The fundamental vibration mode of a tuning fork radiates sound as a longitudinal (or linear) quadrupole sound source with a well-defined transition between the near-field and far-field radiated patterns. But, it can be effectively demonstrated by touching the stem of a vibrating fork to a table top, door, or piano soundboard. This stem motion is very small, and difficult to feel if you place a finger tip at the bottom of the stem. However, the stem actually vibrates up and down at the fundamental frequency as well as at the second harmonic, 852 Hz - twice the frequency of the fundamental (even there is no vibrational mode of the fork at this frequency). When vibrating in the fundamental mode, it would appear that the stem of the fork is stationary. This is a symmetric mode, since the two tines are mirror images of each other.Ī video on my YouTube Channel shows the slow-motion oscillation (shot with a high speed camera at 1200fps) of a 125 Hz tuning fork vibrating in its fundamental mode of vibration. The two tines of the fork alternately move toward and away from each other, each bending like a cantilever beam, fixed at the stem and free at the other end. The fundamental mode of vibration is the mode most commonly associated with tuning forks it is the mode shape whose frequency is printed on the fork, which in this case is 426 Hz.
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