At the low frequencies, there are only a small number of narrowband bins which fall within the corresponding octave band limits. This linear frequency spectrum is then apportioned among the octave bands to produce the corresponding octave spectrum.Īs you would expect, the FFT size directly determines the number of octave bands that can be computed. SpectraPLUS-DT is a digital analyzer and utilizes the FFT algorithm to compute the narrowband (linear) frequency spectrum. ![]() Most octave analyzers are analog instruments and use analog filters for each band. These standards are used by SpectraPLUS-DT. The 'one-third-octaves' have been standardized for scientific instruments commonly used for tasks like to determine NC-criterion and similar. Standard octave band center frequencies and bandwidths have been agreed upon by the international community. The naturally occurring ambient spectra will be found to slope downwards at the highest frequencies, and a constant percentage band (CPB) spectrum of the same data will usually be more uniform in level over a broad frequency range. It can be argued that the frequency resolution in octave band analysis is too poor to be of much use, but a major advantage of constant percentage band analysis is that a very wide frequency range can be displayed on a single graph and the frequency resolution at the lower frequencies can still be fairly narrow. 1/1 and 1/3 octave spectra are the most frequently used formats in acoustical measurements - three 1/3 octave bands encompass one octave. In other words, the analysis bands become wider in proportion to their center frequencies. This type of spectrum is called constant percentage band (CPB) because each frequency band is a constant percentage of its center frequency. For example, if the frequencies are 8 Hz, their ratio is 1.500. ![]() ![]() Octave band analysis has been defined as a standard for acoustic analysis.
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