I have a lot of background examples and information supporting this fact, that I won't bore you with at this time. So my answer to you is -6dB is half as loud and +6dB is twice as loud. All my audio listening experience tells me +6dB is twice as loud. I was taught +6dB is twice as loud by a well educated mastering engineer. The problem is the books have never been rewritten and many people use the audio encyclopedia as a reference book, so the 10dB refernce continues to flourish. Today music is not mixed the same way and the music is much closer in level to the voice level. So what happened is when you raised the music level, your ear would go into a compression state due to the high mid-range level of the voice over the music, and it actually took a boast of +10dB to sound twice as loud.

The problem there is that this study was done using music of that time in the 1930's, where the voice was mixed much louder than the background music.

It is further supported by the original audio encyclopedia which stated +10dB is twice as loud. Most of the people that believe the 10dB twice as loud number base this information off of the 'Fletcher/Munson equal loudness curves', which was a study done by Fletcher and Munson using pure tone sinewaves, which are not representative of a true complex waveforms like music. Most audio reference books say that a +10dB boast is twice as loud and a -10dB reduction is half as loud. Actually this is a debatable area as to what is 'half as loud' or 'twice as loud' is. 'The bottom line is whether the sound coming out of the PC's speakers should be half as loud when I cut a waveform down by 6 dB or 3 dB.' In digital audio 0dB is always as high as the waveform can go to, so the very top of the scale you are looking at is 0dB.or more precisely 0dBFS. This scale is used particularly in applications where sound travels in water.The dB levels in Forge are referenced to 'Full Scale' or dBFS. It should be noted at this point that there is another decibel scale in use, called the sound pressure level, based on the ratio of the pressure amplitude to a reference pressure. For example, a 56.0 dB sound is twice as intense as a 53.0 dB sound, a 97.0 dB sound is half as intense as a 100 dB sound, and so on. In equation form, intensity I is I=rac\ is given (and not the actual intensities), this result is true for any intensities that differ by a factor of two. Power is the rate at which energy is transferred by the wave. Intensity is defined to be the power per unit area carried by a wave. The relevant physical quantity is sound intensity, a concept that is valid for all sounds whether or not they are in the audible range. High noise exposure is hazardous to hearing, and it is common for musicians to have hearing losses that are sufficiently severe that they interfere with the musicians’ abilities to perform. In cartoons depicting a screaming person (or an animal making a loud noise), the cartoonist often shows an open mouth with a vibrating uvula, the hanging tissue at the back of the mouth, to suggest a loud sound coming from the throat Figure 2. We are all very familiar with the loudness of sounds and aware that they are related to how energetically the source is vibrating. But when a passing motorist has his stereo turned up, you cannot even hear what the person next to you in your car is saying. After settling into bed, you may hear your blood pulsing through your ears. In a quiet forest, you can sometimes hear a single leaf fall to the ground. Noise on crowded roadways like this one in Delhi makes it hard to hear others unless they shout.

Who made the Decibel Scale