Reported April 2007

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 patient's 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 pattern -- 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 "lub-dub" sound from a heartbeat with the first beat being the strongest. An ultrasound stethoscope yields a "ta-da-ta" 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 -- a noise level comparable to an alarm clock or a busy street -- and are useless above 90 decibels. 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.

If you would like more information, please contact:

Adrianus J.M. Houtsma, Ph.D.
US Army Aeromedical Research Laboratory
Fort Rucker, AL
(334) 255-6959
adrianus.houtsma@us.army.mil

Acoustical Society of America
Melville, NY 11747-4502
(516) 576-2360
http://asa.aip.org
asa@aip.org

DID YOU KNOW?

The stethoscope was invented in France in 1816 by René-Théophile-Hyacinthe Laënnec. In 1852, George Camman perfected the instrument design for commercial production, which has been the standard ever since.

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