Abstract
Methods hitherto in use have proved unsatisfactory for recording murmurs, except such as are loud and low-pitched or of a sustained musical character and not very high-pitched. Einthoven's method, in which a carbon microphone is used, gives murmur records which are often more complicated than the sounds themselves, a circumstance which appears to be due to lack of damping in the moving parts of the microphone, especially of the carbon particles. In most microphones the diaphragms have a natural period too low for best results, but the undamped motion of the carbon particles is the most serious drawback. The methods not involving use of a microphone all suffer from a common defect. They are insensitive if the instruments are made of high natural period and if made sensitive they give records so distorted as to be almost valueless except for time-relation studies of the first and second sounds and of very intense murmurs.
To overcome these difficulties an electromagnetic telephone has been used to convert the sound oscillations into electrical oscillations. This telephone is provided with an air-damped diaphragm of high natural period. A shallow ring fastened to the telephone cap serves as “mouthpiece” and has a small lateral opening to maintain atmospheric pressure within it except for the sudden variations due to sound. This device is applied directly to the chest wall and the air column confined between the diaphragm and the chest wall is very short and has a high natural period.
The currents produced by the telephone are amplified by a four-stage amplifier used in such a manner as to give practically distortionless amplification. It is necessary to protect the vacuum tubes against extraneous sound and mechanical vibration.
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