Sound Level Meter Calibration

Sound Level Meter Calibration

 We calibrate most brands of sound meter

 We can calibrate both type 1 and type 2 sound meters.

 Fast turnaround means little downtime

 We can calibrate all makes and models of sound metres and noise metres accurately, quickly, and affordably.

 Calibration of your sound equipment on a regular basis ensures that all test results are as accurate as possible. Our professional calibration laboratory technicians can compare your sound metre to a known standard to ensure that it is operating within acceptable sound level parameters.

Our calibration laboratory can perform calibrations on sound level metres as well as calibrators of both Class I and Class II variations.

For more information on our sound calibration services, give us a call or fill out the calibration form to the right, and we'll get back to you as soon as possible. We can calibrate sound metres of both types 1 and 2, as well as personal noise dosimeters and type 1/type 2 acoustic calibrators.

 Types of Sound Equipment,

We offer calibration on the following anemometers and wind speed meters,

Sound Calibrators,

Sound Level Calibrator,

Class 1 Acoustic Calibrator,

Class 2 Acoustic Calibrator,

Sound Meter Acoustic Calibrator,

Sound Meters,

Sound Meter,

Sound Level Meter,

Noise Meter,

Decibel Meter,

Audiometer,

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 ?Pa, which is usually considered the threshold of human hearing (at 1 kHz). Sound Pressure Level (Lp) is calculated according to the following formula:

where:

pref is the reference sound pressure, and

prms 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:

The conventional sound level meter that measures exponential time-weighted sound level.

The integrating-averaging sound level meter that measures time-average sound level.

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:

Sound Level Meter Calibration

Having analyzed what a sound level meter is and how it works, we can now see, how it is calibrated

First of all, according to IEC 61672-1, the reference environmental conditions of a laboratory performing sound meter calibrations should be:

Temperature: 20 to 26 oC

Static Pressure: 80 to 105 kPa

Relative Humidity: 25 to 70%

The environmental conditions of the laboratory shall be measured and recorded during the calibration.

Prior to any measurements, the sound level meter and all accessories shall be visually inspected, paying particular attention to damage to, or accumulation of foreign materials on the protection grid or diaphragm of the microphone. Also, before conducting any test, the voltage delivered by the power supply for the sound level meter shall be checked to ensure that it is within the specified operating limits.

In order to calibrate sound level meters, a sound calibrator must be used. According to IEC 61672-1, at least one model of sound calibrator shall be stated in the sound level meter’s instruction manual for checking and maintaining the correct indication on its display. For class 1 sound level meters, a sound calibrator conforming to class 1 specifications of IEC 60942 is required. For class 2 instruments, the sound calibrator shall conform to either class 1 or class 2 specifications of IEC 60942.

The sound calibrator usually provides a reference sound pressure level of 94 dB or/and 114 dB at the frequency of 1 kHz. The sound calibrator must have a traceable and documented calibration certificate from a higher level laboratory (often from a national metrology institute).

When calibrating a sound level meter, most of the laboratories perform the calibration frequency check. In order to conduct this test, the operator must connect the sound meter to the corresponding calibrated sound calibrator and check that the instrument is within the tolerance limits. If necessary, the sound level meter can be adjusted to indicate the required sound pressure level.

Sound Level Meters – Periodic Tests

Besides the aforementioned test, there are some more tests that can be performed (depending on the type and the construction of the sound level meter) which are described in IEC 61672-3 “Sound level meters – Periodic Tests”:

Self-generated noise with microphone replaced by an electrical input signal device

With the microphone replaced by an electrical input signal device and terminated in the manner specified in the instruction manual for measurements of the level of the corresponding self-generated noise, the indicated level of the time-averaged or time-weighted self-generated noise would be recorded for all frequency weightings available in the sound level meter.

Acoustical signal test of a frequency weighting

The sound level meter shall be set for frequency weighting C, if available, otherwise, it would be set for frequency weighting A. In order to perform this test, a calibrated multi-frequency sound calibrator (class 1) is required. The test is performed in three frequencies, 125 Hz, 1 kHz and either 4 kHz or 8 kHz (depending on the free-field adjustment data that are provided, or at the discretion of the calibration laboratory).

Electrical Signal tests of frequency weightings

With this test, the frequency weightings are determined relative to the response at 1 kHz, by using steady sinusoidal electrical signals. On the reference level range and for each frequency weighting to be tested, the level of a 1 kHz input signal shall be adjusted to display an indication that is 45 dB less than the upper limit stated in the instruction manual for the linear operating range at 1 kHz on the reference level range. The test must be performed, for class 1 instruments, at the 10 frequencies at nominal octave intervals from 31,5 Hz to 16 kHz and for class 2 sound level meters, at the 8 frequencies at nominal octave intervals from 63 Hz to 8 kHz.

Frequency and time weightings at 1 kHz

The test is performed by applying a steady sinusoidal electrical signal of 1 kHz with amplitude set to indicate 94 dB at frequency weighting A. Then the performance of C-weighting, Z-weighting and FLAT response are checked, with the sound level meter set to display F time-weighted sound level, S time-weighted sound level and time average sound level (depending on the functions available).

Level linearity on the reference level range

This test is performed by using steady sinusoidal electrical signals of 8 kHz with the instrument set to frequency weighting A and time weighting S. The amplitude of the signal, initially set to give an indication at the reference level of 94 dB in each case, is increased in 5 dB steps up to within 5 dB of the upper limit of the linear operating range. The signal is then increased in 1 dB steps until, but not including, the first indication of overload. The tests are then continued, again starting from the 94 dB level with the signal decreasing in 5 dB steps down to within 5 dB of the specified lower limit. The signal is then decreased in 1 dB steps down to, but not including the first indication of under-range or the lower limit of the linear operating range.

Level linearity including the level range control

This test is performed by using a steady sinusoidal electrical signal of 1 kHz with the sound level meter set to frequency weighting A. The signal is adjusted to display a reference level of i.e. 94 dB at the reference range of the instrument. The signal is kept constant while the instrument changes ranges and the readings at each range are checked.

Toneburst response

The response of the sound level meter too short duration signals shall be tested on the reference level range with 4 kHz tone bursts that start and stop at zero crossings and are extracted from steady 4 kHz sinusoidal input signals. The sound level meter shall be set for frequency-weighting A.

Peak C sound level

Indications of peak C sound level shall be tested on the reference level range. The test signals are a single complete cycle of an 8 kHz sinusoid starting and stopping at zero crossings, and positive and negative half cycles of a 500 Hz sinusoid that also start and stop at zero crossings.

Overload indication

This test of overload indication is only to be performed for sound level meters capable of displaying time-average sound level. Overload indication shall be tested on the reference level range with the sound level meter set to display A-weighted time-average sound level. Positive and negative one-half cycle sinusoidal electrical signals at a frequency of 4 kHz shall be used. The one-half cycle signals shall be extracted from steady signals of the same signal level and shall begin and end at zero crossings.

Of course, not all of the above tests are applicable to all sound level meters. Only the indication at the calibration check frequency is mandatory. The rest of the tests depend on the meter’s capabilities.

The calibration laboratory must provide in the calibration certificate the uncertainty of measurement.

Calculation of the measurement uncertainty of sound level meters calibration contains at least the following components:

The measurement uncertainty from the calibration certificate of the standard instruments (i.e. sound level calibrator).

The uncertainty resulting from any environmental effects.

The repeatability of measurements.

Uncertainty due to the limited resolution of the sound level meter under test.

According to IEC 61672-3, the actual expanded u