Interactive physics simulator
Ultrasound
Explore sound waves above 20 kHz — used in medical imaging, SONAR, and bat echolocation. Watch a piezoelectric probe emit ultrasound pulses into human tissue, track echoes returning from organ boundaries, and see how depth, frequency, and wave speed combine to build a real-time scan image.
Ultrasound Pulse-Echo Simulator
Watch ultrasound pulses travel through tissue layers, reflect at organ boundaries, and return to the probe. The B-scan display builds in real time, showing depth from echo timing using d = vt / 2.
Live Telemetry
- Frequency
- 5.0 MHz
- Wavelength
- 0.308 mm
- Wave Speed
- 1540 m/s
- Echo Time
- 65 µs
- Target Depth
- 5.0 cm
- Pulse Rate
- 1.0 kHz
What is Ultrasound?
Ultrasound is a longitudinal mechanical wave whose frequency exceeds 20,000 Hz (20 kHz) — above the upper limit of human hearing. Like audible sound, ultrasound requires a medium to travel through and cannot propagate through a vacuum. It obeys the same wave equations but its very short wavelength allows it to interact with tiny structures that ordinary sound would pass straight through.
The higher the frequency, the shorter the wavelength, and the smaller the detail an ultrasound system can detect. This trade-off — better resolution at higher frequency but less tissue penetration — governs every practical ultrasound design, from medical probes to bat biology to industrial scanners.
Key Facts
- Frequency: above 20,000 Hz (20 kHz).
- Longitudinal wave — needs a medium.
- Short wavelength → high resolution.
- Piezoelectric crystals emit and detect it.
- Cannot be heard by humans.
Key Formulas
v = f λ
d = v × t / 2
Where d = depth, v = wave speed in medium, t = round-trip echo time.
Speed in soft tissue: ~1540 m/s | Air: ~340 m/s
Speed in Different Media
| Medium | Speed (m/s) | Typical use |
|---|---|---|
| Air | 340 | Bat echolocation, parking sensor |
| Water / seawater | 1480–1500 | SONAR, fish finding |
| Soft tissue | 1540 | Medical imaging |
| Fat | 1450 | Body composition scans |
| Bone | 4080 | Bone density, NDT |
| Steel | 5960 | Industrial flaw detection |
Real-life Applications
- Medical imaging (sonography): Fetal scans, organ imaging, echocardiography.
- SONAR: Submarine detection, seabed mapping, fish finding.
- Bat echolocation: Navigating and hunting prey in darkness.
- Industrial NDT: Finding cracks and flaws in metal without cutting it.
- Ultrasonic cleaning: Microscopic cavitation bubbles clean surgical tools.
- Doppler ultrasound: Measuring blood flow speed and direction.
Solved Examples
An ultrasound pulse travels at 1540 m/s in soft tissue. The echo returns after 0.000065 s (65 µs). Find the depth of the reflecting structure.
- Use the pulse-echo formula: d = v × t / 2.
- d = 1540 × 0.000065 / 2.
- d = 0.10010 / 2 = 0.05005 m ≈ 5.0 cm.
Answer: Depth ≈ 5.0 cm
A medical ultrasound probe operates at 5 MHz. The speed of sound in soft tissue is 1540 m/s. Find the wavelength.
- Use v = f λ, so λ = v / f.
- λ = 1540 / (5 × 106) m.
- λ = 0.000308 m = 0.308 mm.
Answer: Wavelength = 0.308 mm
A sonar system emits ultrasound at 40 kHz into seawater (v = 1500 m/s). An echo returns in 0.20 s. Find the distance to the submarine.
- Distance = v × t / 2 (pulse-echo, total travel is there and back).
- d = 1500 × 0.20 / 2 = 300 / 2 = 150 m.
Answer: Distance to submarine = 150 m
Common Mistakes
- Forgetting to divide by 2 in d = vt/2 (the pulse travels there AND back).
- Using the speed of sound in air (340 m/s) instead of in tissue (1540 m/s) for medical problems.
- Confusing ultrasound (mechanical wave) with X-rays (electromagnetic radiation).
- Thinking higher frequency always gives deeper penetration — it actually gives less penetration but better resolution.
- Forgetting that ultrasound cannot travel through a vacuum — it needs a medium.
