If I have a 2volt input for the below circuit (LM741 opamp), the gain would be 1+586/1000 = 1.586 giving me an output of 3.172 volts

So 20 log (vout/vin) = log(1.586) = 0.2

Therefore 20*0.2 = 4db on the pass band as per the AC analysis.

So for the corner frequency I calculated as

3.172*0.707 = 2.24volts

Therefore 2.24/2 =1.121302 = log(1.121302) = 0.0492

Therefore 20*0.0492 = 1db as per the AC analysis.

But on the transient analysis the output is 2.14v not 2.24v at 1Khz, what causes this voltage drop ?

The same is for at 3Khz, the attenuation is -15.152db on the AC analysis

But when I calculate results from transient analysis I get,

20log (326mv/2) so (326mv/2) = 0.163, therefore log (0.163) = -0.7878

Therefore 20*-0.7878 = -15.75db.

thanks matt.