Circular hole punched on a magnetic circuit used in microspeakers to reduce flux leakage

Abstract

With the increasing market demand on mobile phone, demand on microspeakers with lower flux leakage has been rising rapidly. As is shown in Fig.1 (a), caused by flux leakage, especially by the maximum flux leakage (B Lmax ) in the side of sound holes, the dust accumulation through sound holes has a harmful impact on the performance of a microspeaker. Nowadays, a lower grade magnet or a smaller magnet volume is usually used to reduce the flux leakage in a microspeaker, but generally produces lower air gap flux density so that degrades the performance of microspeaker [1]. In this study, a circular hole punched on the magnetic circuit was proposed in order to reduce the flux leakage and maintain the performance of a microspeaker. Electromagnetic analysis of the flux density (B) and flux leakage was performed by a quarter of 3D FEM (Finite Element Method) as the axisymmetric of the modeling, as shown in Fig.1 (b1-b3). A conventional magnet circuit with N48H grade magnets shown in Fig.1 (b1) [2] was set as the default type in this study. Based on the default type, it could be seen in the Fig.1 (b2) that an IM (Inner Magnet) with N33H magnet grade was used in the grade type, because the center area has a higher flux leakage than other areas in the flux leakage distribution of default type, which is shown in Fig.1 (c). Moreover, in Fig.1 (b3), a circular hole was punched at the center area on both IM and IP (Inner Plate) in the proposed type. Samples were manufactured based on the simulations, and a test was employed by using a Kanetec Gauss meter and B&K equipments. As shown in Table I, the test results were well matched with those of the simulation, and the proposed type reduced the B Lmax by 2.3 times with less sacrifice of the flux density compared with that of the grade type. In addition, the tested SPL (Sound Pressure Level) of the proposed type decreased by only 0.30dB, which is an indiscernible gap for the human ear [3], but the resonance frequency was maintained. In conclusion, with punched holes on a magnetic circuit, both the simulated and experimental results confirmed that the proposed type was considerably more effective in reducing the flux leakage while maintaining the performance of microspeakers in an acceptable range.