Noise performance of new conductors in Waikanae

The noise qualities of the old conductor previously installed on the transmission lines through Waikanae had improved over time due to aging. Transpower undertook extensive research to obtain a replacement conductor that provides the best achievable noise performance for a new conductor, (see factsheet at bottom of page). However, in wet conditions only, the new conductor will still initially sound different in character and level. 

As with test conductors, which we installed near Foxton in March 2019, we expect the new conductor noise performance to improve in the months following installation. From week to week, we anticipate recorded levels will go up and down but that the overall trend should be a downward one.

The monitors are placed approximately midway between pylons, where the conductors come closest to the ground. The microphone is elevated on a pole to be closer to the conductor, so that it better detects conductor noise rather than other sounds in the general environment.

The sites also monitor the weather including rainfall, which allows conductor noise during wet conditions to be identified.

Pictured below: Waikanae 1 monitor (top) and Waikanae 2 monitor.

Assessment of performance in wet conditions

The monitoring stations record sound levels and weather data for every 15-minute period throughout the day and night.

Periods of ‘wet’ conductor are identified as any 15-minute period during which rainfall is recorded, together with the following 15-minute period (which is considered the ‘drying period’ when the conductor is wet but there is no longer rain falling).

Conductor sound is relatively quiet compared to natural sounds such as from wind and vegetation movement, as well as other sounds such as road-traffic in the general area. Therefore, despite the close proximity of the conductor, only a small fraction of the monitoring data reliably represents the sound from the conductor itself. The monitoring data is processed to identify only those periods at night under calm weather conditions with steady sound levels, which should best represent the conductor. These periods include both wet and dry conditions.

We report two measures, firstly the overall “average” (LAeq) noise level. We calculate this from the average of all valid 15-minute periods every two weeks. This best enables general trends over time to be observed.

In wet conditions, in addition to a normal “crackling” sound new conductors can generate a hum, particularly in the initial months. This hum mainly results from sound at a frequency of 100 Hz. This is the second measure reported. The new conductor in Waikanae is designed to minimise this hum but it still may be audible at times and is therefore being specifically monitored.

Measurement of tonality

Environmental sound is made up of a mixture of sound occurring at many different frequencies, from lower frequencies like a bass drum to higher frequencies like a shrill whistle. The 100 Hz conductor hum is a lower frequency. Analysers can be used to split up overall sound and determine how loud each frequency component is.

The following figure shows the sound levels in frequency bands for wet conductor conditions. In this figure lower frequencies are represented by bars towards the left and higher frequencies are towards the right. As shown in the figure, a noticeable “tone” can occur where the sound at a particular frequency is prominent (taller on the graph) compared to the neighbouring frequencies. 

We measure the degree of prominence of the tone and report this as “100Hz tonality”. Again, we report this as the average of all valid 15-minute samples recorded over each two-week period.

The ongoing conductor performance measured at both the Waikanae 1 and Waikanae 2 monitors is shown below. The first graph shows the average sound level and the second the prominence of the 100Hz tone (hum). This information will be updated every two weeks for the first year after the conductors are installed.

If there are no, or too few valid measurements, this is noted in the results. A lack of valid measurements occurs if there are extended dry or windy conditions during a two-week period for example. The Waikanae 1 monitoring site has been temporarily removed while earthworks proceed at the site.