Electrical audio signal processing systems and devices – Stethoscopes – electrical
Reexamination Certificate
2000-07-14
2002-08-20
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Stethoscopes, electrical
C181S131000, C600S528000, CD24S134000
Reexamination Certificate
active
06438238
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a stethoscope, more broadly sometimes referred to as a vital signs monitor, and more particularly to a stethoscope which has improved fidelity of reproduced body-generated signals.
BACKGROUND OF THE INVENTION
For the purpose of this application, a stethoscope is defined as a device having a body signal transducing device which is placable on the body, coupling to the skin and serving as the pick-up for sounds generated in the body and transmitted through the flesh which, for the purpose of this application, is defined as the skin and underlying tissue. Since the stethoscope is frequently used to monitor sounds due to action of the heart, in which case the body signal transducing device is placed on the chest, the body signal transducing device is frequently referred to a chest piece, and is hereinafter referred to as such. The stethoscope as used in this application is considered in its broadest sense. The stethoscope, as it is defined in the present application, also has an output element which allows a user of the stethoscope to perceive the sounds being generated by the body. This output element can be provided by a variety of devices such as earphones, speakers, recording mediums (capable of being played back), visual monitors, etc., all hereinafter referred to as earphones. In the stethoscope, the chest piece and the earphones are operatively connected by a transmission medium which, for a classical mechanical stethoscope where the output of the chest piece is transmitted through a gaseous medium such as air, can be a tube containing a column of air. Alternatively, for an electronic stethoscope where the output of the chest piece is an electronic signal, the transmission medium can be composed of a variety of electronic components.
There are a variety of stethoscopes which have been developed. All of these are based on techniques for sensing the sound emitted from the body by monitoring the sound that radiates from the body and is transmitted through a confined volume of a fluid. Historically, the fluid employed in the stethoscopes was air, and attempts were made to mechanically amplify the sounds generated by the body. Sound, being a wave form of energy, is classically described in terms of its intensity, reported in decibels (dB) with larger values corresponding to greater intensity, and its frequency, reported in cycles per second or Hertz (Hz).
The stethoscope was invented by Rene Laennec in France in 1819. His stethoscope was a monaural device which was fashioned from wood and had an input opening and an associated sound chamber or so-called “sound accumulator”, which was modeled on the shape of a musical horn. A sound transmission conduit (also fabricated from wood) was a straight, smooth bore which coupled the sound chamber to the physician's ear. A major obstacle when using Laennec's monaural stethoscope was the tendency of unwanted sounds from the surrounding environment masking the desired sounds, since one ear was directly subject to sounds from the surrounding environment.
A binaural stethoscope was developed by George Camman in 1850 in an effort to overcome or, at least, mitigate the impact of the sounds from the surrounding environment thereby enhancing the ability of the user of the stethoscope to listen to the sounds generated by the heart and other body organs such as the lungs. This stethoscope had flexible tubes which transmitted sound to both ears of the user, reducing the user's exposure to sounds from the surrounding environment.
Both of these stethoscopes employed open-ended sound chambers which, when pressed against the skin of the patient, provided a confined volume of air for transmitting sound waves from the body of the patient to the earpiece. A significant problem with such open-ended stethoscopes is the low intensity of the sound transmitted to the observer's ear(s), due to loss in intensity of the sound as it crosses the interface between the flesh of the patient and the air-filled sound chamber of the chest piece. This signal loss is due to the differences in mechanical impedance of the flesh, which is substantially liquid, and the air employed in the chest piece. This difference in mechanical impedance between the flesh and the air causes only a small portion of the sound wave energy to be transferred from the flesh of the body to the air in the chest piece. As a result, the signal received is low in intensity, and may be difficult to detect or distinguish over ambient noise from the surrounding environment.
Another problem associated with these stethoscopes is that the frequency response of these devices is dependent on the pressure with which the chest piece is applied to the body of the patient. As the pressure applied to the chest piece increases, the chest piece causes distortion of the flesh. This distortion results in the flesh of the patient filling a portion of the chamber and altering the volume and pressure of air in the sound chamber, altering its mechanical performance in amplifying sounds. Thus, the characteristic response of the chest piece is dependent on technique (the touch of the user), so the sounds heard may differ for different users, making comparison of results problematic.
A significant advance in overcoming the problem of low intensity (dB) of the sound transmitted to the user of the stethoscope was made by Dr. R. C. M. Bowles (Massachusetts General Hospital) and was patented in 1901. His improvement to the stethoscope was the addition of a thin semi-rigid diaphragm attached to the chest piece and covering the opening to a conical-shaped sound chamber. This was found to significantly increase the intensity (dB) of sounds generated by the heartbeat and transmitted to the user, achieving such increase by selectively amplifying sounds in the frequency band centered near 90 Hz, the band associated with many of the sounds associated with the heart. The Bowles type stethoscope design is one of the most common currently used, and few significant design changes have been made since its invention.
While the Bowles stethoscope significantly increases the intensity of sounds in the frequency range of many of the sounds generated by the heart, the use of a diaphragm creates frequency-dependent distortion due to the natural vibrational frequencies of the diaphragm. This creates a response which is highly dependent on frequency, having significantly increased sensitivity to sounds in the frequency bands centered at about 90 Hz and 300 Hz, and greatly reduced sensitivity in the frequency band centered near 200 Hz and above about 500 Hz. Thus, while the Bowles type stethoscope offers a significant increase in the intensity (dB) of many of the sounds generated by the heart and other sounds of similar frequency heard by the user of the stethoscope, it reduces the intensity (dB) of transmitted sounds generated by the heart or other organs which are in the frequency bands of reduced sensitivity, thus rendering the resulting stethoscope unsuitable for the observation of many sounds which may be of interest to the user. For this reason, many modem stethoscopes employ a combination chest piece which includes both an open-ended sound chamber, either conical or spherical in shape, and a Bowles type diaphragm-covered conical-shaped sound chamber, enabling the user to select the chamber best suited for monitoring the frequencies of interest.
Furthermore, if the diaphragm of the chest piece receives sounds from the surrounding environment which are in the frequency band of increased sensitivity, these sounds are also amplified, thus tending to obscure the sounds monitored by the user of the stethoscope.
The use of a thin semi-rigid diaphragm also results in a frequency-dependent response of the diaphragm which varies with the pressure on the chest piece as it is engaged with the body. Thus, when a thin semi-rigid diaphragm is employed in a stethoscope, a variation in response of the stethoscope results from the diaphragm changing its natural harmonic frequency as it
Mei Xu
Semprebon Jeffrey E.
Weins Michael J.
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