it does this because human ears dont perceive all frequencies at the same
loudness even if they have the same amplitude.  you could find a table in
almost any acoustics book that shows the relative loudness for all freqs.  you
could then make a table of the inverse of that function that would be used to
raise or lower the amplitude at different freqs so that it would sound the
same loudness across the spectrum.  however, every ear is different, so not
everyone would perceive it properly.

pete


Rosati wrote:

> Greetings-
>
> If, for example, a sine tone was gliss. from 64 to 20000 cps at a constant
> amplitude, it  appears to get louder as the pitch increases. Does anyone
> know what is the easiest way to balance this so that loudness remains
> constant across the frequency spectrum?
>
> thanks,
>
> dante