## 1 Introduction  
In the productive agricultural sector, new technological innovations are continuously developed and integrated to improve and facilitate the farmer's work. The broiler industry is no exception and the producers are interested in availing every opportunity to add efficiencies into their production systems. Modern technologies, together with precision engineering, can achieve real-time information on animal health. This new management system is called Precision Livestock Farming (PLF) and is now used to monitor animals, ensuring their excellent health and well-being [1]. Through the electronic collection of data, their processing and application, precision farming has the objective and potential of improving production efficiency, reducing costs, and safeguarding animal health [2]. An animal showing excellent health can be the guarantee of product quality, which is why the PLF is based on the assumption that the continuous and direct monitoring or observation of the animals allows the farmers to detect and control their health status at any given time.  
Different technologies are currently available to the farmer to measure different parameters on the farm, such as ventilation rate, feed supply and temperature. The introduction of equipment such as microphones, cameras and sensors (for example, body temperature sensors or skin conductivity sensors), has managed to focus the attention on the core of the production process, the animal [3].  
Recent studies have focused on animal's vocalisations. This parameter can provide information on their sex, age, mother and offspring's relationship, emotional state and growth rate [1]. This study is centred precisely on this parameter, and its aim is to verify how the levels of carbon dioxide (CO\({}_{2}\)) within a farm can influence the vocalisations of broilers. CO\({}_{2}\) is naturally emitted by the animals, as a result of their metabolism and their litter, and it is therefore used as a tracer gas in order to determine the ventilation rates in animal houses [4]. CO\({}_{2}\) is an anaesthetic acid gas, which causes unpleasant pain sensations in animals [5]. Its increase inside the farm due to poor ventilation is directly related to the increase of other noxious gases, which together can cause the chicken to rapidly lose brain functions, leading to convulsions and death [6].  
Therefore, it is important to accurately study the acoustical features of the farm sounds to understand and describe how they are related with the CO\({}_{2}\) concentration. These indicators, obtained in a near real-time environment, can be used as a warning signal for the farmer about the need or not to increase the ventilation rate inside the farm in order to reduce greenhouse gas concentration, that can cause, among others, chronic respiratory disease, which affect the growth rate of broilers. Production meat quality is affected by high gas concentration and for other stress sources as for example acute external noise exposure [7]. Forecasting gas values and applying proper ventilation, higher animal well-being an production quality can be ensured.  
This paper is structured as follows. The related work used as baseline of this project is detailed in Section 2. The specifications of the farm deployment, the carbon dioxide concentration and acoustical features measurements are detailed in Section 3. The filtered features, the acoustical feature importance and correlations of the data and the representation of the most important ones can be found in Section 4. Discussion of the acoustical features analysis behaviours linked with the gas concentration found in this work are detailed in Section 5 and the conclusion, as well as future work, is detailed in Section 6.