## 1 Introduction  
Acoustic emission (AE) is one of the noninvasive methods used for the detection and localization of partial discharges (PD), occurring in the insulation systems of power transformers [1; 2; 3]. PD may pose a serious risk to the proper functioning of this type of equipment. In the field of high-voltage power transformer diagnostics, there are various other existing methods to calculate their technical condition estimation, e.g., optical spectrophotometry [4; 5; 6], or UHV [7; 8; 9] methods. The advantage of non-invasive PD detection methods is their resistance to electromagnetic interference, which makes them appropriate for application under on-line operation [10; 11; 12].  
Apart from the detection of the PD occurrence, its localization inside the device is crucial [11; 13]. Knowing the speed of the waveform of AE is the key parameter in the application of methods for locating incomplete discharges. The location of the PD source is calculated by assuming the known AE velocity value and coordinates of the measuring sensor. Regarding the AE velocity, within literature, various values are defined, e.g., the speed of 1417 m/s is assumed in [13], 1431 m/s in [14] or 1400 m/s in [15; 16]. Both within literature and in the IEEE standard C57.127-2007 [17], the impact from temperature and moisture content is mentioned.  
The speed of AE wave has an important impact on the localization result, as shown in [18], where authors measured AE velocity changes at various distances while propagating in concrete. Therefore, in this study, experiments have been carried out to determine the AE wave velocity in the chosen propagation centers. At the present time, new electro- and thermo-insulating materials are being developed. Aside from mineral oils, this also includes natural and synthetic esters [19; 20; 21].  
PD emit AE waves in a wide-frequency spectrum. Therefore, the effect of modulation frequencies was studied in this paper. In practice, the surveyors do not know where the AE source is located,thus, they place the sensors at various locations. For that reason, the AE velocity at different distances between the source and receiver was determined. Transformers are usually large in size, but the distance from the isolation system to the sensor can be up to several meters long. As the AE wave undergoes significant attenuation, especially in the HF range, it is impossible to record pressure changes over long distances. The distances considered in this study were chosen experimentally, as part of basic works [22]. In real transformers, the AE wave will undergo additional phenomena such as damping, refraction, reflections, etc. In this work, we did not analyze the barriers to propagation as the primary objective was to determine the AE velocity in barrier-free centers.  
The main contribution of the here presented research study lies in the determination of the influence of distance and modulation frequency of the AE wave on its velocity, in various dielectric liquids. The results of preliminary works were obtained for natural ester and are presented in [22]. In this article, comparative analyses for the velocity measurements of AE waves propagating in three types of dielectrics are presented. In particular, mineral oil type TRAFO EN [23], natural ester type eN1204 [24], and synthetic ester type 7131 [25] are considered.