BACKGROUND: A new type of stethoscope relies on ultrasound to enable doctors to hear the sounds of the body in extremely loud situations, such as during the transportation of patients in MedEVAC helicopters, or wounded soldiers in Blackhawk helicopters.
HOW IT WORKS: These new ultrasound models transmit a sound signal into the patientıs body. This sound is reflected back to the stethoscope at a slightly different frequency because it bounces off the internal organs, changing the sound wave patternı essentially, the Doppler effect. The difference in frequencies between the transmitted sound wave and the returning sound wave received by the instrument can be computed to determine the motion of the internal organs. This difference in frequency is then converted into audible sound. Ultrasound stethoscopes produce a markedly different sound than conventional ones. An acoustic stethoscope yields a ılub-dubı sound from a heartbeat with the first beat being the strongest. An ultrasound stethoscope yields a ıta-da-taı pattern with the second beat being the strongest.
THE PROBLEM: Traditional stethoscopes transmit and amplify sound within the range of human hearing: from 20 hertz to 20,000 hertz. Most body sound, such as that of the heart and lungs, fall into the 100 to 200 hertz range. Current acoustic stethoscopes detect and amplify vibrations that allow doctors to hear the heart and lungs better. However, they become difficult to use around 80 decibels ı a noise level comparable to an alarm clock or a busy street, -- and are useless above 90 decibels. Modern electronic stethoscopes improved that threshold to 95 decibels by replacing the earpieces with loudspeaker inserts, which provide a better seal over the ear canal. They also have electrical cables instead of the conventional tubing, decreasing acoustic noise. But this is still not sufficient to make the instruments useful in very noisy environments. The ultrasound stethoscope is nearly impervious to loud noise and can make accurate readings at noise levels up to 120 decibels, similar to the volume experienced in the front row at a rock concert.
THE DOPPLER EFFECT: Both sound waves and light waves exhibit the Doppler Effect. Just as a train whistle will sound higher as it approaches a platform and then become lower in pitch as it moves away, light emitted by a moving object is perceived to increase in frequency (a blue shift) if it is moving toward the observer; if the object is moving away from us, it will be shifted toward the red end of the spectrum.
The Acoustical Society of America contributed to the information contained in the TV portion of this report.