# Subtracting background noise from measurements?



## Mama70 (Dec 21, 2010)

I've been trying to lower the background noise levels so I thought how nice it would be to subtract one frequency response from another. 

I saved first some normal background noise with RTA and then measured the same noise with my projector on. Then saved the measurements as text files, imported to LibreOffice Calc, linearized and subtracted the background noise and finally converted the values back to dB-scale. 

Could it be possible to have this kind of feature also in REW? It would be nice to see for instance the waterfall chart without the background noise.


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## JohnM (Apr 11, 2006)

The All SPL plot has trace arithmetic in the Controls panel, however that would be of no help in lowering the noise floor of a waterfall. The waterfall is generated from the impulse response, which is a time signal. Removing tonal interference from the impulse response would require subtracting a signal in which the tonal interference had exactly the same phase. The bulk of the noise in a waterfall is random content which cannot be subtracted. To improve the signal to noise ratio of the impulse response try longer sweeps, higher levels and multiple sweeps.


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## Mama70 (Dec 21, 2010)

JohnM said:


> The All SPL plot has trace arithmetic in the Controls panel, however that would be of no help in lowering the noise floor of a waterfall. The waterfall is generated from the impulse response, which is a time signal. Removing tonal interference from the impulse response would require subtracting a signal in which the tonal interference had exactly the same phase. The bulk of the noise in a waterfall is random content which cannot be subtracted. To improve the signal to noise ratio of the impulse response try longer sweeps, higher levels and multiple sweeps.


Ok, typical RTFM, thanks


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