The bode function of Control System Toolbox will plot the frequency response of a transfer function in dB. The function can also return numerical data of the magnitude of the response, and that it returns in absolute units, not dB. So, for example:
% create transfer function
H = tf([1 0.1 7.5],[1 0.12 9 0 0]);
% plot the frequency response - not the magnitude is in dB
bode(H)
% extract the magnitude data
[mag,phase,freqs]= bode(H);
% mag contains the responses at freqs, expressed in absolute units.
% convert to dB
magdb = 20*log10(mag);
See the reference page for more details.
doc bode
