L=numel(t); % how long is t vector???
f = Fs*(0:L-1); % zero-based frequency of same number elements
NOTA BENE:: The "-1" correction for 0-based counting instead of 1-based to match total number of elements.
It is peculiar to base the frequency vector on the length of the time vector, however; generally one uses a fft to convert a time-based sampled data set into frequency and the zero-based one-sided frequency vector is then half the length of the input time trace. See the examples at the doc link above for typical application/use.
db in Signal Processing Toolbox will convert to decibels assuming voltage or optional based on power (square input).
As virtually all MATLAB functions are, it is vectorized to operate over an input vector or array.
