Nils Arthur asked in another post:
While we are talking volume controls. Could you explain why it's only possible to lower the volume in Windows (i.e. setting a volume between 0% and 100%) and not raise it (i.e setting it higher than 100%)?
Before I get into the the answer, let me define some terms: Attenuation means reducing the amplitude of a signal from a baseline - so if the signal is a full range sine wave going from 1.0 to -1.0, if you attenuate it to 50%, you get a sine wave from 0.5 to -0.5. I wrote about it (with pictures :)) in this post.
The answer to Nils' question is both simple and complicated.
The simple part of the answer: Because most PC audio hardware only supports attenuation and not amplification.
Now for the complicated parts of the answer: We only support what the hardware allows for master volume. And most hardware only supports attenuation. There are a lot of reasons for that, but the primary one is that it's dramatically cheaper (and uses less power) to attenuate signals than it is to amplify them.
The other issue w.r.t. amplification/attenuation is signal quality. As I mentioned in the post on volume above, you can attenuate a sample in the digital domain without loss of fidelity. However when you attempt to amplify a signal in the digital domain, it clips. That means that amplification MUST be done in the analog domain. Again, this goes to hardware costs - because amplification needs to be done in the analog domain, it means that the audio hardware needs to have an amplifier that can be digitally controlled, which is (again) more expensive. The audio hardware doesn't even have to support hardware volume - if Windows doesn't find a hardware volume control, it simply inserts a master volume into the audio pipeline.
Some audio hardware DOES support amplification, but the audio volume controls map the volume control from low to high into a range from 0..100 because it's dramatically simpler to represent that to the user.
That means that if an audio solution presents a hardware volume from -96.0dB to +3dB (there are a number of them that do that), we'll map that 99dB range into a 0.0 to 1.0 range that maps nicely into a slider. We've thought about differentiating between attenuation and amplification in the volume UI, but the reality is that the net effect is the same whether we represent amplification or not.
You can see if your audio hardware supports amplification by going to the multimedia control panel. Select the audio endpoint you want to check, go to the "Properties" dialog. On that dialog, check the "Levels" tab, the hardware master volume control is present there. You can right click on the text box and change the units from linear to dB, you can then move the slider around to see the dB range. Or you could write some code and call the IAudioEndpointVolume::GetVolumeRange API, which will return the information directly.