An exponential distance model which calculates distanceGain according to: pow(distance / refDistance, -rolloffFactor)
An inverse distance model which calculates distanceGain according to: refDistance / (refDistance + rolloffFactor * (distance - refDistance))
A linear distance model which calculates distanceGain according to: 1 - rolloffFactor * (distance - refDistance) / (maxDistance - refDistance)