Lower measurement limit of sound level meter and self-generated noise
The definition of the total range of the sound level meter in the new international standard IEC61672-1:2002 for sound level meters and the new measurement and verification regulations for sound level meters JJG188-2002 is: the response to a sinusoidal signal, starting from the minimum on the maximum sensitivity level range The A-weighted sound level range that can be tested when there is no overload or underrange indication and the level linearity error is within the specified tolerance range. At the same time, it is stipulated that within any frequency weighting or frequency response range of the sound level meter, on all level ranges of any frequency, after the level linear error plus the expanded uncertainty caused by the measurement (0.3dB), the level 1 sound level Level 2 does not exceed ±1.1dB, level 2 does not exceed ±1.4dB. Accordingly, in order to ensure the level linear error requirements, after deducting the influence of uncertainty, the self-generated noise of a level 1 sound level meter should be at least 8dB lower than the lower measurement limit, and that of a level 2 sound level meter should be at least 6.7dB lower. They are all at least 5dB lower than the old standard and require higher standards.
However, many manufacturers currently stipulate the self-generated noise (noise floor) value as the lower measurement limit of the sound level meter, which is obviously misleading users. Users should pay attention when choosing. The actual measurement lower limit of this type of sound level meter is 6.7dB~8dB higher than what they give. Some manufacturers still stipulate that the lower limit of measurement is 5dB higher than the background noise according to the old national and international standards for sound level meters, which is also not accurate enough.
The lower measurement limit of a sound level meter is mainly determined by the sensitivity of the microphone and the self-generated noise of the sound level meter. To lower the lower limit of measurement, we must start from these two aspects. In the new international standards and new regulations, manufacturers are required to give the maximum self-generated acoustic noise and self-generated electrical noise respectively. It is required that the sound level meter be placed in a low-noise sound field to measure self-generated sound noise. Since some are low-noise sound fields only for A sound level, only the A sound level of self-generated sound noise can be measured at this time. Self-generated electrical noise is measured by using equivalent electrical impedance in place of the microphone. We know that microphones also produce self-generated noise (thermal noise), so generally the self-generated acoustic noise of a sound level meter is greater than electrical noise. The equivalent electrical impedance of a microphone is basically a capacitor. Its capacitance is about 50pF for a 1-inch microphone and about 15pF for a 1/2-inch microphone. The self-generated noise measured by different capacitances will be different. When testing self-generated electrical noise, adapters used for electrical signal switching should not be used. The built-in capacitor of this adapter is 0.01μF or 0.1μF, and the electrical noise measured with it will be significantly lower. In addition, when measuring self-generated noise, the time-weighted sound levels of F and S should be the arithmetic average of 10 indications randomly read within 60 seconds, rather than the maximum reading value. For the time-averaged sound level, the average time should be at least 30 seconds.
