Ultrasonic Probe Explained | Piezoelectric Element, Near Field, Beam Spread + 2 Worked Problems

Every ultrasonic inspection starts inside a probe the size of your thumb. What sits inside it, and what shape of sound comes out, decides what you can find and what you will miss. This video opens up an ultrasonic probe in 3D and follows its beam into steel. You'll learn how the piezoelectric effect lets one disc send and receive sound, why the element is half a wavelength thick, what the backing and matching layer do, why the near field is uneven and how long it is, how far the beam spreads, how frequency trades resolution against penetration, and how to compare echoes in decibels. 0:00 Intro 0:27 The idea: piezoelectric element, probe layers, near field, beam spread, frequency, decibels 2:29 Problem 1: near field and beam spread of a 10 mm, 5 MHz probe in steel 4:04 Problem 2: attenuation from back-wall echoes, and a flaw echo in dB 5:40 Recap Key equations Element thickness: t = λ/2, so f₀ = v/2t Wavelength: λ = v/f Near field: N = D²/4λ Beam spread (half-angle to the beam edge): sin γ = 1.22λ/D Decibels (amplitude): ΔdB = 20 log₁₀(A₁/A₂); 6 dB = ×2, 20 dB = ×10 Answers Problem 1: λ = 1.18 mm; N = 21.1 mm; γ = 8.3°; beam width at 100 mm ≈ 29 mm. With a 2 MHz probe: N = 8.4 mm, γ = 21°, width ≈ 78 mm. Problem 2: 20 log(80/20) = 12.0 dB over 100 mm of extra path, so α ≈ 0.12 dB/mm (before beam-spread and surface corrections). A flaw needing +18 dB is 10^(−18/20) = 0.126, about 1/8 of the back-wall echo. Useful for students of physics, mechanical and materials engineering, and anyone learning ultrasonic testing (UT) or non-destructive testing (NDT). #UltrasonicTesting #Piezoelectric #NDT #Physics #Engineering