Accreditatie ENMO Services lab – discipline Sound

The ENMO Services lab is accredited according to ISO17025, with the accompanying technical standards IEC61672-3:2013 and IEC60472, annex B. Various national guidelines encourage users of sound level meters or sound calibrators to have an accredited calibration performed every 1 or 2 years to ensure legal and accurate measurements. With each calibration, the customer receives a full certificate stating that the equipment complies and falls within the specified tolerances of the relevant standard.

Sound calibrator

A sound calibrator, calibrated according to IEC60472, annex B, is measured in the ENMO Services lab using a reference pressure-field microphone. Each pressure point of the offered calibrator is measured three times. The average of these three measurements is calculated and will be the newly established pressure value.

Next, each frequency point is measured to determine whether the sound pressure accurately matches the correct frequency, ensuring it functions as a valid reference point.

Lastly, the Total Distortion is determined. The measured pressure value is broadband, but in an ideal world, it would exist only at the specific frequency point. However, in practice, this is not possible. Every frequency introduces harmonics, and the self-noise of the electronics in the calibrator is also present in the broadband signal. It is essential that the fundamental pressure value is indeed the primary source of the broadband pressure value.

Sound level meter

A sound level meter is calibrated according to IEC61672-3:2013 and must logically undergo different calibration tests compared to a calibrator. Since a sound level meter is more versatile than a calibrator, its calibration is much more comprehensive. The first part of the calibration is entirely electrical. The microphone is replaced by a corresponding capacitance, allowing the necessary signals to be sent into the device via a function generator. The first check of the electrical tests is to verify whether the device is correctly set to the reference values and whether these match the present frequency and time weightings.

The next step is to check the linearity of the sound level meter. Measurements will be performed at 8kHz and A-weighted. Problems with the sound level meter and preamplifier are often detected faster at higher frequencies. The commonly used A-weighting filter is also tested for linearity in this way.

The next check involves the frequency weighting of the sound level meter. Each weighting is tested per octave, from 63Hz to 16kHz (class 1) or 8kHz (class 2).

In addition to frequency weightings, there are also time weightings and peak detectors, as these have a significant impact on measurement results. To verify this, bursts are applied, and the sound level meter must display the expected value within tolerance. These signals are also used to check LAE or LAeq.

A final detector that needs to be checked is the overload. It is important for the device to accurately indicate whether a measurement is outside its measurement range or not. Operating outside the measurement range can quickly lead to incorrect but seemingly plausible readings. Therefore, the overload detection must also be verified.

The next tests are focused on the stability of the device. A measurement is conducted to see whether the sound level meter remains stable over approximately 30 minutes, to detect any drift in the device. Additionally, a 5-minute high-level measurement is performed to ensure the device continues to function correctly, even when operating near the overload threshold.

For the final electrical test, the device’s self-noise is checked to ensure it is still below the maximum values specified by the manufacturer. This ensures that the sound level meter’s electronics do not induce excessive noise into the measurements.

In the next phase of the calibration, the corresponding microphone of the sound level meter is used again. A calibration is carried out with a B&K 4226 multifunction calibrator to ensure that the reference point at 94dB is accurate. After that, three measurements are performed at 1kHz, 125Hz, and 8kHz. The average of these measurements is documented.

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