It is often necessary to restrict the magnetic field around an inductor to prevent coupling between that inductor and another one close by. This is achieved by enclosing the inductor in an earthed metal can, this being known as 'screening'. The can should be at least 1.5 times the coil diameter from the coil, otherwise the Q of the latter will be degraded. Generally, it is not necessary to screen coils which are separated by at least three coil diameters or which are tuned to different frequencies.
Similarly it is often required to screen one part of a circuit from another, eg to prevent feedback from the output of a high-gain amplifier to its input, or to prevent an unwanted strong signal getting to the input of an amplifier.
Aluminium and preferably copper are satisfactory screening materials at radio frequencies, but a high-permeability alloy such as mu-metal is necessary at audio frequencies.
In the photo above the larger metal box on the left (lid removed), prevents interaction between the electromagnetic fields within the box, and those outside it. The screening provided on the IF-transformers (the three smaller metal boxes) serves a similar purpose.
