The attraction material is used to absorb the airflow inside the speaker, reducing the standing wave and resonance. Wool, glass wool, felt, etc. are generally used.
For the closed box, since the sound waves behind the speaker are completely absorbed, the sound absorbing cotton generally fills the entire box, which is equivalent to increasing the volume of the box.
For a phase-inverted box, how much sound-absorbing cotton is required depends on the Q value of the different speakers.
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