The Intricacies of the Stethoscope
Whenever we make a trip to the physician, they almost always will put a certain device on the chest to listen to the heart. This device is loved by most patients and if this routine is omitted by the physician, the patient may feel like they have not been helped by the doctor. This device, the stetho- scope, has evolved from a rolled-up paper tube to digital devices that even have noise cancellation. Ironically, the man who invented the stethoscope brought it up as a means of preventing doctors, who were all male at that time (1816), from putting their ears close to the chests of female patients. (This man deserved a medal) The French doctor who made this invention, solved the inaccuracy of the method of the doctor placing their ear on the patient’s chest.
Evolution of the stethoscope:
Leaennac’s Le Cylindre (1816)
This clever doctor, got 24 sheets of paper and rolled them up and used them to transmit sound. The first stethoscope simply used the principles of transmission of sound through air. The sound waves from the chest cause vibration of air within the hollow tube and these sound waves travel to the ear and are heard by the doctor. The sounds heard were amplified compared to when the plain ear was placed on the chest. Various models of the single-tube stethoscope were made with each being better than the last.
Cammann’s binaural stethoscope (1852):
This modification of the stethoscope had two ear- pieces which were aimed at cancelling out noise interference from the environment that would have been picked up by the second ear. This stethoscope also had a hollow triangular chest piece that improved the amplification of sounds from the body.
Bowle’s modern-day stethoscope (1925):
This variation was also binaural and had a plastic diaphragm which replaced the hollow chest piece. The stem (the part which connects the diaphragm to the rubber tubing) was made of brass which is non-compressible. This provided better sound quality than previous models whose stems were made of compressible materials like rubber. In this model, the plastic on the diaphragm vibrates from the sounds produced by the organs of the chest. The vibration of the diaphragm causes vibration of the air in the hollow rubber tubing which is transmitted through the earpieces to the ears. Later, this design was modified by other scientists, who introduced a bell for sounds of lower pitch.
This design has been maintained and improved by current scientists and biomedical engineers.
The modern day stethoscope:
If a survey was done among medical professionals about the best stethoscope brand, chances are that 4 out of 5 of them will tell you about the Littmann. This brand has dominated the medical market that some people refer to all stethoscopes as ‘littmanns’.
Parts of the stethoscope and how it works:
This beautiful construction has four main pieces; the diaphragm, the bell, the PVC or rubber tubing and the ear pieces. The principles of activity are very similar to those of Bowle’s model.
When the stethoscope is placed on the body, the diaphragm creates a seal that blocks out external noise and allows the healthcare professional to hear the sounds of the body more clearly. The chest piece is typically made of metal and has a diaphragm and a bell. The diaphragm is used to listen to high frequency sounds, such as the sounds of the heart and lungs, while the bell is used to listen to low frequency sounds, such as the sounds of the abdomen. This difference in sound transmitted is due to the variation in the structure of the two parts. The diaphragm/drum has a larger surface area than the bell, as well as a flat surface, which transmits sounds of higher frequency better because of their short wave length. The stethoscope bell, on the other hand, is a smaller, cup-shaped surface that is designed to pick up low-frequency sounds. The bell is designed to create a seal against the skin, which prevents high-frequency sounds from entering the tubing. Instead, the bell is sensitive to low-frequency sounds, which have a longer wavelength and are better picked up by a small surface area. The bell is often used to listen for abnormal sounds such as bruits (abnormal noises), which are caused by turbulent blood flow in blood vessels and produce low-frequency sounds.
The tubing of the stethoscope is typically made of rubber or plastic and connects the chest piece to the earpieces. The tubing is designed to transmit sound waves from the chest piece to the earpieces without distorting the sound. The earpieces of a stethoscope are designed to convert the sound waves picked up by the diaphragm or bell into audible sound waves that can be heard by the listener. The earpieces consist of a pair of hollow tubes that are connected to the stethoscope’s tubing. The tubes are angled towards the listener’s ears and have soft ear tips that fit comfortably into the ear canal. The earpieces are designed to block out ambient noise and enhance the volume of the sounds being transmitted through the tubing. Talk about noise cancellation. Inside the earpieces, the vibrations transmitted through the tubing are converted into audible sound waves by a small, lightweight diaphragm. The diaphragm in the earpiece vibrates in response to the sound waves and creates a miniature sound wave in the air inside the earpiece. This miniature sound wave is then transmitted into the listener’s ear canal, where it is heard as sound. In the present day, the stethoscope has been digitalized to make the user experience better. These digital stethoscopes can even transmit the sounds heard onto other devices like computers. This beautiful construction has been a great help to medical professionals around the globe, as well as saving patients’ lives. For without it, how would doctors be able to effectively tell the difference between asthma and pneumonia which can be so similar? For the engineer, you should pick up interest in the mechanics of sound and how it can be engineered to make daily life easier. An- other lesson can also be picked up by you reading about the workings of a fetoscope, which is a modification of the stethoscope.
by Birungi Beatrice
Training in Medicine and Surgery