Sound analysis has been applied to evaluate pig physical condition such as body temperature changes, pain, hunger and thirst. Moi et al.
[18] identified the differences in swine vocalization patterns according to different stress conditions (thirst (no access to water), hunger (no access to food), and heat stress). Pig was found to be thirsty when sound intensity ranged from 73.87 dB to 80.18 dB. With a value higher than 80.18 dB, it indicated that the pigs were hungry or under heat stress. For further confirming the pig's status, pitch frequency presented a difference, with the hunger of 212.87~276.71 Hz and heat stress of higher than 276.71 Hz
[18]. The abnormal conditions by analyzing grunt, frightened screams and feeding howl sounds were also detected
[19, 20]. The results showed that the total sound recognition rate could achieve 95.5%
[19]. Besides, vocalization is a valuable tool for identifying situations of stress in pigs during the castration procedure
[21, 22]. Without local anaesthesia, piglets uttered almost twice screams during the experiment. Also, screams characteristics are significantly different from grunts
[23]. Moreover, different acoustic parameters were beneficial for evaluating the level of pain in piglet management. The results showed that the values of pitch, intensity, and maximum amplitude were enhanced from pigs in normal status to castration
[24]. Based on the researches of screams characteristics, representative features were focused on and taken into consideration to define pig screams for constructing a more accurate classifier. And pig screams were defined when the pig sound duration was longer than 0.4 s
[25]. A simple voting system was constructed to classify the screams with a precision of 83%
[25]. With respect to specific emotion analysis of pigs, Riley et al.
[26] and Moura et al.
[27] proved that phonations increased with fear and distress piglets. Grunting was also found to be highly variable, with the lowest grunting for happy emotion.