About Sound Meters

About Sound Level Meters

Sound Level Meters (or Sound Meters) are the instruments that measure sound pressure levels. Sound Pressure Level (or Sound Level) is a logarithmic measure of the effective sound pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. The standard reference sound pressure in air is 20 μPa, which is usually considered the threshold of human hearing (at 1 kHz). Sound Pressure Level (L_p) is calculated according to the following formula:

where:

p_ref is the reference sound pressure, and

p_rms is the rms sound pressure being measured.

Sound level meters are commonly used for measuring several kinds of noise, especially for industrial, environmental and aircraft applications.

According to IEC 61672-1 “Electroacoustics – Sound level meters, Part1: Specifications”, there are three kinds of sound level meters:

1. The conventional sound level meter that measures exponential time-weighted sound level.
2. The integrating-averaging sound level meter that measures time-average sound level.
3. The integrating sound level meter that measures sound exposure level.

A sound level meter can perform any one, or all, of the above three kinds of measurements. Frequency weighting A is mandatory for all sound meters since its use is required in all countries in order to protect workers against noise-induced deafness.  A frequency weighting A is also mandatory in all civil aircraft noise measurements.

Frequency weighting C is only mandatory for sound level meters conforming to class 1 tolerance limits (classes are explained below). The Zero frequency weighting (Z-weighting) is optional.

Sound level meters are commonly used to measure sounds generally in the range of human hearing. Their main components are a microphone, a signal processor and a display device. A sound level meter may be a self-contained hand-held instrument with an attached microphone and a built-in display device or it may comprise of separate components in one or more enclosures and may be capable of displaying a variety of acoustical signal levels. Depending on their construction, sound level meters are also classified into three groups as follows:

• Group X sound level meters: self-contained instruments which operate, in normal mode, with internal battery power, requiring no external connections to other apparatus.
• Group Y sound level meters: self-contained instruments which require connection to a public supply of electric power for the normal mode of operation, requiring no external connections to other apparatus.
• Group Z sound level meters: instruments which require two or more items of equipment, which are essential constituent parts of the sound level meter, to be connected together by some means for the normal mode operation. The separate items may be operated from internal batteries or from a public power supply of electric power.

Depending on their accuracy, sound level meters are divided into two performance categories, class 1 and class 2. Sound level meters of both classes have the same functionality but they differ mainly in the tolerance limits. Class 1 instruments have a wider frequency range and a tighter tolerance than class 2 sound level meters which are lower cost instruments. In the following table the tolerance limits (in dB) of each class are presented in some indicative frequencies, as stated in IEC 61672-1: