Requirements for speech recognition is crucial for effective communication.  The good speech recognition is in the frequency range from 500 to 2000 Hz.  It is directly related to the arithmetic average of hearing loss in decibels at 500, 1000 and 2000 Hz.  In practical a hearing loss of 25 dB will allow speech to be just understood satisfactorily, while a loss of 92 dB is regarded as total hearing loss.  In many plant rooms, normally compose of many noise sources, the reflection of the sound from wall, and solid panel can add the level of noise to be much higher.  A high quality of sound absorber can will used to absorb noise at the solid wall  in order to reduce the power of the sound in the plant room.