## 4 Conclusions  
The research carried out reveals a novelty in Portugal, whether in the social sciences or the more exact ones, such as environmental acoustics. The originality of the research derived from considering the health impacts on the population due to the noise emitted by the power poles and power lines in residential areas, from the sound level measurements, besides distributing surveys and adapted audiometric test performance. Regarding the first question formulated in the introduction to this paper, \"Do power poles and power lines cause discomfort due to low-frequency noise?\" the distance between the receiver and the source may interfere with the degree of discomfort experienced by individuals. Especially due to the specific characteristics of high propagation of low-frequency noise  
Figure 8: Hearing threshold for pure sounds—“exposed” group. Figure 9: Hearing threshold for pure sounds—“unexposed” group. and its low absorption by materials and the environment. For the results presented here, the sound was recorded to 5 m of distance from the source (400 kV power pole) and was audible to the \"exposed\" and \"unexposed\" volunteers during the adapted audiometric tests. The noise level measurements exceeded the criterion curve of the DEFRA methodology [30]. The difference in the exceeded average of 4.3 dB shows a considerable difference between the two villages. If the source evaluated was only from the power poles and power lines, it could be induced that in the \"exposed\" group, the source more than doubled compared to the \"unexposed\" group. Regarding the second question, \"Do power poles and power lines affect the population's quality of life\"?, the results of the audiometric tests showed a significant difference between the two groups, mainly the hearing threshold below 39 Hz, which shows that the \"exposed\" volunteers are not very sensitive to the low frequencies. This implies that the \"exposed\" group needs a greater sound intensity to perceive the noise, especially in the lower frequencies. As most interviewees have lived most of their lives in Serzedelo, this low sensitivity to low frequencies could be a consequence of the long period of exposure to the noise emitted by power poles and power lines. In turn, the \"unexposed\" group tends to perceive the noise at a slightly lower sound intensity due to the fact that this village is free from these infrastructures. Additionally, the \"exposed\" group has a greater number of respondents with health problems (e.g., cardiovascular diseases, insomnia and depression), which may be caused from the characteristics of the place where they live and, probably mainly, from exposure to low-frequency noise emitted by power poles and power lines. Case-control type studies seem to be the most appropriate for the continuation of this type of research. It is essential to monitor the daily life of the interviewees and their lifestyles (for example, diet and sleep quality), aspects related to work dynamics such as current occupation and previous occupation, genetic predispositions, that is, the history of cases of illness in the immediate family circle, and the characteristics of the structure of the house. However, it is worth mentioning the complexity of conducting a case-control study, due to the difficulty of accessing information on patient morbidity and access to the patient database that is subject to confidentiality. In addition, a study of this nature would take several years. It is expected that in future studies this type of evaluation can be explored by varying the distance between the receiver and the source. This may provide important information on the definition of the limits for installing power poles and power lines in residential areas, based not only on the objective component (measurement of sound levels and setting the minimum limit of evaluation below the usual 50 Hz), but also considering the subjective component, that is, the perception of discomfort reported by the exposed population.