The sound pressure level (SPL) of two sound sources A and B are measured in an anechoic chamber with background noise. The measured sound pressure level when only sound source A is switched on is 85 dB; when only sound source B is switched on is 90 dB; and when both sound sources A and B are switched on is 91 dB. Find the SPL of the background noise in the anechoic chamber, assuming all sound sources are uncorrelated. Keep your calculations to the 4th decimal place.