Class 1 vs. Class 2 SPL Measurement

Class 1 vs. Class 2 SPL Measurement

What is the difference between Class 1 and Class 2 SPL measurement?

In short, Class 1 SPL systems are required to have a tighter accuracy tolerance than Class 2 SPL systems. For most live events, Class 2 systems are all that is required, but some municipalities and governing agencies require Class 1 compliance. 

Check with any ordinances for language discussing tolerance requirements and measurement standards, as they very greatly between municipalities.

The longer answer is that Class 1 SPL systems are required to be able to measure over a wider frequency range than a Class 2 system while meeting tighter tolerances for performance criteria and documentation. Standards document IEC 61672 contains a full run-down of the specific criteria, and can be viewed/purchased here.

The differences between Classes are not limited to microphone and interface tolerances, but also the format and detail with which the measurement data is logged and presented in a legal setting (the entire signal acquisition chain is subject to specified tolerances).  It is for this reason that the UI (user interface) displaying the measurement results are also factored into Class compliance.

The stipulations for SPL documentation 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 1k 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 be 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.

The tolerances are wider at the upper and lower extremities of the frequency range, 

as can be seen.

Which Class should I use?

This largely depends on your specific measuring application, in addition to any regulations that you are required to meet. Typically, a Class 2 meter will be satisfactory when assessing basic levels in standard commercial, educational, recreational, industrial, or residential applications. Workplace noise measurements following OSHA or NIOSH guidelines may utilize Class 2 sound level meters. These noise assessments often require noise monitoring stations. 

Most sound level meters offer a range between 30-130 dB, but specific contexts may require more different ranges. For instance, environmental noise assessments might need a range that goes as low as 25dB, but industrial settings could require a range that extends to at least 140dB. Class 1 meters tend to be more expensive because of their high accuracy, which should be considered, as well.

Where precision is critical, such as in laboratory environments or law enforcement scenarios, a Class 1 meter is recommended. Building and room acoustics typically use Class 1 meters with capabilities for measuring reverberation time. Engineering sound measurements, which can include specialized tasks like machinery noise evaluation or product development testing, often require meters with advanced frequency analysis features like FFT or third-octave-banded analysis, as well.

Most sound level meters offer a range between 30-130 dB, but specific contexts may require more different ranges. For instance, environmental noise assessments might need a range that goes as low as 25dB, but industrial settings could require a range that extends to at least 140dB. Class 1 meters tend to be more expensive because of their high accuracy, which should be considered, as well.