# Detection of Respiratory Phases in a Breath Sound and Their Subsequent Utilization in a Diagnosis  
[PERSON]  
1 Department of Anatomy and Biomechanics, Faculty of Physical Education and Sport, Charles University, [PERSON] 269/31, 162 52 Prague 6, Czech Republic; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Peediatrics, 2 nd Faculty of Medicine and University Hospital Motol, Charles University, V VUvalu 84, 150 06 Prague 5, Czech Republic; [PERSON]S]  
[PERSON]  
1 Department of Anatomy and Biomechanics, Faculty of Physical Education and Sport, Charles University, [PERSON] 269/31, 162 52 Prague 6, Czech Republic; [EMAIL_ADDRESS]  
[PERSON]  
5 Department of Information and Communication Technology in Medicine, Faculty of Biomedical Engineering, Czech Technical University in Prague, [PERSON]DRESS]  
[PERSON]  
4 Correspondence: [EMAIL_ADDRESS] (D.S. ); [EMAIL_ADDRESS] (F.L. ); Tel. : +420-602-341-582 (F.L.) [PERSON]  
1 Department of Anatomy and Biomechanics, Faculty of Physical Education and Sport, Charles University, [PERSON] 269/31, 162 52 Prague 6, Czech Republic; [EMAIL_ADDRESS]  
Footnote 1: [[http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci)]([http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci))  
Footnote 2: [[http://www.mdpi.com/journals/](http://www.mdpi.com/journals/)]([http://www.mdpi.com/journals/](http://www.mdpi.com/journals/))  
signal carries certain modifications as the sound propagates through the tissue, for example, the lung parenchyma acts acoustically as a low-pass filter or through the detection equipment [5]. These modifications directly influence the significant frequencies for the analysis which mostly lies bellow 250 Hz [2,5]. In addition to these modifications, there are other significant sources of noise in the sound such as the sounds from the respiratory muscles (\(\sim\)20 Hz) and from the heart (\(\sim\)50 Hz) [2,4,5,7]. As indicated above, not only do the tissue affect the signal, but also the equipment can negatively influence the recording while using a poor quality sensors and electronics. The common methods to eliminate the noise in the recording are based on ambient noise reduction, which is very difficult in a general practitioner's office, on correct manipulation with the equipment or on the proper design and selection of the electronics and hardware including solid acoustic shielding or suitable sensors [8]. On the other hand, the recording is always affected by some unwanted noise which originates either form from the patient's body, the surrounding or from the equipment. To reduce the unwanted noise and obtain relevant and useful information from the recording, it is necessary to filter the respiratory sound recording. The common approach to reduce the noise in the recording is the application of the high-pass and low-pass filters of nth order [6,9,10]. The properties of such a filter reflect the type of obstruction that the physician suspects during the examination. The cutoff frequency for the low-pass filter can reach 100-500 Hz and the cutoff frequency for the high-pass filter can be set between 800-5000 Hz [6,9,10]. Some studies even adopted voice activity detection (VAD) to filter the signal [11]. Sadly, none of these approaches is sufficient to detect the respiratory phases, and therefore the algorithm requires additional features, for instance, observation of multiple sound parameters [6] or creation of recording's envelope [9]. It is known that the respiratory tract of a healthy person produces a normal respiratory sound without adventitious sounds. The sound from normal lung parenchyma detected on the chest wall (called the vesicular breathing) is characterized by a quiet low-frequency noisy sound during inspiration and it is hardly audible during expiration. The sound detected on the trachea (so called tracheal breath sound) is characterized by a broader spectrum of noise and it is audible both during inspiration and expiration [2,4].