Tonal noise detection is a useful function for environmental noise measurements and can aid in the location and identification of problematic noise sources. Noise sources that are tonal in nature or that have tonal content can often be more annoying or disturbin and so having the ability to detect and record tones can be very useful. The CR:171C and CR:172C optimus sound level meters, which are Class 1 and Class 2 respectively, can use one of two different methods to detect and identify noise tones and these are outlined below.
The default method for the Tonal Noise Detection is the Cirrus Improved Method.
Tonal noise detection methods
Method 1 – ISO 1996-2:2007
In the ISO standard, the definition of a tone is that if a band is higher than its direct neighbours by an amount that is dependent upon frequency, it is highlighted. The standard defines different levels of threshold depending upon the frequency of the 1:3 octave band. When a tone is detected using the ISO 1996-2:2007 method, it is highlighted in blue on the A weighted graphical screen and also the A weighted numeric screen.
Method 2 – Cirrus improved method
There are several limitations with the Simplified Method described in ISO 1996-2:2007 Annex D.
These are that the frequency bands used are limited to 25Hz to 10kHz, only A-weighted data is used and tones between bands may not be detected accurately
The Cirrus improved method takes the ISO method and expands upon it by using all of the frequency bands available in the instrument (6.3Hz to 20kHz), detecting tones between bands, using both A and Z weighted spectra and allowing the thresholds for detection to be adjusted within the NoiseTools software
The implementation of these methods is described in detail in Technical Note 32 – Tonal noise detection with the optimus sound level meters.Download Technical Note 32