OK.
There can be frequency bins with negative values.
And so when you say you 're adding something that has the overall shape of speech , is that in a  in a particular frequency bin ? Or you 're adding something across all the frequencies when you get these negatives ?
For each frequencies I a I 'm adding some , uh , noise , but the a the amount of  the amount of noise I add is not the same for all the frequency bins.