Stethoscopes is an important device to detect the tones of the acoustic wave from the client's body, hence complaints of a client can be diagnosed by doctors in auscultation. 
As shown in FIG.  1, the conventional stethoscope is composed of a body with two microphones, a hollow shaft which can be rotated around the body and connected with the ear pipes to transfer the sound from the client's body (the ear pipes are not shown in the FIG.  1).  There is a opening in each microphone to recess the acoustic wave and transfer it to the body.  In the hollow shaft, there are through holes whose sizes matches with that of the openings.  The through holes and the inner part of the hollow shaft form a passageway for the recessed acoustic wave travelling into the inner part of the hollow shaft when the hollow shaft rotates around the body and the through holes are directly aligned with the openings.  In order to fix the related positions of the body and the hollow shaft, there are a ball and a pressed spring inside the hollow shaft and a curved groove inside the body.  When the ball is directly aligned with the curved groove, the pressed spring pushes the ball outward to clamp the body and the hollow shaft. 
It is important that, when the ball clamps the related locations of the body and the hollow shaft, the opening and the through hole must be exactly aligned.  Otherwise, the through holes and the inner part of the hollow shaft will not form a passageway, and the recessed acoustic wave can not travel into the inner part of the hollow shaft.  For the above-mentioned reason, a substantially high precision is needed in the fabrication of the conventional stethoscopes, which additionally increases the cost. 
According to the U.S. Pat.  No.  4,770,270, which discloses an indexing detent which is composed of a hollow cylindrical element with a beveled-shape seat end and a pressed spring.  When the hollow cylindrical element is aligned with the opening of the body, the pressed spring pushes the hollow cylindrical element outward to clamp the body and the hollow shaft.  The sound from the microphone then travels through the inner space of the hollow cylindrical element and into the hollow shaft.  Hence, the positioning and acoustic connecting functions are immediately achieved.  The beveled-shape seat end of the hollow cylindrical element however induces a large friction force when the hollow shaft is rotated around the body, this stethoscope will not work smoothly and will easily induce large abrasion.