Unit Three: Measurement and Analysis III: Advanced

Question 1

AC coupling

Answer 1

a connector between a transducer and a data collection device with an analog high pass filter having a low cutoff frequency, e.g. 1 Hz, to eliminate any DC voltage in the signal.

Question 2

DC coupling

Answer 2

a connector between a transducer and a data collection device without any filtering, which allows both DC and AC signals to pass.

Question 3

Pretriggering

Answer 3

continuous sampling of the input signal to allow capturing the signal leading up to the trigger event which occurs at the maximum designated signal level.

Question 4

Frequency response function

Answer 4

the relation between an output to an input as a function of frequency

Question 5

Impulse response function

Answer 5

the response, expressed as a function of frequency, of a system to an impulse

Question 6

Autospectrum

Answer 6

the power spectrum density of an input signal as a function of frequency.

Question 7

Cross-spectrum

Answer 7

a complex function containing both amplitude and phase as a function of frequency, containing information of both input and output signals.

Question 8

Coherence function

Answer 8

a measure of the effect of the extraneous noise in the system in the measurement of the frequency response function.

Question 9

Coherent output power

Answer 9

that part of the power of the output that is coherent with the input to a system.

Question 10

Overlap data processing

Answer 10

digital signal processing in which time records are overlapped to avoid gaps in the records that could result in the failure to capture a transient signal.

Question 11

Zoom transform

Answer 11

a digital processing method in which a specific frequency band is expanded to increase the frequency resolution.

Question 12

Waterfall plots

Answer 12

graphic representations of spectral data continuously updated at periodic intervals, capturing the change in the spectrum as the source changes.

Question 13

Instrumented impact hammer

Answer 13

a hammer with a force gage attached to the tip, used in experimental modal analysis.

Question 14

Experimental modal analysis

Answer 14

a method of determining frequency response for a system based on measuring the frequency response
due to input and using reciprocity to minimize the number of data points taken.

Question 15

Frequency response function matrix

Answer 15

a matrix of frequency response functions for each combination of excitation and response at locations on a structure or in a space.

Question 16

Short-Time Fourier transforms

Answer 16

a Fourier transform that contains a window function that limits the integration to a short time.

Question 17

Wavelet transform

Answer 17

a transform used to increase the frequency resolution of signals with short time duration. A wave-like expression is used in the transformation.

Question 18

Cepstral analysis

Answer 18

the transform of the variation in amplitude of a spectrum. Useful in harmonic analysis to uncover relationships between many discrete tones.

Question 19

Multiple input/single output analysis

Answer 19

relating a combined spectrum composed of multiple noise sources to the individual noise sources making up the spectrum through a cross spectrum two-dimensional matrix analysis.