A specially constructed microphone that responds only when the protected object is disturbed is known as a:

The term "contact microphone" refers to a device designed to pick up sound through vibrations directly from a surface rather than through the air. This feature makes it particularly useful for security applications, as it can detect disturbances in the protected object itself, such as tampering or unauthorized access. When a protected object is moved or struck, the vibrations travel directly to the microphone, allowing it to capture the sound and alert security personnel. This precise operation is essential in environments where minimizing false alarms is crucial.

In contrast, the other options represent different types of microphones or technologies that do not specifically focus on detecting vibrations from a surface contact. For example, a special acoustic microphone may capture sound from the air without the specificity of surface disturbance detection. A parabolic microphone is designed to capture sound from a distance and focuses sound waves in a concentrated manner, making it ineffective for the purpose of detecting direct disturbances on a surface. A radio frequency microphone primarily operates by transmitting audio signals via radio waves, which does not relate to the disturbances of physical objects. Thus, the functionality and application of the contact microphone stand out in contexts requiring immediate response to disturbances.