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Wavelength at the ultrasonic fundamental frequency

Aviation Mechanic General (AMG) practice question 15 of 21 on Inspection Concepts and Techniques. ACS element AM.I.K.K3 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONIn ultrasonic resonance thickness testing, the fundamental frequency is the frequency at which the wavelength is twice the thickness of the specimen. For a 0.250-inch-thick plate, what wavelength corresponds to the fundamental frequency?

  1. A0.125 inch
  2. B0.500 inch
  3. C0.250 inch, equal to the plate thickness
Show the answer and explanation

Correct answer: B. 0.500 inch

  • A — incorrect Half the thickness is not the fundamental condition; the wavelength must be twice the thickness.
  • B — correct At the fundamental frequency the wavelength is twice the specimen thickness, so for a 0.250-inch plate the wavelength is 0.500 inch and the reflected wave returns in phase with the transmitted wave.
  • C — incorrect A wavelength equal to the thickness occurs at a higher frequency that also produces resonance, but it is not the fundamental frequency defined in the handbook.

BACKGROUND

The resonance method of ultrasonic inspection differs from pulse methods in that the transmission frequency may be continuously varied. It is used principally for thickness measurement when the two sides of the material are smooth and parallel and the back side is inaccessible. If the frequency of the ultrasonic wave is such that its wavelength is twice the specimen thickness, called the fundamental frequency, the reflected wave arrives back at the transducer in phase with the original transmission and strengthens the signal through constructive interference. Increasing the frequency so that three times the wavelength equals four times the thickness produces cancellation, and a further increase to a wavelength equal to the thickness produces resonance once more.

STUDY TIP

For resonance calculations, remember that the fundamental wavelength is exactly two times the thickness; everything else in the series follows from that.

KEY POINTS

  • Resonance testing varies frequency to measure thickness from one side.
  • At the fundamental frequency, wavelength equals twice the specimen thickness.
  • A 0.250-inch plate resonates at a 0.500-inch wavelength.

SOURCES

  • FAA-H-8083-30B ch10 "Resonance"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.