Air-coupled microphone
Problem you are aiming at is connected with Phonocardiography (recording bodily sounds). Traditionally contact sensors (such as stethoscopes) were used for this task, but problem is that you must raise and lower the device between two measurements. In case of air-coupled microphone (meaning that it is not touching the body) you can move it constantly, performing a continuous measurement. 

Obviously there is some disadvantage, such that it is prone to environmental noise, but shielding (tubes) are used, as well as microphone arrays for noise cancellation. Problem of coupling the membrane with patient body is very hard to tackle, as big mismatch exists between air characteristic impedance and body impedance, but using a conical shape is helping to transform these impedances. 

What kind of microphones were being used? These are almost the same used widely in recordings:


Condenser
Electret
Piezoelectric


Some of these were adapted for recording of infra-sounds, i.e. Brüel &amp; Kjaer, model 4117 or other great, 4135. Although people are also using quite cheap electret microphones for this task.

This article can shed some light on that:


  Algernon D., et al. - Signal Processing for Air-Coupled Impact-Echo
  using Microphone
  Arrays.