You can change the base of the logarithm by dividing it by the desired base, then use the appropriate code to change the tick labels:
frex = 2.^((10:76)/10)
frex2 = log(frex)/log(2)
N = 1024;
n = 0:N-1;
w0 = 2*pi/5;
x = sin(w0*n)+10*sin(2*w0*n);
% Compute the short-time Fourier transform using the function defaults. Plot the spectrogram.
figure
spectrogram(x,'yaxis');
figure
spectrogram(x,'yaxis')
Ax = gca;
yt = Ax.YTick;
Ax.YTickLabel = log(yt)/log(2);
This example is from the spectrogram documentation, with my changes to the y-axis labels.
.
