Null (physics)

In physics a null is a point in a field where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be scalar, vector or tensor in nature. Common situations where nulls arise are in the polar patterns of microphones and antennae, and nulls caused by reflections of waves.

A common polar pattern for microphones is the cardioid. This has a single direction in which the microphone does not respond to impinging sound waves. Highly directional (shotgun) microphones have more complex polar patterns. These microphones have a large, narrow lobe in the main direction of sound reception but also a smaller lobe in the opposite direction and usually also several other smaller lobes. This pattern is achieved by wave cancellation inside the body of the microphone. Between each of these lobes is a null direction where no sound at all is detected.

A common, and basic, radio antenna is the dipole. This has a figure-of-eight polar pattern with two nulls on opposite sides. Highly directional antennae, such as the Yagi have polar patterns very similar to highly directional microphones and for similar reasons. That is, they have multiple small lobes off the main direction with nulls between them.

Standing waves can be caused by a wave being reflected back through the transmission medium in which it arrived. If the incident and reflected waves are transmitted without loss then there will be points along the transmission path where the incident and reflected waves exactly cancel each other due to them being in antiphase.

Standing waves are found in wind musical instruments. Wind instruments consist of a tube which acts as an acoustic transmission line in which standing waves are set up. Open ended tubes must have zero air pressure change at the end of the tube so this point is a pressure null. Closed tubes must have zero air velocity at the end of the tube so this point is a velocity null. There may further nulls along the tube depending on the vibration mode that has been set up by the instrument player. Higher modes have more nulls.

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