In this study, we propose an application for immersive virtual reality experiences, which integrates three-dimensional (3D) head-mounted displays (HMDs) with a precise indoor position tracking algorithm based on ultrasound. Our method provides a natural virtual reality experience with interaction by precisely matching the physical movements in the real world with those in the virtual environment, unlike other methods that require external input devices to move around in the virtual environment. Users can move within the assigned indoor space while carrying a wireless client device with the HMD, without the risk of colliding with obstacles or structures. The system is designed to provide the accurate 3D X, Y, and Z coordinate values of translational movements in real-time as well as the pitch, roll, and yaw values of rotational movements supported by the HMD, resulting in the six degrees of freedom required by immersive virtual reality. In addition, the system utilizes ultrasonic transducers in a grid format, which makes it simple to expand the position tracking coverage area, and supports simultaneous tracking of multiple users. Through experiments and a user study we show that the system obtains the accurate position of the moving objects and delivers a highly immersive virtual reality experience.