The intensity level of a sound source is defined as the logarithm of the ratio of the intensity of the sound to a reference intensity. The reference intensity is usually taken to be 10^(-12) W/m^2.

The intensity of a sound wave is proportional to the square of the amplitude of the wave and inversely proportional to the square of the distance from the source. Therefore, if the distance to the sound source is halved, the intensity of the sound will be increased by a factor of 4.

The intensity level will then increase by a factor of 10log(4) = 6 dB.

Therefore, if the distance to a sound source is halved, the intensity level will increase by 6 dB.