Open Fit Instruments Detail Specification The Behind-the-Ear Open Fit device is a discreet, small and lightweight instrument. The ultra-thin transparent soft tubing and small silicone dome snap into the BTE housing as a single unit. An open system ensures that the ear is well ventilated and that you experience a more natural sound, including the sound of your own voice. These are extremely comfortable hearing aids to wear. They are particularly suited to mild to moderate high frequency hearing losses.