Octave band filters are used in basic acoustic engineering design and analysis work. By observing the frequency band in which the maximum sound pressure level occurs, the system characteristics that relate to the noise generation may be identified. Octave band filters has the ability to split the audible spectrum into smaller bands, identifying the frequency content of the noise. An octave band is defined as a frequency range in which the ratio of the upper and lower frequency limits for the range is equal to 2. Octave band filters are often included as a feature of sound level meters. If the filters were perfect all the energy outside the octave band frequency range would be attenuated. Typically the attenuation of the octave band filter is on the order of -25 db at a frequency one octave below or one octave above the center frequency of the octave band.