# On the Control of Low-Frequency Audible Noise from Electrical Substations: A Case Study
## 3 Results and Discussion
### Noise Masking  
The narrow band spectrum at the balcony of the receiver's house, predicted by convolving the new impulse response at the receiver and the source noise, is given in Figure 19. It can be observed that despite the presence of the barrier, the 300 Hz component appears to be well audible since it crosses the 19 phon equal loudness contour. The height of the barrier cannot be further increased for safety reasons, thus other classes of actions must be undertaken.  
Broadband signals have been used to improve room acoustic quality since the late 1970s. It is possible to conceal a tonal component within a broadband noise overlapping it, to avoid its clear perception by the human ear [27]. In the case at hand, the residual tonal component after the installation of the barrier features an SPL of 28.7 dB at a frequency of 300 Hz.  
Figure 19: Predicted narrow band sound pressure spectrum at the receiver after the installation of the baffled barrier.  
Figure 18: Impulse response in case of barrier with lateral baffles.  
The use of white noise as a masker signal is reviewed by [PERSON] [27]. When a second sound is added to an existing one, the detection threshold of the latter increases. The shift between the threshold in quiet and the masked threshold is called \"masking\". The estimation of the masking effect is based on the masked threshold contours first introduced by [PERSON] and [PERSON] in 1950 [39]. These curves show the masked thresholds produced at each frequency by a white noise at various spectrum levels. In Figure 20 the residual disturbance at 300 Hz is slightly above the threshold in quiet, thus a masking action could be convenient. If a white noise with 10 dB spectrum level is chosen, one can estimate the perception difference between the tone and the masking noise by the comparison between two equal loudness curves: with reference to Figure 20, the highest curve touching the tone to be masked (19 phon), and the highest curve touching the masking noise (24 phon), leading to a difference of 5 dB. It is worth remarking the white noise spectrum level must be chosen so that the \"sound blanket\" is not more annoying than the masked disturbance. The overall A-weighted SPL introduced by the 10 dB spectrum level signal is equal to 30 dB(A), which is 16 dB below the background noise and, therefore, can be considered negligible.  
The background noise detected in the area may be sufficient to mask the disturbance during the daytime, but, coming from vehicle traffic, its masking effect is likely to disappear at night. In this case, the introduction of a broadband signal can be seen as an artificial background noise that ideally could be turned on only at night, thus gradually replacing the daytime traffic noise. A practical implementation of this technique can be achieved either with distributed loudspeaker systems, to be installed in the premises of the disturbed dwelling, or with soundscaping approaches. For example, the spectrum of a water fountain [40] seems particularly suitable to replace the traffic noise, as the energy content of the two signals is very similar in the high frequency range (Figure 21).  
Figure 20: Masking of the tonal component using a white noise. Dashed lines: equal loudness contours at 19 phon and 24 phon. Threshold in quiet adapted from [27].  
It is worth noting that the introduction of intentional noises should be evaluated from a psychoacoustical perspective. In this respect, the possibility to virtually reproduce the sound signal at the receiver in different scenarios is an interesting possibility that the impulse response reconstruction offers.