Early stealth aircraft designs like the F-117 and B-2 were optimized for signature reduction in the frequency bands most typically used by air defense and fighter radar sets. Newer designs like the B-21 and F-47 supposedly have "broadband stealth" which reduces radar cross section across a broader range of radar bands but details are classified.
I'm pleased at the discussion in this chain of replies, as it illustrates the cat-and-mouse game stealth (low observability?) is. Stealth never was a perfect defense, as it's detractors claim. It forces the adversary to make tradeoffs, such as increasing the costs needed to detect such craft, which can then be exploited for gain.
