# Design and Analysis of a Novel Microspeaker with Enhanced Low-Frequency SPL and Size Reduction
## 4 New Microspeaker Design
### Magnetic Circuit Structure Design  
The Lorentz force (electromagnetic force) is the force that drives the coil vibration. To increase the Lorentz force, the flux density passing through coil B or the total coil length \(l\) need to be increased. As the design of the speaker coil determines both the flux density and the total coil length, it is important to appropriately design the coil to maximize the Lorentz force. In addition, the low-frequency SPL can be improved by using a heavier coil. By adopting a large-diameter coil, the mass and total length of the coil are increased simultaneously. However, this increased coil diameter leads to an increased thickness and height of the coil, which causes a low flux density, resulting in a decreased Lorentz force. The schematics of the coil are shown in Figure 12.  
For the proposed microsppeaker, 1410_SPK, a 0.077-mm coil diameter was used, unlike the 0.070-mm coil used in 1610_SPK. By using this coil with a larger diameter, we increased the mass and length of the coil by approximately 51% and 25%, respectively, compared with those of the prototype 1610_SPK. The coil specifications of 1610_SPK and 1410_SPK are listed in Table 6. Because a coil with a larger diameter and the same resistance of 6 \(\Omega\) was used, the height of 1410_SPK exceeded that of 1610_SPK, making it unsuitable for newer smartphones. The yoke thickness around the coil and also the distance between the upper part of the yoke and the bottom surface of the coil were reduced for solving the height problem. Figure 13 shows the component thicknesses for 1610_SPK and the newly designed 1410_SPK. As shown in this figure, the distance between the bottom of the yoke and the top surface of the coil in 1410_SPK was less than that in 1610_SPK. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multirow{2}{*}{**Diameter**} & **Winding** & **Coil** & **Coil** & **Mass** & **Total** \\  & & **Layer** & **Height** & **Thickness** & **Length** \\ \hline
1610\_SPK & 0.070 mm & 4 & 1.02 mm & 0.328 mm & 46.41 mg & 1339.3 mm \\
1410\_SPK & 0.077 mm & 4 & 1.16 mm & 0.356 mm & 69.99 mg & 1675.7 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Coil specification of 1610_SPK and 1410_SPK. Figure 12: Schematic of the coil design. The magnets in the speaker are the main source of magnetic flux. The generated magnetic flux is concentrated on the coil through the top plates. The prototype 1610_SPK has a total of three permanent magnets: one center magnet and two outer magnets. Figure 14 depicts the design of 1610_SPK and the average flux density distribution on the coil, which is divided into eight sections. Because the design of 1610_SPK is symmetric, the coil flux density is also symmetric. As shown in Figure 14b, the longer part of the coil is labeled as section 2, the corner part as section 3, and the shorter part as section 4 for comparison purposes. Over the entire coil, 1610_SPK has an average flux density of 0.560 T.  
As in 1610_SPK, one center magnet and two outer magnets were deployed in 1410_SPK. The design of 1410_SPK with a total of three magnets, called 1410_SPK_3 mag, is shown in Figure 15b. Owing to the reduction in the length of the speaker from 16.0 to 14.0 mm, we used smaller outer magnets compared with those in 1610_SPK. Owing to the smaller outer magnets, the flux density of 1410_SPK_3 mag was lower than that of the coil in 1610_SPK. Although the average flux density in 1410_SPK was 13.4% lower than that in 1610_SPK, the longer coil length resulted in a 4.6% larger force. As shown in Figure 14b, the shorter part of the coil (section 4) has a lower flux density than that of the longer part (section 4). This is because there are no outer magnets in section 4. This can also be seen in Figure 15b. To increase the flux density in section 4, we added two additional outer magnets, as shown in Figure 15c; thus, 1410_SPK features a total of five magnets. This resulted in a 10.8% increase in the total force compared with that when using the 1410_SPK with two outer magnets.