[Openal] Can someone explain signed vs. unsigned audio

[Kyle D. Jansen]

>
> For signed 8 bit:
> Silence would be 0. Full positive would be 127. Full negative would be -128.
>
> For unsigned 8 bit:
> Silence would be 127. Full positive would be 255. Full negative would be 0.
>
> Similarly with 16 bit signed: 0 is silence, 32767 is full +ve, -32768 is full -ve.
> 16 bit unsigned: 32767 silence, 65535 full +ve, 0 full -ve
>
> --
> Best regards,
> Peter                            mailto:darkmatter at blueyonder.co.uk

Then what is the difference between signs, since, as far as the bits, [signed] -128 == [unsigned] 0?
Further, how is volume heard? ie, is -127 as "loud" as 127? I'm doing calculations with the psychological volume, and I want to be clear on this.

Thanks.