Reflection at Boundaries: When ultrasound waves encounter a boundary between two different media (e.g., muscle and bone), a portion of the wave is reflected back as an echo while the rest is transmitted.
Transducers: A device called a transducer is used to both emit high-frequency pulses and detect the returning echoes. It converts electrical energy into sound energy and vice versa.
Distance Calculation: By measuring the time delay between the emission of a pulse and the detection of its echo, the distance to the boundary can be calculated using the speed of sound in that medium.
Medical Sonography: Ultrasound is used to create real-time images of internal organs or a fetus. It is preferred over X-rays for prenatal care because it is non-ionizing and considered harmless to biological tissue.
SONAR (Sound Navigation and Ranging): Ships and submarines use ultrasound to map the ocean floor or detect underwater objects. The system calculates depth by timing how long it takes for a pulse to bounce off the seabed.
Industrial Testing: Ultrasound can detect internal cracks or flaws in metal components. A reflection occurring earlier than expected indicates a boundary (crack) within the material.
Natural Sources: Infrasound is produced by large-scale events such as earthquakes, volcanic eruptions, and lightning strikes. It is also used by some large animals, like elephants, for long-distance communication.
Seismic P-Waves: Primary waves are longitudinal infrasound waves that travel quickly through both solids and liquids. They are the first waves detected by seismometers during an earthquake.
Seismic S-Waves: Secondary waves are transverse waves that travel more slowly than P-waves and can only pass through solids. The inability of S-waves to pass through the Earth's outer core provides evidence that the core is liquid.
| Feature | Infrasound | Ultrasound |
|---|---|---|
| Frequency | Below | Above |
| Wavelength | Very long (meters to kilometers) | Very short (millimeters) |
| Primary Use | Seismic monitoring, animal comms | Medical imaging, SONAR, cleaning |
| Medium | Travels well through Earth/Air | Reflects easily at small boundaries |
The 'Divide by Two' Rule: In echo-location or SONAR problems, the time provided is often the 'round-trip' time. You must divide the total distance () by 2 to find the actual depth or distance to the object.
Unit Conversion: Always check if frequency is given in . Remember that . Ultrasound must be greater than .
Wave Types: Remember that P-waves are longitudinal (like sound) and S-waves are transverse. This distinction is critical when explaining why only P-waves pass through the liquid outer core.
Safety Justification: If asked why ultrasound is used for fetal scanning, focus on the fact that it is non-ionizing radiation, unlike X-rays which can damage DNA.
