## 1 Introduction  
Corona discharge is a kind of gas discharge phenomenon in extremely uneven electric fields, which often occurs around power equipment with large curvature such as burrs and defects [1]. Audible noise accompanies corona discharge, which is one of the important control indicators of the electromagnetic environment, and many standards also clearly limit the noise level at the boundaries of transmission line corridors and substations [2, 3, 4]. Compared with the problems of total electric field and radio interference, audible noise can be perceived directly, and it is different from other environmental noise; the noise is most likely to attract people's attention because of its unique frequency characteristics [5]. Corona-generated noise generated by power transmission and transformation equipment has become one of the key factors limiting power construction [6, 7]. In order to study the audible noise characteristics of corona discharge, scholars have conducted many experiments to study the relationship between audible noise and corona parameters. [PERSON] et al. [8] studied the relationship between sound waves and corona current, and the results showed that as the applied voltage increased, the sound wave amplitude continued to increase, whereas the amplitude of corona current remained almost constant. The statistical results of the amplitude ratio of the sound wave to the corona current and the number of pulses followed a normal distribution. [PERSON] et al. [9] studied the characteristics of the amplitude spectrum of a single current pulse and audible noise pulse, and they found that the current spectrum was mainly below 30 MHz, while the audible noise spectrum reached its maximum value at about 10 kHz and attenuated along both sides of this frequency. [PERSON] et al. [10] conducted a DC corona experiment using needle-plane electrodes. The experiment results showed that the discharge voltage and the electrode polarity had a direct effect on the rising time and pulsed width of the current pulse. The waveform of sound pressure was a bipolar microsecond pulse, which lagged behind the current pulse in the time domain. In addition, due to the different development process of positive and negative corona discharge, their acoustic characteristics demonstrated an apparent polarity effect [11; 12]. Scholars have found that positive sound had large values at some frequencies, but the noise spectrum fluctuated greatly. The negative sound had a large value at the repetition frequency point of the current pulse, and with the voltage increased, the sound energy shifted toward high frequencies and the noise spectrum became smoother. The noise amplitude generated by the positive corona under the same voltage and conditions is much higher than that of the negative [13]. From the discharge waveform, the comparatively large current amplitude and pulsewidth of the positive corona leads to larger audible noise than negative. In terms of the propagation characteristics of corona noise, [PERSON] et al. [14] built a single-point corona-generated audible noise measurement platform. The measurement results showed that the audible noise had no directionality, and sound propagated outward in the form of spherical waves. Every time the distance doubled, the sound pressure decreased by about 6 dB. For the audible noise from transmission lines, [PERSON] et al. [15] believed that spatial sound waves were the result of the superposition of multiple point sources on the line. In general, the propagation of sound in the air was also influenced by the effect of atmospheric absorption and ground reflection, and these factors need to be fully considered in numerical simulation. Meanwhile, if we wish to predict and analyze the amplitude of corona-generated audible noise, in addition to establishing an accurate spatial propagation model of sound, it is also necessary to build a sound source model which is suitable for corona discharge characteristics.