It is unrealistic to design a completely enclosed BLS. Designers usually need to open some
holes on the BLS for air ventilation, or signal traces pathing.

In low frequencies, BLS products are often used to restrain magnetic fields which are usually
caused by low impedance sources (e.g., inductors, transformers). In these cases, the
mechanism of EMI shielding becomes more complicated. In the quasistatic magnetic field,
the magnetic flux will be induced to go through the materials with higher permeability.
Therefore, the shielding effectiveness is determined by the structure of the shielding, the
material thickness, and the permeability. When the frequency gets higher, the reflection loss
caused by eddy currents will become more dominant. That means the material conductivity
becomes the key factor influencing the shielding effectiveness.

Examined as a structure only, the BLS is simply is a piece of bent meta. In terms of EMI
shielding performance however, there remains a multitude of factors which could determine
actual SE. In general, the primary consideration of BLS is the apertures / holes. Factors
pertaining to material conductivity and permeability are not as important but should be
considered for low frequency shielding. In higher frequencies, the cavity resonance makes the
shielding effectiveness decrease sharply. However, it has been proven that full wave EMI
simulation is especially useful in addressing these problems. Combined with absorber
materials, high frequency EMI radiation could be mitigated effectively and significantly.