Ultrasound and Echo Sounding
Ultrasound is sound above 20 kilohertz. With known sound speed and travel paths, echo times locate reflectors in medical scans, metal testing and sea-floor surveys. Acoustic-impedance differences explain reflected pulses and the use of gel against the skin.
What a learner can do afterwards
- Gives the approximate 20 Hz to 20 kHz hearing range of a young person and defines ultrasound as sound above 20 kHz
- Calculates depth or thickness from a round-trip echo time and known sound speed using the stated travel path
- Explains reflection at a boundary with different acoustic impedances and why gel fills air gaps during a scan
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A young person's hearing range is roughly 20 Hz to 20 kHz. Hearing limits vary between people and often change with age. Ultrasound is defined as sound above 20 kHz, or 20,000 Hz. Being too faint for someone to hear does not make a sound ultrasonic. Bats and dolphins use ultrasonic echoes. Ultrasound also helps locate cracks and form medical images.
For a straight path in one uniform material, let v be sound speed and t the time from pulse emission to echo return at the same probe. The total path is vt, twice the reflector distance d, so d = vt/2. A vertical path gives depth. A path perpendicular to a plate's back face gives thickness. At fixed speed, doubling the return time doubles the distance.
A ship sends a pulse vertically downward. Use a constant seawater sound speed of 1500 m/s. The echo returns after 0.4 s. The total path is 1500 × 0.4 = 600 m. Depth is half that distance: 600 / 2 = 300 m. Reporting 600 m as the depth would count both journeys.
Acoustic impedance is density times sound speed. In an ideal model of a pulse hitting a flat boundary head-on, different impedances cause reflection; matched impedances give no reflection. The material names alone do not decide this. Gel fills air gaps between probe and skin. Air and tissue have very different impedances, so an air gap would reflect much of the pulse before it entered the body.
In soft tissue, higher ultrasound frequencies usually give finer image detail but are absorbed more strongly, limiting penetration. Frequency selection balances detail and depth. Scanners combine echo times and strengths from many paths to form images, including images of a fetus. An echocardiogram uses repeated ultrasound measurements to show heart structures and their movement.
Ultrasound is above 20 kHz. For a straight round trip at known uniform speed, reflector distance is vt/2. Impedance differences explain boundary echoes and the need to fill air gaps.
Ultrasound lesson, questions, worked solutions and flashcards written for this dataset.
Reading passages and flashcards written for this dataset. Ultrasound medical imaging is a linked Khan Academy video.
More to read
What an echo timer measures
The timer records an interval between sending sound and receiving its echo. It does not measure depth directly. To calculate a distance, you need the sound speed and enough information about the route.
Suppose a pulse crosses a uniform plate perpendicular to its back face and returns to the sending probe. With sound speed 5000 m/s and return time 0.00002 s, the total path is 0.10 m. The plate thickness is half that distance, 0.05 m. A tilted route would be longer than the thickness. Several layers with different sound speeds would also need more information than this calculation uses.
The echo's arrival time and strength describe different features of the measurement. Time helps locate a reflector. Strength tells how much sound returns. In the ideal head-on boundary model, equal acoustic impedances produce no reflection even if the materials have different names.
Building a moving heart image
An ultrasound probe sends pulses and receives returning sound. Echo timing helps place reflecting structures along each measured path. Repeated measurements allow a scanner to show movement, including the movement of heart walls and valves. An ultrasound examination of the heart is called an echocardiogram.
For a scan through the chest, gel fills air gaps between the probe and skin. Without good sound transmission into the body, useful echoes from deeper structures would be much weaker.
A two-dimensional image shows a slice through the heart. Other probe positions provide other views. Collecting several ordinary views does not automatically make a three-dimensional image; three-dimensional echocardiography acquires data for that purpose.
The physics explains how signals contribute to images. Interpreting a scan also requires clinical information and professional training. This passage is not a guide to diagnosing a heart condition.
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