Null (physics)

Wind instruments consist of a tube which acts as an acoustic transmission line in which standing waves are set up.

There may be further nulls along the tube depending on the vibration mode that has been set up by the instrument player.

These waves will cancel periodically along the line causing nulls every half wavelength.

Nulls on transmission lines are very sharp, in contrast to the peaks which are broad and flat.

With the measurement of VSWR and the null positions the value of the terminating impedance's magnitude and phase can be calculated.

The measurement is carried out by placing an instrument to detect voltage or current in the central branch.

An advantage of the bridge method is that this instrument does not need to be calibrated since it is only required detect nulls, not to actually measure a voltage.