## 4 Discussion
It is reasonable to assume that the differences shown in 661W cell proliferation between the control and the emotional voice case, a read-aloud version of the gratitude letter, was caused by some particular properties of the participant's voice (\({}^{*}p=0.004\)). However, this possibility was controlled using a non-emotional stimulus, a read-aloud modern version of _Don Quixote_, which did not produce significant changes compared to the control group (\(p=0.179\)). To isolate the physical aspects of the gratitude emotion in the participant's voice, a synthesized emotional stimulus was created. This consisted of a synthesized version of the same gratitude letter read aloud by the participant and showed strong temporal synchrony and acceptable spectral similarity (MSC = 0.21, 0-5000 Hz band). The synthesized case was not statistically significantly different from the control (\(p=0.481\)). As introduced earlier, emotion in speech is mainly characterized by F0, amplitude, timbre, and temporal aspects. Having carefully considered all of these aspects, we hypothesize that the biophysical emotional parameters driving the biological changes may be the F0 and timbre components. The amplitude and temporal aspects showed a high degree of similarity, whereas F0 and timbre showed more variability. Although we cannot rule out that some of these differences may be due to the method of synthesis, variations in F0 are closely linked to emotional expression. For instance, emotions such as happiness often lead to increased F0, whereas tenderness states may lower it [57]. Moreover, the harmonic structure contributes to the timbre of the voice and plays a crucial role in conveying emotional nuance, as different emotional states modulate both the amplitude and distribution of these harmonic frequencies [58; 59]. As a future improvement to our protocol, an actor would read aloud the gratitude letter written by the participant, and the resulting AAW signal would be radiated over cells. This new control would provide us with more information about the frequency components. From a biophysical perspective, there is evidence in the literature that AAW stimuli are capable of inducing significant changes in cellular dynamics and genetic expression. [PERSON] et al., 2018 demonstrated that sound stimulation (94 dB, sine-wave sound with a frequency range of 55 Hz to 4 kHz) resulted in a significant reduction in Ptgs2 mRNA levels in the bone marrow-derived ST2 cell line. This was attributed to an increase in RNA decay [34]. In another study by [PERSON] et al. (2021) on in vitro cardiac muscle HL1 cells, significant differences between emotional and non-emotional AAW stimulation were found using alpha-tubulin staining after the different 20 min sound exposures. Compared to the control condition, non-acoustic stimuli, the light emission intensity displayed by phalloidin (F-actin), beta-actin, and alpha-actinin-1 increased when the cells were exposed to meditative music, mantras, and love signals, but decreased with hate signals [60]. Focusing on the mechanotransductional response, the potential biological mechanisms that could trigger the cellular responses to AAW can be classified into two main groups: biochemical signaling pathways and cytoskeletal dynamics. As expected, both groups are interconnected, making it quite difficult in many cases to differentiate between these mechanisms. First, the signal must be captured from the extracellular environment and transmitted into the cell through mechanosensitive sensors. At this point, the plasma membrane contains numerous mechanosensitive biomolecules/structures, notably integrins [61], mechanosensitive ion channels [62], primary cilia [63], cadherins [64], and ICAMs [62; 65], among others. When the biosignal reaches these mechanosensors, it induces conformationalchanges that initiate intracellular signaling.