Separating noise by frequency
Octave bands group a frequency interval whose upper limit is twice its lower limit. They are identified by a centre frequency, such as 125, 250, 500 or 1000 Hz. Analysis shows how sound energy is distributed between low and high frequencies, information lost when noise is summarised as one overall level.
Related concept: Workplace noise.
What they are used for
The spectrum helps select controls, examine interference with communication and estimate hearing protector attenuation. Two environments with the same A-weighted level may need different solutions if different frequencies dominate. An absorbent material, barrier or earmuff does not necessarily work equally well in every band.
What must be measured
Establish whether band results are unweighted or already include A or C corrections, and which time interval they represent. The instrument needs suitable filters and functions. Combining one band measured during one operation with another from a different process can produce a spectrum corresponding to no real situation.
Decibels are not added arithmetically
To obtain an overall level from bands, energies are combined through logarithmic addition. Directly adding the dB numbers produces an incorrect result. Avoid applying a weighting twice. A clear table distinguishes centre frequency, measured level, correction applied and the predicted result after a control measure.
Hypothetical example
A compressor and a cutting tool produce similar overall levels at two workstations. The compressor concentrates more energy at low frequencies. Choosing the same protector solely by its SNR may conceal protection differences. Octave-band calculation compares the workplace spectrum with the protector’s declared attenuation in each interval.
Resolution and limits
One-third-octave bands provide more detailed analysis but do not themselves replace a sound sampling strategy. A spectrum characterises the sample taken, not automatically the entire shift. Assessment must relate frequency, duration and task, retaining enough data for another specialist to review how the preventive conclusion was reached.
