Voice is a common and frequently used word. When an object vibrates, sound waves are generated around it, and the sound waves continue to spread outward and are heard by people as sound.

The lower limit of human hearing is defined as 0db (decibel), normal speech is about 70db, and the upper limit of hearing is 120db, which will cause earache and is unbearable. Sounds above 140db will make people lose hearing.

However, when sound propagates in the room, it will continue to follow the law of propagation in the outdoor atmosphere, and it will also be reflected by the ceiling, ground, and walls of the room.

When the sound source continuously emits sound, the incident sound wave and the reflected sound wave are superimposed to form a complex indoor sound field. This is a common acoustic phenomenon in room acoustics.