CVA Signal Processing & FFT Analysis 1 — Questions and Answers
Question 1: What does FFT stand for in vibration analysis?
- Fast Fourier Transform (Correct answer)
- Frequency Field Testing
- Full Force Transmission
- Filtered Frequency Trace
Correct answer: Fast Fourier Transform
FFT stands for Fast Fourier Transform, a mathematical algorithm that converts a time-domain signal into its frequency-domain components.
Question 2: What is aliasing in digital signal processing?
- Signal amplification above the Nyquist frequency
- False frequencies appearing when the sampling rate is too low (Correct answer)
- Overlap between adjacent frequency bins
- Leakage of energy into neighboring spectral lines
Correct answer: False frequencies appearing when the sampling rate is too low
Aliasing occurs when the sampling rate is insufficient (below twice the highest frequency of interest), causing high-frequency signals to appear as false low-frequency components.
Question 3: What is spectral leakage in FFT analysis?
- Loss of signal energy due to sensor placement
- Spreading of energy into adjacent frequency bins caused by non-integer cycles in the time record (Correct answer)
- Noise entering the spectrum from electrical interference
- Attenuation of high-frequency components
Correct answer: Spreading of energy into adjacent frequency bins caused by non-integer cycles in the time record
Spectral leakage occurs when a signal's frequency does not fall exactly on an FFT bin, causing energy to spread into neighboring bins and masking nearby peaks.
Question 4: Which windowing function is most appropriate for analyzing synchronous (shaft-speed) vibration components?
- Hanning window (Correct answer)
- Flat Top window
- Rectangular (uniform) window
- Kaiser-Bessel window
Correct answer: Hanning window
The Hanning window is the most common general-purpose window for rotating machinery analysis as it provides a good balance between frequency resolution and leakage reduction.
Question 5: What is the frequency resolution of an FFT spectrum?
- Sampling rate divided by the number of spectral lines
- Maximum frequency divided by the number of averages
- 1 divided by the time record length (Correct answer)
- Bandwidth divided by the overlap percentage
Correct answer: 1 divided by the time record length
Frequency resolution equals 1 divided by the time record length (in seconds), so longer time records provide finer frequency resolution.
Question 6: What is the purpose of averaging multiple FFT spectra?
- To increase the maximum detectable frequency
- To reduce random noise and improve the signal-to-noise ratio (Correct answer)
- To extend the frequency resolution beyond the Nyquist limit
- To correct for transducer sensitivity variations
Correct answer: To reduce random noise and improve the signal-to-noise ratio
Averaging multiple FFT spectra reduces random noise by a factor proportional to the square root of the number of averages, improving the visibility of true vibration components.
CVA Signal Processing & FFT Analysis 1
What does FFT stand for in vibration analysis?