Describe the uses of ultrasound in nondestructive testing of materials, medical scanning of soft tissue and sonar including calculation of depth or distance from time and wave speed
Describe the uses of ultrasound in nondestructive testing of materials, medical scanning of soft tissue and sonar, including calculation of depth or distance from time and wave speed.
Key Concepts
Sound is a longitudinal wave that requires a material medium to travel.
Ultrasound refers to sound waves with frequencies above the upper limit of human hearing (typically > 20 kHz).
The speed of sound, \$v\$, depends on the medium’s density and elasticity.
When an ultrasonic pulse is reflected from a surface, the travel time can be used to determine the distance to that surface.
Ultrasound in Nondestructive Testing (NDT)
Ultrasound is used to detect internal flaws in solids without damaging the part. A transducer generates a short pulse that travels through the material; any discontinuity (crack, void, inclusion) reflects part of the pulse back to the transducer.
Typical frequencies: 1 – 10 MHz (higher frequencies give better resolution but lower penetration).
Information obtained: location of a flaw, size (approximate), and sometimes the nature of the defect.
The distance to a flaw is calculated from the round‑trip travel time:
\$\$
d = \frac{v\,t}{2}
\$\$
where \$d\$ is the depth of the flaw, \$v\$ is the speed of sound in the material, and \$t\$ is the measured time between emission and reception of the echo.
Medical Ultrasound Scanning
Medical ultrasound uses high‑frequency sound waves (2 – 15 MHz) to produce images of soft tissues such as organs, blood vessels, and a developing fetus. The principle is the same as NDT: a transducer emits pulses and receives echoes from tissue boundaries.
Soft tissues have acoustic impedances that differ enough to produce detectable echoes.
Real‑time images are formed by scanning the beam across the body and displaying the echo intensity versus depth.
Safety: ultrasound does not use ionising radiation and is considered safe for routine diagnostic use.
Depth calculation uses the same formula as above, but the speed of sound in soft tissue is taken as approximately \$1540\ \text{m s}^{-1}\$.
Sonar (Sound Navigation and Ranging)
Sonar systems emit ultrasonic pulses into water and listen for echoes from objects or the seabed. It is used for navigation, depth sounding, and detection of underwater objects.
Active sonar: emits a pulse and measures the return time.
Passive sonar: listens only for sounds produced by other objects (not covered here).
Typical frequencies: 10 kHz – 1 MHz, depending on required range and resolution.
Depth or distance is found from the travel time of the pulse:
\$\$
\text{Depth} = \frac{v_{\text{water}}\,t}{2}
\$\$
where \$v_{\text{water}} \approx 1480\ \text{m s}^{-1}\$ at 20 °C.
Worked Example: Depth from Sonar Echo
A sonar system emits a pulse that returns after \$0.12\ \text{s}\$. Calculate the depth of the seabed.
Use the speed of sound in seawater: \$v = 1500\ \text{m s}^{-1}\$ (approximate).
Therefore, the seabed is \$90\ \text{m}\$ below the sonar transducer.
Comparison of Ultrasound Applications
Application
Typical Frequency
Medium
Speed of Sound Used
Key Use
Nondestructive Testing
1 – 10 MHz
Solids (metal, composite)
Varies (e.g., steel \$≈ 5900\ \text{m s}^{-1}\$)
Detect internal flaws
Medical Scanning
2 – 15 MHz
Soft tissue
≈ 1540 m s⁻¹
Image organs and fetus
Sonar
10 kHz – 1 MHz
Water
≈ 1480 m s⁻¹ (fresh water) / 1500 m s⁻¹ (seawater)
Measure depth, locate objects
Suggested diagram: Schematic of an ultrasonic pulse travelling through a material, reflecting from a flaw, and returning to the transducer (for NDT). Include labels for the transducer, pulse, flaw, travel time \$t\$, and calculated depth \$d\$.
Suggested diagram: Cross‑section of a medical ultrasound probe scanning soft tissue, showing emitted pulses, reflected echoes from tissue boundaries, and formation of a 2‑D image.
Suggested diagram: Sonar depth sounding – a ship‑mounted transducer emitting a pulse, the pulse reflecting off the seabed, and the round‑trip time \$t\$ used to calculate depth.
Summary
Ultrasound frequencies are far above the audible range and are chosen to balance resolution and penetration.
All three applications (NDT, medical imaging, sonar) rely on the same fundamental relationship between distance, speed, and travel time.
Knowing the appropriate speed of sound for the medium is essential for accurate depth calculations.