Visualizing the invisible
Smart aliasing
WHY:
For all measurements the Nyquist limit (Nyquist frequency = sampling frequency / 2) sets the maximal frequency that can be measured without aliasing. Aliasing is the phenomena when frequencies under-sampled with a lower sampling frequency get aliased onto a lower apparent frequency, often over top of actual low frequencies, thereby making the measurements unusable.
The sampling frequency, in case of high-speed cameras, is the frame rate (fps). In order to measure structural dynamic responses up to e.g. 2500 Hz, a sampling of at least 5000 Hz or fps, would be required. Cameras, capable of such rates alone, cost upwards of multiple tens-of-thousands (even substantially surpassing a hundred thousand) euro.
WHAT:
Smart aliasing is a proprietary technique we have developed to enable visual modal analysis (VMA) measurements of high frequency structural responses using our more affordable and easy-to-use USB 3.1 high-speed systems running our Motion Scope tracker software. The high-frequency capability enables the use of our systems for acoustic analysis.
HOW:
The method works by forcing aliasing in a controlled manner using a shaker and matching the excitation to the camera acquisition. The measured response then gets dealiased to produce the true and genuine high-frequency spectrum far beyond the Nyquist limit.
EXAMPLE:
For a demonstration of a smart alias measurement, see the following video.
The Nyquist limit for that case was at 185 Hz, however a frequency response from 200 to 2500 Hz, almost 14x above the Nyquist limit, was appropriately measured from a single recording!