A device that displays how much energy an input waveform has at different wavelengths. The classic spectrum analyzer used a narrow bandpass filter that swept through the audio range, graphing the level of signal that passed through the filter at different frequencies. These devices were accurate but slow; they were not capable of accurately measuring a changing waveform. In the late 1970s, the first “real-time” analyzers were developed, using banks of bandpass filters tuned at specific intervals (typically one octave or 1/3 octave apart) and using an array of indicator lights to display the results in bar-graph style. Most current spectral analyzers use Fast Fourier Transform to perform the analysis.