Resonance is a phenomenon that structures capable of oscillation will oscillate in sympathy with relatively feeble external forces which act periodically and whose oscillation period coincides with that of the resonating structure. While it is resonating, the structure stores up energy.

Under certain circumstances the amount of energy stored up in this way may become so great that it brings about the destruction or collapse of the structure. A simple example of a resonating structure is a child’s swing (Fig 1a). It is a pendulum, which is given a push or a thrust in the swinging direction each time it reaches its maximum deflection. Its energy build up i.e., its resonance, is directly evident from the increasing amplitude of the deflection of the swing. Another example is a liquid in a U-shaped tube (Fig.1b). The liquid can be set in motion by blowing into one end of the tube, and by blowing it periodically at the appropriate instant, the amplitude of its oscillations is progressively increased.

The effective length of the tube can be varied by means of an adjustable disc at the other end. The vibrations i.e sound waves are reflected by this disc, and on suitably adjusting its position, a stationery wave will be produced in the tube, and resonance occurs.

This happens when the distance between the two discs is equal to an odd multiple of one-quarter of the wavelength of the sound waves set up in the tube, and vibration nodes and antinodes are formed. These can be indicated by introducing a small quantity of some suitably light powder e.g., lycopodium powder, into the tube. The powder congregates in a heap at each node. The nodes are thus made Visible, and the distance between them can be measured. The distance between two successive nodes is equal to half the wavelength of the sound waves set up in the tube.

Resonance effects are also observed in connection with electromagnetic phenomena. The most well known and important example is the excitation of an electromagnetic oscillatory circuit, comprising a self-inductance L and capacity C by an alternating voltage (Fig.3) In the circuit the energy oscillate between its electrical state in the condenser (Fig.3a) and its magnetic state in the magnetic field of self-induction (Fig.3b) If the natural period of vibration and therefore the frequency of the oscillatory circuit corresponds to that of the alternating voltage, resonance will occur.

The circuit will in that case absorb the maximum amount of energy from the source of energy that produces the excitation. Radio transmitters and receivers are turned with the aid of this resonance effect. To prevent the energy attaining disastrously high values, resistances are included in the circuit; these cause energy losses in the form of heat.

Another phenomenon that acoustic and electric vibrations have in common is echo, i.e., the reflection of sound waves or electromagnetic waves from obstacles they encounter (Fig.4).