FILTERS AND EQUALIZERS

 Correct 


The cut-off frequency is reduced by - 3 dB, an amount that is deemed to be significant (where the intensity level is reduced by half).
That may sound redundant when the statement refers to attenuation, like a double negative, but in practice it is clear to say either "attenuated by 3 dB" or

"attenuated by - 3 dB"