ambienBeat is a mobile heart rate regulator which provides closed-loop ambient biofeedback via subliminal tactile stimulus based on a user's heartbeat rate variability (HRV). We applied the principle of interpersonal physiological synchronization to achieve our goal of effortless regulation of HRV, which is tightly coupled with mental stress levels. ambienBeat provides various patterns of subliminal tactile stimuli, which mimics the feeling of a heartbeat pulse, to guide a user's HRV to resonate with its rhythmic, tactile patterns. The strength and rhythmic patterns of tactile stimulation are controlled to a level below the cognitive threshold of an individual's tactile sensitivity on their wrist so as to minimize task disturbance. Here we present an acoustically noiseless, soft voice-coil actuator to render the ambient tactile stimulus and present the system and implementation process. We evaluated our system by comparing it to ambient auditory and visual guidance. Results from the user study shows that the subliminal tactile stimulation was effective in guiding a user's HRV to resonate with ambienBeat to either calm or boost the heart rate using minimal cognitive load.