## 2 Background  
The human heart is comprised of four chambers. Two of them are called atrias and they make the upper portion of the heart while remaining two chambers are called ventricles and they make lower portion of heart, and blood enters the heart through atrias and exit through ventricles. Normal heart sounds (Figure 1a is an example of normal heart sound signal) are generated by closing and opening of the valves of heart. The heart sound signals produced, are directly related to opening and closing of the valves, blood flow and viscosity. Therefore during exercise or other activities which increases the heart rate, blood flow through the valves is increased and hence heart sound signal's intensity is increased and during shock (low blood flow) the sound signal's intensity is decreased [2]. Figure 1f also shows the spectrum of a PCG signal. Figure 1: _Cont._  
The movements of heart valves generate audible sounds of frequency range lower than 2 kHz, commonly refer to as \"lub-dub\" The \"lub\" is the first portion in the heart sound signal and is denoted by (S1), it is generated from the closing of mitral and tricuspid valve. One complete cardiac cycle is the heart sound signal wave which starts from S1 and ends to the start of next S1 and it is described as one heartbeat. The duration, pitch, and shape of the heart sound shows us detail about the different conditions of heart [4]. Closing of mitral valves is followed by closing of tricuspid valve and usually the delay between this operation is 20 to 30 ms. Due to the left ventricle contraction first, the mitral valve component occurs first followed by tricuspid valve component in the signal. If the duration between these two sound components is in between 100 to 200 ms, it is called a split and its frequency range lies from 40 to 200 Hz and it is considered fatal if the delay is above 30 ms [7]. \"Dub\", is generated (the second heart sound component denoted by \"S2\"), when the aortic vales and pulmonary valves are closed. S1 is usually of longer time period and lower frequency than S2 which has shorter duration but higher frequency ranges (50 to 250 Hz). A2 is another heart sound signal component generated due to movement of Aortic valve and P2 is another heart sound signal component generated due to movement of pulmonary valve. Aortic pressure is higher compared to pulmonary pressure hence the A2 component appears before P2 in the heart sound signal. The presence of cardiac diseases such as pulmonary stenosis and atrial septal defect can be identified by analyzing closely the split between aortic component A2 and pulmonary component P2 and their intensities. During deep emotional and excitement state, the duration between A2 and P2 may be longer than usual, causing long and wide splitting in between the components. This state is considered fatal if the delay between these component is longer than 30 ms [7]. Beside \"lub-dub\", some noisy signals may be present in the heart sounds called murmurs. Murmurs, both normal and fatal, are continuous vibrations produced due to the irregular flow of blood in cardiovascular system. Murmurs can be of two types \"normal murmurs\" and \"abnormal murmurs\", normal murmurs are usually present in heart sound signal component of infants, children and adults during exercise also in women (during pregnancy), this type of murmur can be detected in the first heart sound. Abnormal murmurs are usually present in heart patience which indicate a heart valve defect called stenosis (squeezed heart valves) and regurgitation (bleeding heart valves) [7]. Murmurs  
Figure 1: (**a**) A normal heart sound signal; (**b**) Murmur in systole (MVP); (**c**) Mitral Regurgitation (MR); (**d**) Mitral Stenosis (MS); (**e**) Aortic Stenosis (AS); (**f**) Spectrum of a PCG signal. are the unusual sound present in heart sound cycle of an abnormal heart sound which indicates some abnormalities. Based on the murmur position in the heart cycle, they can be called systolic murmurs, diastolic murmurs and continuous murmurs [4].