Coefficients of high frequency samples (29 mm).

3.2.2. Results of the Acoustic Absorption Testing with Larger Sample Size

*Low-frequency sound absorption coefficients:* To obtain a better understanding of how the two best-performing samples in low-frequency sound absorption, SCL and FCL, compare with each other, we created six replicates for each sample group and tested their acoustic absorption. Three formworks were filled for each substrate mixture, resulting in two replicates per formwork, thus six replicates per substrate mixture (Table 2). Figure 9 presents the test results of the six SCL replicates grown in three separate formworks. Figure 10 presents the test results of the six FCL replicates grown in three separate formworks.

**Table 2.** Samples Tested for Low-Frequency Sound Absorption.


**Figure 10.** Sound Absorption Coefficients of FCL replicates (100 mm).

Of the low to mid-frequency samples, the shredded cardboard samples (SCL) follow two general trends. Half of the replicates' sound absorption coefficients peak between 450 Hz and 650 Hz and then begin to drop, while the other half has a much higher absorption rate, and the absorption peak shifts to between 750 Hz and 1050 Hz. Of the mid-range frequencies (500 Hz to 2 kHz), SCL performs well, with half of the replicates reaching over a 0.9 sound absorption coefficient at some frequency.

The fine cardboard samples (FCL) similarly follow two general trends in the low to mid-frequency ranges. Four of the replicates' sound absorption coefficients peak between 400 Hz and 700 Hz, then drop and remain constant, while the other two have a much higher absorption rate, and the absorption peak shifts to between 550 Hz and 850 Hz. Of the mid-range frequencies (500 Hz to 2 kHz), some of the FCL also perform well, with two of the replicates reaching a 0.9 sound absorption coefficient at some frequency.

As can be seen in Table 4, the test results show that in the selected low to mid frequencies (125 Hz, 250 Hz, 500 Hz, 1000 Hz), the sound absorption trends of both low-frequency sample groups (SCL and FCL) are statistically similar (*p* > 0.05).

*High-frequency sound absorption coefficients:* To obtain a better understanding of how the two best performing samples in high-frequency sound absorption, SCH and FCH, compare with each other, we created nine replicates for each sample group and tested their acoustic absorption. Three formworks were filled for each substrate mixture, resulting in three replicates per formwork, thus nine replicates per substrate mixture (Table 3). Figure 11 presents the test results of the nine SCH replicates. Figure 12 presents the test results of the nine FCH replicates.

**Table 3.** Samples Tested for High-Frequency Sound Absorption.


**Figure 11.** Sound Absorption Coefficients of SCH replicates (29 mm).

Of the high-frequency samples, the shredded cardboard samples (SCH) follow a single trend. The samples generally peak between 500 Hz and 1.5 kHz, dip and then gradually rise again. The samples have a medium to high absorption rate from 500 Hz to 1.5 kHz and 4 kHz to 5.5 kHz. The fine cardboard samples (FCH) also follow a single trend. However, as can be seen in Table 4, in the selected high frequencies (2000 Hz, 4000 Hz), high-frequency sample groups (SCH and FCH) have different sound absorption trends (*p* = 0.019 and *p* = 0.011, respectively). Shredded cardboard samples (SCH) had better sound absorption performance than fine cardboard samples (FCH).

**Table 4.** Sound absorption coefficients of shredded and fine cardboard samples (mean ± Standard Deviation).


### *3.3. Mycelium-Based Acoustic Panel Prototypes Cultivated with Fine Cardboard Substrates*

The results of the acoustic panel prototypes revealed that mycelium growth is still consistent even in larger formworks (Figure 13). However, the durability of the material proves to be a problem on a larger scale. After dying and handling, the edges of the panels began to show signs of deterioration. In order to ensure durability with the fine cardboard

material, additional support may be necessary. This could potentially be remedied with additional substrate materials, internal support, or external backing.

**Figure 13.** Acoustic panel prototype cultivated with Fine Cardboard substrate.

Concurrently with panel fabrication, a customizable panel system was generated using parametric modeling software (Rhinoceros 3D, Version 7.0. Robert McNeel & Associates, Seattle, WA, USA). Figure 14 shows a custom acoustic wall configuration generated using this system and illustrates how the panel configuration can be altered. The use of the parametric system aids in random wall configurations. Using a three-dimensional truchet tile in the parametric system allows for a number of wall configurations with only one panel, thus reducing the need for different formworks.

**Figure 14.** Parametric acoustic panel wall prototype design: (**a**) Acoustic panel installation illustrated (**b**) Example wall configuration.

### **4. Discussion**

The results of this study indicate that mycelium-based composites grown on waste shredded and fine cardboard show potential as sound-absorbing materials, specifically in the mid to high-frequency ranges. Shredded cardboard samples (SCH) slightly outperform fine cardboard samples (FCH) in high-frequency ranges.

### *4.1. Comparison to Commercial Sound Absorbing Materials*

The impedance tube test results show that mycelium-based composites cultivated on shredded cardboard and fine cardboard can both be considered sound-absorbing materials (α > 0.2) and have the potential to compete with the performance of synthetic sound ab-

sorbers (Table 5). The sound absorbing coefficients of three types of commercially available synthetic sound absorbing products made with fiberglass, polypropylene, and plaster were compared with the fine cardboard samples (FCL + FCH) and shredded cardboard samples (SCL + SCH). The comparison revealed that the fiberglass insulation board shows better sound absorption than both sample groups in all the frequencies except 125 Hz. When compared with the polypropylene product, both sample groups have better sound absorption at low frequencies (125 Hz, 250 Hz, 500 Hz). Compared with plasterboard, one of the most common interior wall finishes, the absorption coefficients of both samples are significantly higher in the mid to high-frequency ranges. These comparisons are helpful in discussing the potential of the two sample groups as sound absorbers. For more accurate comparisons, commercially available sound-absorbing materials need to be tested by the authors for each frequency range using the same testing model.

**Table 5.** Sound Absorption Coefficients comparing commercial sound absorbing materials with the fine cardboard samples (FCL + FCH) and shredded cardboard samples (SCL + SCH).

