Background noise.Generally, it refers to all interferences irrelevant to the existence of signals in the occurrence, inspection, measurement or recording system.However, in the measurement of industrial noise or environmental noise, it refers to the ambient noise outside the measured noise source.For example, if traffic noise is measured in the street near the factory, the factory noise is the background noise.If the purpose of the measurement is to determine the factory noise, the traffic noise becomes the background noise.During noise measurement, attention must be paid to the interference degree of background noise.[1]
The background noise cannot affect people's listening to the main voice, especially when it is necessary to be quiet, music is not only sound, but also a silent melody - at this time, it is important not to introduce any noise, such as the rumbling of electric fans and the noise of traffic.Noise evaluation curve（NC）It is a common standard used to identify background noise.
However, the noise evaluation curve is only applicable to continuous and time-varying sound waves without information.When sound waves carry important information, such as wonderful speeches or beautiful melodies, there must be stricter standards.
In some cases, the quieter the better.If the background noise is reduced, other previously hidden noises may be highlighted.Background noise can be enhanced to the level of noise evaluation value without people's awareness.This noise masking method is much more economical than the noise control method.
It is necessary to establish special standards for places where special activities are held.For example, in the recording studio, the accuracy of the noise standard should reach the minimum noise level of the recording equipment, but it should be higher than the recorder's hearing threshold. In this way, the recorder can clearly distinguish the noise sources at the beginning of the recording, and these noises will not be heard from the tape.[2]
Influence of background noise
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When measuring the noise level of a noise source at a certain site, it is usually impossible to eliminate the noise transmitted to the site by other noise sources (this noise is called background noise), but only to conduct overall measurement, which will cause a certain deviation in the sound level of the noise source to be measured.This is the effect of background noise.
If other noise sources can be excluded, the same problem does not exist. But for most occasions, the interference source is not specific, and there are many, so it is difficult to divide.
However, exclude the noise source to be measured (so-called "image"noise source）It is relatively easy.First, stop the determination of the noise source, only measure the noise level of the background noise, and then measure the sound level of the compound noise formed by the target noise source and other noise sources.If the intensity of the background noise is more than 10 dB lower than that of the combined noise, then the impact caused by the background noise will be less than 1 dB, that is, it will be regarded as interference without background noise.When the difference between the two is less than 10 dB, the approximate noise source of the object can be obtained by correcting the intensity of the compound noiseNoise level。[3]
Deduction of background noise
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During noise measurement.Even if the measured sound source stops making sound, there is still some noise in the environment, which is called ambient noise or background noise.The background noise will inevitably affect the noise measurement results.In other words, the measurement result is essentially the composite result of the noise and background noise under consideration.Only after deducting the background noise from this result can the correct sound pressure level of the noise under investigation be obtained.
If the sound pressure level of the measured noise source and theSound pressure levelIf the sound pressure level is 10 dB higher than the background noise and the sound pressure level of each frequency band, the influence of background noise can be ignored;If the difference between the measured noise and the background noise is 3~10 dB, it shall be corrected;If the difference is less than 3dB, the measurement result is invalid.[4]
