FFT Window Functions

A window function is a mathematical function that is normally symmetric around the middle of the interval, usually near a maximum in the middle, and usually tapering away from the middle.

Each window function has its own characteristics and suitability for different applications (some are more frequency accurate, others are more amplitude accurate). To choose a window function, you (the user) must select the most appropriate one. Some suggestions:

Sine wave or combination of sine waves: Hann
Sine wave (amplitude accuracy is important): Flat Top
Narrowband random signal (vibration data): Hann
Broadband random (white noise): Rectangular
Closely spaced sine waves: Uniform, Hamming
Excitation signals (hammer blow): Force
Response signals: Exponential
Unknown content: Hann
Two tones with frequencies close but amplitudes very different: Kaiser-Bessel
Two tones with frequencies close and almost equal amplitudes: Rectangular

Hann Window

Hann (also known as Hanning, raised cosine, or von Hann) window, is named after the Austrian meteorologist Julius von Hann. The math looks like:

w[n] = 1/2(1 - cos({2 pi n}/N))

Rectangular window

The rectangular window (also known as as boxcar, uniform or Dirichlet window) is the simplest window, equivalent to replacing all but N values of a data sequence by zeros, making it appear as though the waveform suddenly turns on and off.

w[n] = 1