## 5 Discussion
### Implications and Limitations  
This study explored the impact of hair dryer sound on users' perceived product quality using fNIRS measurements. Our findings indicate that high-sound hair dryers received higher perceived quality scores and induced greater changes in users' \(\Delta\)HbO in the DLPFC compared to soundless hair dryers. When participants evaluated the perceived quality of low-sound hair dryers, differential activation between the left and right hemispheres was observed, with increased left-brain activity relative to the right hemisphere. These findings emphasize the beneficial impact of multisensory product design on perceived quality. Additionally, the study provides an objective method for assessing product design quality through fNIRS measurements, with DLPFC activity, particularly in the left hemisphere, serving as a potential marker for evaluating product quality. The study contributes to the literature on multisensory product design and neuroscience applications in perceived quality and product development. By identifying that higher sound levels can correlate with increased perceptions of quality, the article challenges the conventional preference for quieter designs. This is particularly relevant in contexts where sound conveys functionality or power. While the study focuses on hair dryers, its findings could be extended to other industries, such as automotive design, where sound plays a crucial role in conveying quality (e.g., engine or interior component noises).  
However, the study has several limitations that warrant acknowledgment. Firstly, only three small hair dryers were used as stimulus materials; it is important to acknowledge that in real-world scenarios, different hair dryers typically possess distinct spectral signatures. These spectral differences can influence user perception and product evaluation. Future research could explore how varying spectral characteristics across devices contribute to perceived quality, offering a more ecologically valid assessment. Secondly, the study solely focused on the effect of sound loudness on perceived quality, neglecting other design features of hair dryer sound, such as pitch. Future research could explore the effects of specific sound frequencies or intensities on DLPFC activity to provide a more comprehensive understanding of the neural mechanisms involved in auditory processing and its relationship with emotional responses. Thirdly, the participants recruited for this study were all college students, which limits the generalizability of the findings. The relatively small sample size in this study may limit the statistical power of the analysis. Future research should consider recruiting a large number of participants from diverse age groups and backgrounds to enhance the applicability of the results and statistical power of the analysis. Fourthly, this study focused solely on the PFC without a whole-brain assessment. Future research should consider examining activity in other brain regions during product quality evaluations. Additionally, perceived quality is heavily influenced by numerous factors, including the environmental context in which the product is used, and the study's controlled laboratory conditions may not fully capture real-world scenarios. Future research should focus on conducting evaluations in more realistic settings to validate the findings.