In this third article in the Shaping Sound series, we explore noise reduction and the “open-fit paradox” - where increasing openness can undermine the very features designed to improve speech in noise. Modern hearing aids are highly advanced, but if sound bypasses the device, their effectiveness is limited. We look at how acoustic coupling, venting, and leakage impact signal-to-noise ratio, and why controlling the acoustic interface is essential for delivering real-world benefit.