What is the difference between Class 1 and Class 2 SPL measurement?
The short answer is that Class 1 SPL systems are required to measure over a wider frequency range than a Class 2 system as well as meet tighter tolerances for performance criteria and documentation. The nitty-gritty details are laid out in the standards document IEC 61672, but since standards documents are not very lite reading, we’ll just highlight the key bits (the entire 48 page specifications document can be viewed/purchased HERE).
It is important to know that the differences between Classes are not limited to microphone and interface tolerances (the entire signal acquisition chain is subject to specified tolerances), but also the format and detail with which the measurement data is logged and kept for presentation in a legal setting. It is for this reason that the UI (user interface) displaying the measurement results are also factored in Class compliance.
The stipulations for documentation of SPL information outlined in the IEC standards state that specific data must be reported and documented over the duration of exposure.
Directly from IEC 61672.1-2004:
“− For class 1 sound level meters, the data shall be stated in tabular form at nominal one- third-octave intervals for frequencies from 63 Hz to 1 kHz and then at nominal one-twelfth- octave intervals for frequencies greater than 1 kHz to at least 16 kHz.
− For class 2 sound level meters, the data shall be stated in tabular form at nominal one- third-octave intervals for frequencies from 63 Hz to at least 8 kHz.
− Corrections for the average effects of a windscreen on the microphone frequency response shall be stated in tabular form at nominal one-third-octave intervals from 1 kHz to 16 kHz for class 1 sound level meters and from 1 kHz to 8 kHz for class 2 sound level meters. ”
As you can see, the amount of bandwidth and data points necessary for collection are greater with a Class 1 system.
You may now be asking yourself: what are the tolerance differences between Class 1, and Class 2? Within the meat of the human hearing range the differences are minimal. However the tolerances vary greatly at the upper and lower extremities (See figure 1).
Figure 1: Class 1 and Class 2 upper and lower tolerances as plotted in Smaart v.7
For example, at the reference frequency of 1 KHz, the frequency tolerance limits for Class 1 are +/- 1.1dB, and for Class 2 +/- 1.4dB
At the Lower and upper extremities of the frequency range, the tolerance are wider. At 20Hz, the tolerances are +/- 2.5dB for Class 1, and +/- 3.5dB for Class 2.
The same applies to extreme high frequencies, to greater degree. At 16kHz, the tolerance for Class 1 is +3.5dB to -17dB, and for Class 2, +6dB to –∞dB.
At high frequencies less extreme, such as 8kHz, the tolerances for Class 1 are +2.1dB to -3.1dB, and +/- 5.6dB for Class 2.