## 1 Introduction
Mean opinion scores (MOSs) are the most widely used direct subjective speech quality evaluation method. The MOS is a categorical judgment method in which listeners rate the quality of the speech test signal using a five-point numerical scale, with five indicating \"excellent\" quality and one indicating \"unsatisfactory\" or \"bad\" quality. The measured quality of the speech test signal is obtained by averaging categorical scores from all listeners, and the average score is commonly referred to as the MOS. This method is recommended by the IEEE Subcommittee on Subjective Methods [2] as well as by ITU [3]. In addition, a subjective test methodology for evaluating speech in communication systems that include a noise suppression algorithm [4], which measures the perceived speech quality, is one of the most adopted methods. However, subjective speech quality measurement is expensive as it requires the recruitment of human subjects. Furthermore, subjective quality measurement is not practically applicable in the production phase for security reasons (communication privacy) and, for instance, in automatic speech quality monitoring in VoIP. Therefore, objective quality measures are developed to model subjective listening tests and mathematically estimate the audio quality perceived by human beings. There are two main categories of objective quality measurement methods: end-to-end (intrusive) and single-ended (non-intrusive) methods. End-to-end audio quality objective evaluation involves the usage of mathematical signal processing techniques to compare the VoIP transmitted/processed speech signals to the original speech signal. Speech objective measures calculate the speech quality by measuring the numerical \"distance\" between the VoIP transmitted/processed speech signal and the original speech signal. Currently, the widely used end-to-end automatic speech evaluation methods are perceptual evaluation of speech quality (PESQ) [5], perceptual objective listening quality analysis (POLQA) [6], PEMO-Q [7], and perceptual objective listening quality prediction proposed in 2018, which supersede POLQA. Furthermore, the perceptual evaluation of audio quality (PEAQ) [8] is used to evaluate speech and other audio signals such as music. These methods have been widely applied for designing and evaluating the audio encoding and decoding algorithms. End-to-end methods for objective audio quality evaluation require a clean (origin) reference signal and received (VoIP transmitted)/processed speech signal to evaluate the speech quality. This limits end-to-end method usability in general applications, as, in most cases in VoIP applications, the original speech (reference) signal is not readily available. The performance of IP networks changes based on the network load, affecting the quality of speech transmitted through the network. In the speech quality monitoring of VoIP applications, the interest is in continuously monitoring the quality of the transmitted speech signals. We only have access to the received signal at the monitoring point, whereas the original speech signal is unavailable. Single-ended objective speech quality measures estimate the speech quality using only received/processed signals without needing the original signal. A single-ended (non-intrusive) objective measure of speech quality is a practically viable method for continuously monitoring the quality of speech delivered to a VoIP endpoint or a particular point in the network. Based on continuous quality assessment results, network traffic can be re-routed through a less congested route, or codec parameters can be changed, improving the service quality. Different single-ended methods have been proposed based on signal processing techniques or machine learning approaches. Single-ended (non-intrusive) quality measures are suitable for continuously monitoring the received speech quality in Internet protocol networks, as there is no need for an original/reference signal.