Lower limit of measurement of sound level meters and self-generated noise
The new international standard IEC61672-1:2002 for sound level meters and the new JJG188-2002 metrological verification procedure for sound level meters define the total range of a sound level meter as follows: the response to a sinusoidal signal, from the smallest sound level on the maximum sensitivity level scale to the highest sound level on the minimum sensitivity level scale, with no over- or under-range indication and with the level linearity error within the specified tolerance, the testable A weighted sound level range. It is also specified that the class linearity error plus the extended uncertainty due to measurement (0.3 dB) shall not exceed ±1.1 dB for Class 1 sound level meters and ±1.4 dB for Class 2 sound level meters for all class ranges at any frequency weighting or frequency response range of a sound level meter at any frequency. Should be lower than the lower limit of measurement 8dB, 2-level sound level meter should be at least 6.7dB lower than the old standard at least 5dB higher than the requirements.
However, at present, there are many manufacturers will be self-generated noise (background noise) value specified as the lower limit of measurement of the sound level meter, which is obviously misleading the user. Users should pay attention to the selection of such sound level meters, the actual measurement of the lower limit than they are given to be higher than 6.7dB ~ 8dB. some manufacturers are still according to the old national standards and international standards of sound level meters, the lower limit of the measurement of the noise floor than the high 5dB, which is also not accurate enough.
The lower limit of measurement of sound level meter is mainly determined by the sensitivity of microphone and the self-generated noise of sound level meter, and to reduce the lower limit of measurement, we should start from these two aspects. In the new international standards and new regulations, manufacturers are required to give the highest self-generated acoustic noise and self-generated electrical noise. Requirements will be placed on the sound level meter in a low-noise sound field to measure the self-generated acoustic noise, due to some only for the A sound level is a low-noise sound field, so this time can only measure the self-generated acoustic noise of the A sound level. Self-generated electrical noise is measured by using an equivalent electrical impedance instead of a microphone. As we know, the microphone also generates self-generated noise (thermal noise), so usually the self-generated acoustic noise of a sound level meter is greater than the electrical noise. The equivalent electrical impedance of a microphone is essentially a capacitor with a capacity of about 50 pF for a 1-inch microphone and about 15 pF for a 1/2-inch microphone, and the self-generated noise will be different for different capacitance tests. In the test of self-generated electrical noise, should not be used for the electrical signal transfer fits, this kind of fit the capacitor contained in the 0.01μF or 0.1μF, with which the measured electrical noise will be significantly lower. In addition, for self-generated noise measurements, for F and S time-weighted sound levels, the arithmetic average of 10 random readings within 60s should be used instead of the maximum reading, and for time-averaged sound levels, the averaging time should be at least 30s.
