Cancellation focuses on constant droning sounds like road noise and is less effective on short/sharp sounds like voices or breaking glass. It also is ineffective in eliminating higher frequency noises like the sound of spraying. Noise-cancelling headphones often combine sound isolation with ANC to maximize the sound reduction across the frequency spectrum. Noise cancellation can also be used without sound isolation to make wanted sounds (such as voices) easier to hear. Noise cancellation to eliminate ambient noise is never passive because of the circuitry required, so references to passive noise cancellation actually are referring to products featuring sound isolation.