What are First Octave and Third Octave Band Filters?

When more detailed information about a complex sound is needed, the frequency range of 20Hz to 20kHz can be split into sections or bands. This is done electronically within a sound level meter.

These bands usually have a bandwidth of one octave or one third octave. More advanced instruments may be able to give a narrow band analysis of the noise data. This may be an FFT (Fast Fourier Transform) or information in 1/12 octaves.

An octave band is a frequency band where the highest frequency is twice the lowest frequency.

For example, an octave filter with a centre frequency of 1kHz has a lower frequency of 707Hz and an upper frequency of 1.414kHz. Any frequencies below and above these limits are rejected. A third octave has a width of 1/3 of that of an octave band.

To learn more about octave bands and other acoustic terms that will help you to use and understand your noise measuring equipment better, check out our FREE guide to noise terminology.

The figures below show a noise level split into both octave and third octaves:

Octave Band Filters - Noise Level Measured in 1:1 Octave Bands
Octave Band Filters – Noise Level Measured in 1:1 Octave Bands


An example of Octave Band Analysis

Noise Level Measured using 1:3 Octave Band Filters
Noise Level Measured using 1:3 Octave Band Filters

One example of where frequency analysis is very useful is in the comparison of the noise generated by a turbine and a compressor.

When the noise is measured using a sound level meter, the overall noise level is 113dB(A).

By using 1:1 octave band filters, we can break the noise from each machine into its component parts and it is very clear that the turbine is producing much more low frequency noise than the compressor.

This has implication when hearing protection is being specified. Different hearing protection will be needed for each machine as the PPE chosen for the compressor may not be suitable for the turbine.

This also applies when considering noise reduction enclosure or sound reduction materials.

The table below shows the noise levels for both the compressor (A) and the turbine (B). The first column shows the dB(A) values followed by the octave band figures. These octave band figures are those shown in the graph above.

  dB(A)31Hz63Hz125Hz250Hz500Hz1kHz2kHz4kHz8kHz16kHz
ACompressor113.680879093991001111069889
BTurbine113.61131221161161159785605742
CEnclosureN/AN/AN/A613252628293333
A-C85.7   8480747483776656
B-C99.8   11010390715731249

The attenuation provided by a “standard” noise reducing enclosure is shown in row (C). The final two rows show the noise levels for the compressor and turbine after the attenuation provided by the enclosure has been added.

The overall dB(A) noise level produced by the turbine is reduced by 13dB(A), whilst the compressor noise is reduced by 27dB(A).

The additional information provided by the frequency analysis gives an understanding of why the compressor noise is reduced by 14dB(A) more than the turbine noise.

A simple dB(A) measurement may not have shown this.

The optimus sound level meters can provide both 1:1 and 1:3 octave band analysis of noise levels. If you would like further information, please call us on 0845 230 2434 from the UK or +44 1723 891655 and we will be pleased to help.

Need to measure noise at work?

Check out the Optimus+ Red

The Optimus+ Red is our purpose-built noise at work sound level meter, designed to IEC 61672 standards. Available as an octave band sound level meter, it’s the all-in-one solution for noise at work assessments.

Measuring personal noise exposure?

Check out the Professional doseBadge5

The Professional doseBadge5 is an advanced noise dosimeter for the measurement of personal noise exposure levels in the workplace. With Bluetooth connectivity and octave band filters as standard, it’s complete solution for noise at work and personal noise exposure assessments.

Monitoring environmental noise?

Check out the Environmental Noise Measurement Kit

Compatible with the Optimus+ Green sound level meter, the Environmental Noise Measurement Kit is the perfect solution for long-term outdoor noise monitoring. With 3G/GPRS connectivity, you can view your data in the cloud and get real-time alerts to your phone.


download a Guide to Noise Measurement Terminology

Download the Noise Measurement Terminology Guide to expand your noise knowledge and make measuring noise simpler.

Clarke Roberts

Senior Marketing Executive at Cirrus Research plc
Clarke is a Senior Marketing Executive and is responsible for helping people to find Cirrus Research's innovative noise measurement solutions by using all aspects of the marketing mix.

He particularly enjoys writing content about product applications, including noise nuisance, occupational noise and environmental noise pollution.

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