Analysis of information sources in references of the Wikipedia article "Vibrato" in English language version.
In the case of string instruments, however, not only are they strongly directional, but the pattern of their directionality changes very rapidly with frequency. If you think of that pattern at a given frequency as beacons of sound, like the quills of a porcupine, then even the slight changes in pitch created by vibrato can cause those quills to be continually undulating.
The "fiery tone" that likely results from this phenomenon is an essential characteristic of good violins.
In the case of string instruments, however, not only are they strongly directional, but the pattern of their directionality changes very rapidly with frequency. If you think of that pattern at a given frequency as beacons of sound, like the quills of a porcupine, then even the slight changes in pitch created by vibrato can cause those quills to be continually undulating.