Pattern approval for measuring instruments

Courtesy: Pattern approval for measuring instruments

Personal noise dosimeter

A common variant of the sound level meter is a noise dosemeter (dosimeter in American English). However, this is now formally known as a personal sound exposure meter (PSEM) and has its own international standard IEC 61252:1993.

A noise dosimeter (American) or noise dosemeter (British) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard or EU Directive 2003–10/EC.

This is normally intended to be a body-worn instrument and thus has a relaxed technical requirement, as a body-worn instrument—because of the presence of the body—has a poorer overall acoustic performance. A PSEM gives a read-out based on sound exposure, usually Pa²·h, and the older ‘classic’ dosimeters giving the metric of ‘percentage dose’ are no longer used in most countries. The problem with “%dose” is that it relates to the political situation and thus any device can become obsolete if the “100%” value is changed by local laws.

Traditionally, noise dosemeters were relatively large devices with a microphone mounted near the ear and having a cable going to the instrument body, itself usually belt worn. These devices had several issues, mainly the reliability of the cable and the disturbance to the user’s normal work mode, caused by the presence of the cable. In 1997 following a UK research grant an EU patent was issued for the first of a range of devices that were so small that they resembled a radiation badge and no cable was needed as the whole unit could be fitted near the ear. UK designer and manufacturer, Cirrus Research, introduced the doseBadge personal noise dosimeter, which was the world’s first truly wireless noise dosimeter. Today these devices measure not only simple noise dose, but some even have four separate dosemeters, each with many of the functions of a full-sized sound level meter, including in the latest models full octave band analysis.

Classes

IEC standards divide sound level meters into two “classes”. Sound level meters of the two classes have the same functionality, but different tolerances for error. Class 1 instruments have a wider frequency range and a tighter tolerance than a lower cost Class 2 unit. This applies to both the sound level meter itself as well as the associated calibrator. Most national standards permit the use of “at least a Class 2 instrument”. For many measurements, it is not necessary to use a Class 1 unit; these are best employed for research and law enforcement.

Similarly, the American National Standards Institute (ANSI) specifies sound level meters as three different Types 0, 1 and 2. These are described, as follows, in the Occupational Safety and Health OSHA Technical Manual TED01-00-015, Chapter 5, OSHA Noise and Hearing Conservation, Appendix III:A,^[11] “These ANSI standards set performance and accuracy tolerances according to three levels of precision: Types 0, 1, and 2. Type 0 is used in laboratories, Type 1 is used for precision measurements in the field, and Type 2 is used for general-purpose measurements. For compliance purposes, readings with an ANSI Type 2 sound level meter and dosimeter are considered to have an accuracy of ±2 dBA, while a Type 1 instrument has an accuracy of ±1 dBA. A Type 2 meter is the minimum requirement by OSHA for noise measurements and is usually sufficient for general-purpose noise surveys. The Type 1 meter is preferred for the design of cost-effective noise controls. For unusual measurement situations, refer to the manufacturer’s instructions and appropriate ANSI standards for guidance in interpreting instrument accuracy.”

Labels used to describe sound and noise level values are defined in the IEC Standard 61672-1:2013 For labels, the first letter is always an L. This simply stands for Level, as in the sound pressure level measured through a microphone or the electronic signal level measured at the output from an audio component, such as a mixing desk.

For Measurements it’s important to what frequency weighting effects (how the sound level meter responds to different sound frequencies), and time weighting (how the sound level meter reacts to changes in sound pressure)