How Do I Determine the Frequencies in an FFT Output?

Understanding FFT Frequency Components

Analyzing the frequency content of a signal requires understanding how to extract frequency information from a Fast Fourier Transform (FFT) output. The FFT typically returns two arrays: real and imaginary components. Determining the frequency associated with each component is a crucial step in signal analysis.

The initial FFT output bin represents the DC component (0 Hz). Subsequent bins represent frequencies that increase linearly. The frequency of the nth bin is calculated as n Fs / N, where Fs is the sampling rate and N* is the FFT size.

For instance, with a sampling rate (Fs) of 44.1 kHz and an FFT size (N) of 1024, the frequencies of the first few bins are:

• Bin 0: 0 Hz
• Bin 1: 43.066 Hz
• Bin 2: 86.133 Hz
• Bin 3: 129.199 Hz

Importantly, only the first N/2 bins (approximately) contain useful information. The N/2 bin represents the Nyquist frequency, which is often discarded. Bins beyond N/2 are mirrored and redundant.

Using this formula, you can easily create a frequency array corresponding to the FFT's real and imaginary components, providing a clear picture of the input signal's frequency spectrum.

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