Basic Concepts of Sensing Respiratory Sounds at the Surface of Human Chest

Acoustic sensors for recording lung sounds at the chest surface should meet some basic requirements—(a) achievement the acceptable sensitivity or sensitivity threshold to the useful signal, (b) a linearity of frequency response, and (c) sufficient noise immunity. Specific condition of recording acoustic wave propagating through human thorax is its registering on the border of the body with air medium which can be regarded as an acoustically soft one. Existing types of acoustic sensors usually used may be divided into contact and non-contact receivers. Non-contact receivers are still exotic to some extent. Currently, three types of acoustic receivers mounted in contact with the chest surface are used. They are accelerometer sensor, stethoscope sensor with microphone, and so called “contact” sensor. Any acoustic sensor having a mass when placed on a layer of soft tissues having hardness should inevitably have an eigenfrequency of suspending. According to ratio of working frequency range and the frequency of suspending all mentioned sensors are modeled as the receivers of oscillatory acceleration, velocity, displacement or dynamic force. Experimental study and theoretical estimates indicate that there is no one optimal sensor meeting all basic requirements for all scenarios of acoustic study of lungs. Hence a passive recording of lung sounds is more frequently performed with a stethoscope or with “contact” sensor, whereas active transmission sounding is performed with an accelerometer or “contact” sensor. However even these sensors are frequently used in non-optimal modes. New studies are welcome to design more optimal sensors.