Practice Questions
1. What is the minimum frequency of ultrasound?
Ultrasound has a frequency above 20,000 Hz (20 kHz) — above the upper limit of normal human hearing.
2. A bat emits ultrasound at 80 kHz. What is the wavelength in air if v = 340 m/s?
λ = v / f = 340 / 80,000 = 0.00425 m = 4.25 mm.
3. Why does higher ultrasound frequency give better image resolution?
Higher frequency means shorter wavelength. A shorter wavelength can detect smaller structures, giving finer detail and better image resolution in medical scans.
4. An echo returns in 0.0002 s in soft tissue (v = 1540 m/s). Find the organ depth.
d = v × t / 2 = 1540 × 0.0002 / 2 = 0.154 m = 15.4 cm.
5. What is the piezoelectric effect and how is it used in ultrasound probes?
The piezoelectric effect converts electrical energy to mechanical vibrations (and vice versa). An electric pulse makes the crystal vibrate to emit ultrasound, and returning echoes compress the crystal to produce a voltage that is measured.
Quick Summary
- Ultrasound: frequency above 20 kHz, below human hearing threshold.
- Longitudinal mechanical wave — cannot travel through a vacuum.
- Produced and detected by piezoelectric transducers.
- Pulse-echo depth formula: d = v × t / 2.
- Higher frequency → shorter wavelength → better resolution but less penetration.
- Medical imaging speed in soft tissue: ~1540 m/s.
- Applications: medical imaging, SONAR, echolocation, NDT, cleaning, welding.
Frequently Asked Questions
What is ultrasound?
Ultrasound is a longitudinal mechanical wave with a frequency above 20,000 Hz (20 kHz) — higher than the upper limit of human hearing. It travels through solids, liquids, and gases.
What frequency range is ultrasound?
Ultrasound ranges from about 20 kHz to several GHz. Medical imaging typically uses 1–20 MHz. Industrial NDT uses 0.5–50 MHz. Therapeutic ultrasound uses 0.8–3 MHz.
How does ultrasound imaging work?
A piezoelectric transducer emits a short ultrasound pulse into the body. The pulse reflects at tissue boundaries and returns to the probe. A computer measures the echo delay and constructs an image of internal structures.
What is the pulse-echo formula for ultrasound?
The depth formula is d = v × t / 2, where d is the depth of the reflector, v is the speed of sound in the medium, and t is the round-trip travel time of the pulse.
What is the speed of ultrasound in soft tissue?
The speed of ultrasound in soft tissue is approximately 1540 m/s. In bone it is about 4080 m/s, in fat about 1450 m/s, and in air about 340 m/s.
What is a piezoelectric transducer?
A piezoelectric transducer is a device that converts electrical energy into ultrasonic vibrations (and vice versa). Crystals like quartz or PZT generate ultrasound when an alternating voltage is applied, and produce voltage when squeezed by returning echoes.
What is SONAR?
SONAR (Sound Navigation and Ranging) uses ultrasound to detect objects underwater. A pulse is emitted, and the time delay of the returning echo is used to calculate the distance to a submarine, ship hull, or ocean floor.
Why do bats use ultrasound?
Bats use biological sonar called echolocation. They emit ultrasound pulses at 20–100 kHz and listen to the returning echoes to detect and locate insects, obstacles, and predators in complete darkness.
What is the difference between A-scan and B-scan ultrasound?
An A-scan (amplitude scan) shows echo amplitude versus depth on a graph. A B-scan (brightness scan) maps echo amplitude to pixel brightness to build a 2D cross-sectional image.
Why is a gel applied before a medical ultrasound scan?
Ultrasound gel removes the air gap between the probe and skin. Air reflects almost all ultrasound due to the large acoustic impedance difference, so gel ensures efficient transmission of the pulse into the body.
What are the medical uses of ultrasound?
Medical ultrasound is used for fetal imaging during pregnancy, detecting gallstones and kidney stones, assessing heart function (echocardiography), guiding needle biopsies, and measuring blood flow velocity using Doppler ultrasound.
What are industrial uses of ultrasound?
Industrial uses include non-destructive testing (NDT) to find cracks in metal, ultrasonic cleaning (removing contaminants from surfaces), ultrasonic welding of plastics, flow measurement, and food processing.