Try this —
Fs = 44100;
t = linspace(0, Fs-1, Fs)/Fs;
s1 = sin(2*pi*t*15);
dB30 = db2mag(30)
s2 = s1*dB30;
figure
plot(t, s1, 'DisplayName','Original')
hold on
plot(t, s2, 'DisplayName','Original With 30 dB Amplification')
hold off
grid
legend('Location','best')
.

