I am building a digital stethoscope as a part of  final year project. We acquire the heart beat through diaphragm and and after amplifying it, the signal is fed to the sound card of laptop through the 3.5mm jack. The signal is then processed in MATLAB and then displayed on the screen. The problem I encountered was the input signal to the sound  card is too noisy. Ambient noise is preventing the accurate acquisition. How to filter out the noise?