A stethoscope is a tool that allows people to hear sound inside an object, and a Doppler stethoscope is a particular variety. Regular stethoscopes, used by many medical professionals, amplify sound produced by the object. A Doppler stethoscope, on the other hand, can send sound waves into the body and register the frequency at which they bounce back. Shift in frequency of a moving object, such as the heart inside a person's chest, or an ambulance traveling toward a traffic junction, is called the Doppler Effect. Compared to a regular stethoscope, a Doppler stethoscope is more useful in environments with high ambient sound, such as in a helicopter.
Sound travels in a wave, and can have different frequencies. A frequency is the number of times a single wave is repeated in a set period of time. Sound is measured in hertz (Hz) units, which individually represent one complete wave cycle per second. Humans can normally hear sounds within the range of 20 to 20,000 Hz, but the noises that the body makes are typically between 100 to 200 Hz. These noises include heartbeat and breathing, both basically caused by movement of the involved organs. The heart contracts and relaxes to pump blood, and the lungs expand and contract to take in air.
Conventional stethoscopes are placed on the skin and the doctor then listens to the amplified sounds that the body makes. A Doppler stethoscope is also placed on the body, but it produces sound in the ultrasound frequency range and sends these waves into the body. Ultrasound refers to any sound above the 20,000 Hz top limit of hearing for humans. Although the doctor cannot hear the ultrasound bouncing back off the relevant organs, the Doppler stethoscope turns the ultrasound into a sound signal that the doctor can hear.
Only moving body parts are suitable for the Doppler stethoscope, such as the lungs and the circulatory system, because movement of the relevant object is required for the Doppler effect to be present. When an object producing sound comes closer to anything that listens to it, the frequency of the sound heard gets higher; when the object moves further away, it becomes lower. This effect is used by emergency services, where sirens sound different if they are coming closer or if they are moving away.
When the ultrasound bounces back off the moving part, it changes in frequency depending on how far away the part is at that time. For example, a deflated lung is further away from the front of the chest than an inflated lung, and therefore the ultrasound detects the change in frequency. When it registers the change, it alters the audible sound that the doctor hears.
Compared to regular stethoscopes, the Doppler stethoscope has one significant advantage. It hears the ultrasound it produces itself, and does not hear most of the noise already present in the environment. In noisy places like an emergency medical helicopter, or in combat situations, the Doppler stethoscope gives a more accurate and clearer picture of the health of an individual patient. It also has applications in fetal ultrasound, where it may be used to detect the heartbeat of a baby in the womb.