The speaker unit of the closed speaker is installed in a completely closed box, so that the reversed sound wave radiated backward by the diaphragm is completely blocked by the box, and does not go outside the box to cancel the positive phase sound wave in front of the diaphragm, solved the "sound short circuit" problem, enabling the bass to radiate effectively. The low-frequency attenuation characteristics of the closed box are flat compared to other types of speakers, and the attenuation curve of a second-order low-pass filter is the same, which means it has the best transient response of all types of speakers. At the same time, the air enclosed in the box forms a strong "air spring", which can effectively suppress the displacement of the diaphragm at the resonant frequency and reduce nonlinear distortion. However, the stiffness of the air also increases the low-frequency resonance frequency of the speaker unit, so that the overall low-frequency lower limit of the speaker is higher than that of the unit under free space conditions. Compared with the design of the inverter box and the transmission line speaker, the low frequency lower limit of the closed box is relatively poor. Also, the backward radiation of the diaphragm is not utilized, resulting in lower efficiency.
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