# On the Control of Low-Frequency Audible Noise from Electrical Substations: A Case Study
###### Abstract
With a network frequency of 50 Hz, tonal components at 100, 200 and 300 Hz are to be expected, which makes the problem of low-frequency noise a sensitive issue. Only a modest reduction in transformer noise can be obtained by acting on its construction, but at the price of dramatically increased cost (10-30% of cost increase against noise abatement of 5-10 dB [12]) and inevitable power reduction. The \"IEEE Guide for the Design, Construction, and Operation of Electric Power Substations for Community Acceptance and Environmental Compatibility\" (IEEE 1127-2013) [13] encourages the use of field measurements and of computer simulations in situations where noise can be a concern, and admits that the cost implications of the noise control strategy may become a significant factor. The most widely adopted control measure is the use of suitable shields and barriers made of bricks or gypsum panels, placed between the substation and the area to protect (vegetation is not considered in this context due to the modest attenuation in the low frequency range against the large thickness required and the safety issues connected to fire hazard and reduced visibility). The shields must be placed as close as possible to the source and must have a certain minimum height, depending on the relative positions of source, barrier and receiver and on the required attenuation spectrum. The main benefits of this approach are that no direct intervention on the components is needed and that fair noise reductions can be obtained at a reasonable cost. The reference document for the construction of barriers for power substations is once again the IEEE 1127-2013 Guide. The flexibility of this solution in a wide range of applications is presented in [14]. Examples of noise barriers designed for the control of noise from transformers can be found in the literature [15]. Traditional noise barriers are not the only way to tackle the problem. For example, [PERSON] et al. [16] recently tested the efficacy of flexible acoustical blankets applied to the case of a 125 kV transformer, showing excellent results. [PERSON] et al. [17] proposed a combination of a traditional barrier with 3D-printed acoustic metamaterial and an active noise control system, obtaining promising results from simulations. Active control is indeed another noise control option that can be used to abate noise from transformers, as reviewed by [PERSON] et al. [18] and by [PERSON] et al. [19] and as demonstrated by the positive results shown by in-field applications [17; 20; 21]. The degree of annoyance with continuous audible noise greatly depends upon the relative level of the background ambient noise. The human ear is normally sensitive only to the most dominant among several noises. Sources of ambient noise in the community include vehicular or railway traffic, factories, aircraft, animals, and appliances such as attic fans, air conditioners, and lawn mowers. If ambient noise is very low, even a small amount of wind can override the other noise sources and become the dominant ambient noise. For this reason, the generation of broadband masking noise, often in the form of natural sound, is a good way to attenuate the impact of disturbance on the population. This technique has already been successfully applied to mask unwanted noise in public spaces [22], industrial noise [23], and traffic noise [24]. Water has been proved to be particularly effective to improve the soundscape, because associated with positive perceptions [25]. In the literature, the problem of noise from electrical infrastructures is mainly treated in terms of design best practices, which, however, is rarely applicable to old existing equipment. In this case, noise barriers are the preferred solution, but not much information can be found about the strict size and shape constraints that limit their attenuation capability, especially at low frequency. As a result, this measure alone may not be sufficient, but most of literature works focus on a single technique rather than exploring the possibility of combining more of them.