## 2 Results and Discussion  
### Yeast Number (Cells in Suspension)  
Water was chosen as the sound transfer medium because sound waves experience substantial transmission loss and interference when passing between media of different densities, such as from air to liquid fermenting beer. In this regard, this study is thought to be the first report of an experiment where sound has been delivered via water to a liquid ferment during yeast fermentation.  
Yeast cells in suspension (viable cells) were measured at various time points during 144 h of fermentation (Figure 1). At time 0 h, the number of yeast cells was 9.1 \(\pm\) 0.2 \(\times\) 10\({}^{6}\) cells/mL in all treatments, which was close to the target pitching rate of 10 million cells per mL. After 10 h of fermentation (post-pitching), the number of viable yeast cells in suspension increased to 16.8 \(\pm\) 2.2 \(\times\) 10\({}^{6}\) cells/mL (low frequency, low intensity; LF_LI), 16.7 \(\pm\) 2.4 \(\times\) 10\({}^{6}\) cells/mL (low frequency, high intensity; LF_HI), 18.3 \(\pm\) 4.1 \(\times\) 10\({}^{6}\) cells/mL (high frequency, low intensity; HF_LI), 18.7 \(\pm\) 2.1 \(\times\) 10\({}^{6}\) cells/mL (high frequency, high intensity; HF_HI), and 21.1 \(\pm\) 2.4 \(\times\) 10\({}^{6}\) cells/mL (silence control; S) (Table S1 Supplementary Material). The treatment had a significant effect on viable yeast cells in suspension at 10 and 24 h (\(p\) < 0.05). Maximum yeast numbers occurred 24 h after fermentation commenced, with the silence group recording the highest number (44.5 \(\pm\) 1.5 \(\times\) 10\({}^{6}\) cells/mL), which was significantly higher than LF_LI, LF_HI, and HF_LI but not different to HF_HI. The yeast numbers in suspension subsequently declined and converged with all other treatments at 48 h and for the rest of the fermentation to 144 h (Table S1 Supplementary Material).  
The application of various sound intensities and frequencies has previously been shown to enhance the growth rates of _S. cerevisiae_[3, 4, 5, 6, 9], bacterial species [5, 8, 10, 11, 12, 13], algae species [14, 15], and plant species [16, 17, 18] compared to control experiments. However, in the current study, the growth rate was not calculated, and rather the number of yeast cells in suspension was reported. Therefore, our current findings are not directly comparable to previous studies. Sound treatments (Figure 1) did not enhance yeast numbers in suspension compared to the control (silence) group, which could potentially be ascribed to the following hypotheses: (1) stress, triggered as a result of sound exposure; (2) the specific frequency band delivered during fermentation did not have an effect on yeast in suspension; (3) the cell densities pitched might be too high to see an effect, thus inhibiting an increase in yeast growth as a function of the sound treatment; or (4) as yeast numbersin suspension were measured rather than optical density as in other studies, it is possible differences in total biomass were missed. Moreover, it has previously been reported that audible sound can exert inhibitory effects (osmotic stressors) on _E. coli_, thus decreasing growth and other biological activity [12].