These can often be easily added to the mould by the audiologist should the need arise. Disadvantages may cause pain if children fall and bump their ear. For this reason some audiologists are reluctant to use hard materials with very young or clumsy children. There are many different flexible materials available which provide very good seals. The amount of acoustic feedback control that the material exhibits can be directly related to its flexibility and grip.
Advantages form an airtight seal which minimises feedback.
Disadvantages are more difficult to insert because of their tendency to grip the ear are not as comfortable as a hard mould. Soft acrylics Soft acrylic is essentially a modification of standard hard acrylic. Advantages are somewhat flexible have good feedback control properties. Disadvantages can be misshapen by temperatures of 33 to 35 degrees and tend to expand and fit much more tightly after being in the ear for 30 to 40 minutes. This group actually includes plastics, vinyl and polypropylene.