###### Abstract  
Low-frequency audible noise generated by the magnetostriction effect inherent to the operation of power transformers has become a major drawback, especially in cases where the electrical substation is located in urban areas subject to strict environmental regulations that impose noise limits, differing for day and night periods. Such regulations apply a +5 dB penalty if a total component of noise is present, which is clearly the case of magnetostriction noise, typically concentrated at twice the industrial frequency (50 Hz or 60 Hz, depending on the country). The strategy used to eliminate the tonal characteristics, therefore contributing to establish compliance with the applicable regulation and to alleviate the discomfort it causes to the human ear, consisted in superimposing to the substation noise a masking sound synthesized from \"sounds of nature\" with suitable intensities, to flatten the noise spectrum while enhancing the soundscape. The masking system (heavy-duty speakers powered by a microprocessor platform) was validated at an already judicialized urban scenario. Measurement results confirmed that the masking solution was capable of flattening the tonal frequencies, whose beneficial effect yielded the cancellation of the public civil action filed by the neighbors. The proposed solution is ready to be replicated in other scenarios. p 2021 13 August 2021 2021 1  
attenuation of acoustic noise generated by the operation of high-voltage transformers, even today, is a challenge for managers, researchers, and manufacturers. Particularly, the noise results from a condition intrinsic to the functioning of the transformer itself, caused by the phenomenon of magnetostriction, with tonal characteristics, therefore not feasible to be completely eliminated so as not to cause discomfort to the human ear. Thus, although it is theoretically possible to attenuate noise by means of acoustic conditioning with attenuating and absorbing technologies, aiming at conforming to the strict normative limits imposed by the environmental regulation, in practice, the success of these solutions is not trivial. Acoustic noise attenuation requires creativity, theoretical and practical experience, and mapping of the sound field, which, in turn, requires access to high-risk environments, use of specialized measurement systems, and georeferenced modeling tools. The risky environment of an electrical substation (whose safety standards impose minimum distances to the high-voltage transformer in operation) limits the possible measurement points inside the substation and near the transformers, making it difficult to map acoustic power levels and to perform a directivity analysis of the noise source [3]. As a result of these difficulties, carrying out a theoretical analysis of the energy transfer process during the operation of the electrical transformers would allow modeling the system and characterizing the sources [4]. Moreover, a frequency analysis allows identifying the fundamental tonal components of the acoustic signal and their harmonics [5]. This paper is based on the results of an ongoing research and development project (R&D Light-Aneel 00382-0128/2019), financed with resources from a research-regulated fund managed by the regulatory body of the Brazilian electrical system (Aneel), aiming at reducing the sound pressure level of a broad-band noise that arises from the operation of electrical substations installed in urban areas. The paper describes an acoustic noise masking system designed to eliminate the tonal characteristic of the spectrum, therefore eliminating the discomfort to the human ear and adjusting the sound pressure level to the limits imposed by the environmental legislation. The active masking system was made possible by superimposing to the global noise (which emanates from the substation's operation) \"sounds of nature\", synthesized in the laboratory, thus masking the perception of the tonal frequencies (around 120 Hz and first harmonics) by the human ear.