Q2. What is the main effect of amplitude modulation on the Doppler signal?
Summation on the photodetector over a changing population of speckles has two important effects on the Doppler signal: amplitude modulation and phase modulation.
Q3. What causes the noise in the photodetector?
Even when adequate signal amplitude is maintained, however, dynamic changes in the sampled speckle pattern cause noise in the photodetector output phase which results in ‘speckle noise’ in the vibrometer output.
Q4. What is the key to locating defects on more complex structures?
Techniques such as 3D-SLDV and its use for strain measurements [56] may hold the key to locating defects on more complex structures, particularly if the goal is to develop a near real-time inspection technique for instant detection of the onset of damage.
Q5. What is the significance of CSLDV for modal testing?
For modal testing, the significance of CSLDV is that it replaces the exclusively temporal view with much greater emphasis on the spatial properties of a structure’s modal model.
Q6. What is the role of the eardrum in the acoustic match between air?
The eardrum and the three ossicles of the mammalian middle ear act as an acoustic impedance match between air and the fluid-filled inner ear, from where electrical impulses are sent to the brain.
Q7. What is the main advantage of SLDV?
While SLDV offers much to vibration testing generally, it is in the area of Experimental Modal Analysis (EMA) that SLDV has been so influential.
Q8. What are the main applications of radial vibration measurements?
Radial vibration measurements are of particular interest in automotive powertrain [69], hard disks and their drive spindles [81, 82], and tool condition monitoring in turning [83] and milling [84].
Q9. What is the problem with optical crosstalk?
Optical crosstalk would be a particular concern because the three beams would have to be positioned so closely together to achieve the required spatial resolution, typically microns.
Q10. What was the first paper to describe the Rayleigh scattering of light?
Their seminal paper described measurement of “Doppler shifts in the Rayleigh scattered light at [flow] velocities as low as 0.007 cm/sec” at a time when the laser was still in its infancy.
Q11. What was the need for the scan frequency to be larger than twice the maximum frequency of interest?
In a study using the lifting approach to extract the natural frequencies and massnormalized mode shapes of a free-free beam under impact excitation [33], the need for the scan frequency to be larger than twice the maximum frequency of interest was demonstrated.
Q12. What is the corollary of the definition of a beam?
In all cases, orientation of the beam(s) determines the component of velocity measured with the corollary that it is the small but inevitable misalignments that usually determine measurement accuracy.
Q13. How many individual transducers can be used to measure a structure’s response?
It would be impractical to attach 100s or 1000s of individual transducers to capture a structure’s mode shapes in detail but with an LDV it is possible to measure rapidly the response with a spatial resolution limited only by the laser beam diameter, typically a few tenths of a mm, and the time required to capture each time record.
Q14. What was the first significant innovation from this period?
A further significant innovation from this period was the introduction of a parallel beam instrument [9] for torsional vibration measurement on rotors.
Q15. What is the challenge of doing full justice to a technique that has been applied so widely?
To do full justice to a technique that has been applied so widely not only across the many sectors of mechanical, electrical and civil engineering, and their underpinning science, but also in areas such as medieval fresco condition [46], fruit ripeness [47] and infant respiratory health [48] is quite a challenge.
Q16. What are the advantages of LDV over microscopy?
LDV’s distinct advantages over microscopy based techniques include three-dimensional measurements [59, 60] of picometre displacements, over a wide area [61] and at GHz frequencies.
Q17. What is the vibration map for a rolling tyre wall?
Figure 9 shows a vibration map for a rolling tyre wall as a function of radius, quantifying the extent of the bulge and recovery in the tyre wall as the tracked point approaches and leaves thecontact point on the roller.