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A sound level meter fitted with Octave Band Filters has the ability to split the audible spectrum into smaller bands, identifying the frequency content of the noise.
Most simple noise measurements are made using the "A" frequency weighting and the results expressed in a single dB(A) value. When a measurement is "A" weighted the sound level meter is responding in a similar way to the human ear, being more "deaf" at the lower and higher frequencies.
When you need a little more information than this, you split the spectrum into about 10 bands. These are called the Octave Bands because there is one octave between the bottom and top of each band. The center frequencies for these bands are usually: 31.5Hz , 63Hz , 125Hz , 250Hz , 500Hz , 1kHz , 2kHz , 4kHz , 8kHz and 16kHz
The sound level meter will measure the noise in each of the octave bands, and it is easy to see dominant bands that contribute the most to the overall SPL.
1/3 Octave Band Filters are very similar in nature to the octave band filters described above. The difference is that each of the octave bands is split into three, giving a more detailed description of the frequency content of the noise.
Although rarely used for occupational noise measurement, 1/3 octaves are useful in many environmental, building acoustics and noise control applications.