What type of material can acoustic sensors not detect?

Acoustic sensors operate by detecting sound waves or vibrations that are produced by various materials when struck or manipulated. Each of the materials listed has different acoustic properties that can affect the sensors' ability to identify them.

In the context of the answer provided, plexiglass is a type of acrylic that has unique properties, such as being lightweight and having a certain rigidity that can lead to very low sound frequency responses compared to others like tempered glass. It may not produce distinct or detectable sounds when manipulated or broken due to its particular molecular structure.

Polycarbonate, acrylic, and tempered glass typically produce more noticeable sound frequencies when impacted, which acoustic sensors can easily pick up. Therefore, these materials are more likely to be detected by acoustic sensors, making them suitable candidates for detection, while plexiglass may not create the same level of detectable vibrations.

This explains why plexiglass is portrayed as a material that acoustic sensors encounter difficulties detecting, distinguishing it from the others listed.