## 2 Machine Hearing
### Key Differences among Speech, Music and Environmental Sounds  
In what concerns the audio input that the machine hearing system is asked to process, speech, music and environmental sounds present specific characteristics. The key differences can be directly observed both in the time and the frequency domains, as well as in the structure and the semantics of the signal. These differences can be then parametrized following a physical or perceptual approach depending on the targeted application. Firstly, music and speech signals present a certain periodicity that can be observed when analyzing these signals in the time domain (see Figure 3). Although with some exceptions (e.g., some natural sounds such as bird chirps or cricket sounds), the periodicity in environmental sounds may not be so evident. Secondly, when analyzed in the frequency domain, it can be generally determined that the complexity of the spectrum of environmental sounds (e.g., the sound of a passing car) is notably larger than that of speech or music signals, as depicted in Figure 4. Moreover, it can be observed that speech and music signals usually present harmonic structures in their spectra, a trait that is not that common in environmental sounds, as mentioned before. Figure 3: Time envelope of a: (**a**) speech signal; (**b**) music signal (trumpet); (**c**) environmental sound signal (traffic street). Thirdly, notice that both speech and music sounds are composed of a limited dictionary of sound units: phonemes and notes, respectively. On the contrary, the range of environmental sounds is theoretically infinite, since any occurring sound in the environment may be included in this category (_i.e._, originated from noise, artificial or natural sound sources, see Figure 1). Furthermore, there exists a key difference between these types of signals. In speech and music, phonemes and musical notes are combined so as to obtain meaningful sequences that are actually transmitting a particular semantic or aesthetic message. As opposed, the sequences on environmental sounds do not follow any rule or predefined grammar, although they may convey some kind of meaning (e.g., bird chirps or cricket sounds). Unlike speech and music, also other important information is unknown, such as the duration of the sound events or the proportion between harmonic and non-harmonic spectral structure. Finally, Table 1 presents a summary of the specific characteristics of speech, music and environmental sounds in terms of several factors. Given the noticeable differences between the nature of these sounds, the research community has proposed diverse feature extraction techniques adapted to the particularities of these sounds. However, some works also make use of well-established approaches to build analogous systems in related research fields, e.g., by borrowing features showing good performances for speech and/or music sounds analysis to parametrize environmental sounds. The following sections put the focus on the central topic of this work, presenting an in-depth review of audio feature extraction techniques divided according to their physical or perceptual basis, together with some specific applications of machine hearing focused on the analysis of speech, music or environmental sounds.