The filling-in phenomenon I’ve described is not just a laboratory curiosity; composers exploit this principle as well, knowing that our perception of a melodic line will continue, even if part of it is obscured by other instruments. Whenever we hear the lowest notes on the piano or double bass, we are not actually hearing 27.5 or 35 Hz, because those instruments are typically incapable of producing much energy at these ultralow frequencies: Our ears are filling in the information and giving us the illusion that the tone is that low.
