The mid-head effect refers to the unnatural feeling of the sound field caused by the headphone unit being too close to the ear because the longitudinal sound field of the headphone is not deep. Mid-head effect is common in headphones, especially in-ear headphones. The specific meaning is that when listening to music, it seems as if the instrument exists in the mind rather than in space. A good sound field should create a good sense of presence, making the listener feel as if they are in the center of a concert hall, surrounded by the sounds of various instruments around them (outside). The mid-head effect refers to the illusion that the instrument is played inside the head. This is a common problem with most headphones. However, the correct sound should be when the performer is in front of the person, and the sound is emitted from the front and spreads into the listener's ears, making it feel like he is listening to the scene. At present, only a few hifi headphones can reduce and eliminate the mid-head effect.