Every 20 dB is a factor of 10, while every 10 dB is a factor of 3, and since dBs are the log theyse add. So 90 dB S/N means that the noise 10x10x10x3, or .003% as strong as the signal. Anything over 70-80 dB is inaudible, while 40 dB sounds like a cheap tape recorder, and 60 dB like a fairly good tape recorder with Dolby.