The mathematical relation between intensity and loudness is:-
$$ I_{L} = 10 \log_{10}\left( \frac{I}{I_{0}} \right) $$
where $I$ is the sound intensity and $I_{0}$ the reference intensity. The unit of sound intensity is expressed in decibels (dB).
The mathematical relation between intensity and frequency is:-
$$ I = 2 \pi^{2} \nu^{2} \delta^{2} \rho c$$
with $I$ the sound intensity, $\nu$ the frequency of sound, $\delta$ the amplitude of the sound wave, $\rho$ the density of the medium in which sound is travelling and $c$ the speed of sound.
This indicates that as frequency increases, loudness must also increase.
Then, why most physics textbooks mention that Loudness does not depend on Frequency?
