Why? Because different frequencies are linearly independent and shouldn't affect eachother? That is only true for linear systems.

I am not sure what causes the other sounds to sound dampened, I figured it had something to do with how the ear works, that the sounds were physically still there.

But it is easy enough to try. Get a speaker or transducer rated for ultrasonic frequencies, and pump it with enough power that would make a real speaker sound painfully loud.