--- title: "Ultrasound and Echo Sounding" description: "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 expl" canonical: https://lightmysky.com/learn/science/ultrasound-and-echo-sounding-mt_nE1uPfjhX- source: https://lightmysky.com/learn/science/ultrasound-and-echo-sounding-mt_nE1uPfjhX-.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Waves, Light & Sound · Ages 15 to 16 Page: https://lightmysky.com/learn/science/ultrasound-and-echo-sounding-mt_nE1uPfjhX- ## Ready when they can - 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 ## Lesson: Seeing with echoes 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. *(drawing: Timing starts at emission and ends at echo reception. The cells show event order, not a scaled travel path.)* **Example.** 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [How Sound Waves Travel](https://lightmysky.com/learn/science/how-sound-waves-travel-mt_DQk9oDE7gr) - [The wave equation](https://lightmysky.com/learn/science/the-wave-equation-mt_wmKhTiXt1p)