What is spectral leakage?

• A. Leakage refers to energy lost due to thermal noise.
• B. Leakage describes the crosstalk between channels.
• C. Leakage occurs when a signal's energy spills into adjacent frequencies due to finite observation time.
• D. Leakage is when a signal's power increases within its nominal band.

Answer: (C)

Spectral leakage happens when you analyze a signal for only a finite amount of time, so its frequency content doesn’t stay confined to a single precise frequency. In the Fourier view, taking a finite-time window is like multiplying the signal by a window function in time, which in frequency becomes a convolution with the window’s spectrum. If the signal has energy at a frequency that doesn’t line up exactly with one of the Fourier bins, that energy spreads into neighboring frequencies rather than staying in one spot. That spreading shows up as nonzero energy in adjacent bins and is especially evident with a rectangular window, which has strong sidelobes. Using other window shapes can reduce this leakage by lowering those sidelobes, though it trades off some frequency resolution.

So, leakage is about energy from one frequency showing up in nearby frequencies due to the finite observation time, not about energy loss to noise, channel crosstalk, or an actual increase of power within its nominal band.