Sound 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 uncertainty of the calibration laboratory shall not exceed the maximum uncertainty given in Annex A of the standard and presented in the following table

Written by Sofia