Sound Part Three:
The third part of our acoustical technical notes will discuss frequency weightings and octave bands.
Frequency weightings also known as frequency networks and frequency filters, correlate measured sound levels with the subjective human response. The human ear is frequency selective, between 500 Hz and 6 kHz, our ears are very sensitive compared with lower and higher frequencies. The frequency weightings used in sound level meters are often related to the response of the human ear, to ensure that the meter is measuring pretty much what you actually hear.
It’s extremely important that sound level measurements are made using the correct frequency weighting – usually A-weighting. At higher levels such as C-weighting, the ear’s response is flatter. Z-weighting is also a flat frequency of 10 Hz to 20 kHz.
Table 1 shows a graph of frequency weightings.
When more detailed information about a complex sound is needed, the frequency range of 20 Hz to 20 kHz can be split into sections or bands. This is done electronically within a sound level meter.
The instrument used to measure the distribution of sound throughout the audible frequency range is called the spectrum analyzer. The bands usually have a bandwidth of one octave or one-third octave. An octave band is a frequency band where the highest frequency is double the lowest frequency.
One example where frequency analysis is very useful is in the comparison of the noise generated by a turbine and a compressor. However, the filters can be used for many applications to help you control and reduce the noise. Here are some examples:
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