The ETH team can, for the first time, provide a satisfactory explanation for the annoying hum originating from high voltage lines under certain weather conditions. Usually such lines are practically free from noise in dry fair weather but generate considerable noise when wet. Besides the hissing and crackling, above all the fixed frequency hum tends to be intensely annoying.

Tonal emission is caused by oscillating water droplets

The relationship between water on the lines and the distinctly audible hum has been clearly demonstrated. Explanations for the origin of the hum offered so far in the literature have been proved to be inadequate. Early on, the ETH team suspected to find the origin of the hum in the periodic deformation of water droplets by the electric field. This hypothesis of oscillating water droplets was assessed in the laboratory with tubular model conductors exposed to realistic surface field strengths and short periods of heavy rain followed by long drying intervals. The ensuing noise levels, as well as changes in the shape and population of water droplets, were measured using a multitude of acoustical, electrical and optical diagnostic methods. These showed clearly that the water droplets are deformed in synchronism with twice mains frequency, thus the principal acoustic emission occurs at 100 Hz.

Conductor surface properties have a bearing on the noise

In the course of the ETH experiments, the surface conditions of the model transmission lines were taken into consideration, as these influence the distribution, size and shape of the water droplets and linked to the noise emission. While the lines are subjected to heavy rainfall, reduction of tonal noise is hardly possible - or called for. Once the rain ceases, the surface properties of the conductors control the rapidity of disappearance of distinct water droplets and thus the duration of persisting 100 Hz hum. With hydrophobic or untreated surfaces, water droplets persisted in the experiments, while a hydrophilic coating drastically accelerated the disappearance of most droplets and the associated noise. Suitable conductor geometry can further enhance water removal.

Hope of noise reduction

People living close to high voltage transmission lines find the 100 Hz hum particularly excruciating, all the more so, as conventional sound insulation is rather ineffective against it. During the night the hum may become the dominating noise and may well exceed permissible limits. Network operators are therefore interested in techniques to reduce acoustic emissions. New insights gained in the course of the work mentioned should be useful to support the development of effective methods for noise reduction, including cost-effective treatment of already existing lines. A detailed report of the investigation, which was supported by German and Austrian electrical energy providers, is to be published in the next few days in the Austrian journal e&i (Elektrotechnik und Informationstechnik, Springer, Wien).