# Development of Direct-Vibration Actuator for
## 1 Introduction  
Nowadays, many people use smartphones and tablets as computers to work and perform certain tasks. Along with portability, these devices provide working tools and entertainment functions simultaneously. The dynamic receiver is a small speaker used in mobile devices to generate sounds during voice communications. It emits sounds through holes located around the display panel. The vibration and sound are two kinds of the important features for consumers while selecting smartphones. For this reason, many research studies have been conducted. In the past, the design trend for smartphones was to make these mobile devices thinner. Thus, vibration motor and speaker was widely adopted. For tactile feedback, a horizontal vibration motor was proposed and analyzed in literature to reduce the thickness of the smartphone [1, 2]. A novel structure of the vibration motor was proposed and analyzed to improve the acceleration of the conventional motor [3]. Analysis of the vibration motor using an electromagnetic and a mechanical finite element method (FEM) was introduced and verified via experiments. For the coupling analysis, nonlinear parameters including inductance and speedance was considered. The electrical, magnetic, and mechanical coupling effectsof vibrating motor are solved by considering nonlinear parameters with FEM and numerical iteration method [4]. With regard to the small speaker used in cellular phones, [PERSON] et al. developed a small rectangular speaker to minimize size while retaining adequate performance [5]. To develop thinner speaker for cellular phones, [PERSON] et al. created a novel magnetic circuit by considering the manufacturing process while minimizing the loss of acoustic performance. Instead of using vertically magnetized outer magnets, horizontally magnetized outer magnets were used to reduce the thickness [6]. A rectangular speaker with a modified structure used in liquid crystal display (LCD) phones was developed to improve sound performance. By adding new structures to the yoke, the flux density was improved [7]. Topology optimization was also used to analyze the magnetic circuit in order to improve the speaker performance in cellular phones [8]. Since the industry has progressed toward development of thinner devices, new materials are being used for speakers. To dramatically reduce device thickness, a piezoelectric speaker was developed in [9]. The piezoelectric speaker is a multilayer structure that is thinner than the dynamic receiver. However, the piezoelectric speaker is not widely used owing to its high manufacturing cost and limited performance. Nowadays, the design trend of the smartphone is shifting toward bezel-less display. Owing to enlarged display, the available space for the dynamic receiver has decreased. The sizes of the holes for dynamic receivers have been reduced, and some parts of the display panels have been either cut or modified to allow for the dynamic receiver holes. However, the dynamic receiver holes are a hindrance to bezel-less display design. The design changes in smartphones are shown in Figure 1.  
The conventional dynamic receiver generates sound by directly vibrating a diaphragm connected to a moving coil, and the sound is transmitted to a listener through a hole located at the top of the display panel. The simplified structure of the dynamic receiver is shown in Figure 2a. Figure 1: Design changes in smartphones. Sound can be generated by vibrating elastic objects. In a smartphone, the vibrating display can generate sound that replaces the dynamic receiver. For a bezel-less-display smartphone, indirect-vibration actuators have been introduced and analyzed [10]. In the indirect-vibration actuator, the oscillating  
Figure 2: Schematics of (**a**) dynamic receiver, (**b**) indirect-vibration actuator, and (**c**) direct-vibration actuator. coil is located inside a case. Then, the coil vibrates the case, which transmits vibrations to display panel. The vibrating display generates the sound that the users can hear.