Breath sounds: Mechanism

 Normal breath sounds originate from the larynx. When the sound leaves the larynx it travels down the trachea and then divides when the airway divides. Eventually the sound travels along the airways of different lengths and therefore becomes out of phase. Next it arrives in the respiratory bronchioles and the alveoli and then get transmitted through the chest wall to your stethescope. The fat layer filters out much of the higher frequency (4 kHz). The resulting sounds are much softer (because the sounds has effectively been diluted throughout of the whole lungs). There is no gap between the inspiration and the expiration because the sound has become out of phase and therefore filled in the gap. Finally, the first third of expiration is now only part that is audible because the latter two thirds are much quieter. 

Reference: Chamberlain’s symptoms and signs in clinical medicine, 13th edition, page no:97