Commonly available stethoscopes provide a monaural sound in that the sound is received in a single sound accumulator such as bell type accumulators or diaphragm type accumulators from which it is "mechanically" provided to ear pieces via ear tubes. A person using such a conventional, monophonic stethoscope will not be able to perceive any directionality of spatiality of the sounds heard. This is because the sound is transmitted to both ear tubes from the one sound accumulator, which acts as the transducer of the sound and therefore becomes a single, intermediate source, in which the sounds generated by the true sources are superimposed.
The perception of directionality and spatiality of the sounds in stethoscopes would allow a differentiation between sounds. E.g cardiac murmurs which are the result of a turbulence of the blood flow in the heart and thus the origins of sounds associated with cardiac murmurs are widely spread whereas the closing and opening clicks of cardiac valves originate from a point source.
To enable perception of directionality and spatiality of sound obtained from a stethoscope, each ear must be provided with sounds transduced by separate sound accumulators and consequently two sound channels must be provided to realise a stereophonic stethoscope.
Stethoscopes providing a stereo signal mechanically have been produced and tested, e.g. reference is made to S. Kazama, "A new Stereophonic Stethoscope", Jpn. Heart J, Vol. 31/6, November 1990.
On the other hand, the use of the conventional, mechanical stethoscope principle has limitations in that sounds can only be heard live in the sense that typically only one person is listening to the sounds and bases his or her diagnosis on his perception of the sounds. To obtain a second opinion of another person, this person must also perform an examination of the patient with the stethoscope therefore the patient is required for a longer time and discomfort may be imposed onto the patient. Therefore, the use of electronic stethoscopes can enable an easy and direct way to record the sounds in one examination which can be replayed to other persons at any time and without further examination of the patient. Further, the electronic audio signal can be more readily made available to more than one person during the examination by transducing the audio signal via speakers. An electronic stethoscope in which a microphone is connected to one precordial stethoscope probe or esophageal probe to monitor a monophonic sound accumulated in the bell or probe electronically via a high quality amplifier is described by J. H. Philip et al in "An electronic stethoscope is judged better than conventional stethoscopes for anaesthesia monitoring", Journal of Clinical Monitoring, Vol. 2/3, July 1986. Previous electronic stethoscopes have employed electronic frequency filtering to selectively amplify sound in the range 37.5-1000 Hz where most heart and lung sounds occur 1, e.g. a bandpass frequency of 0.7-5000 Hz with selectable bass and treble boost/cut circuitry. Other electronic stethoscopes use an upward frequency shift of the entire audio band, aiming to allow improved detection of low frequency sounds. However, the electronic manipulation involved makes for complex and expensive circuitry, as well as increased size, particularly if two units are to be employed for stereophonic stethoscopes. It also requires a significant degree of retraining as the familiar heart and lung sounds may be quite altered.