There are many positions and shapes of openings, but most also have a sound conduit inside the hole. The relationship between the internal volume of the box and the hole of the sound conduit, according to the principle of resonance vibration, produces vibration at a specific frequency, which is called the resonance frequency. The sound waves radiated backward by the speaker are inverted through the conduit, radiated to the front from the sound outlet, and superimposed in phase with the sound waves radiated forward by the speaker. It can provide a wider bandwidth than the sealed type, with higher sensitivity and smaller distortion. Ideally, the lower limit of the low-frequency playback frequency can be as much as 20% lower than the maximum vibration frequency of the speaker.

This kind of speaker can reproduce rich bass with a smaller cabinet