I have come across the paper that deals with spatial positioning of mobile sensors to optimally detect sound source, or position mobile cellphone towers to maximize the coverage.
The region $Q$ is partitioned into mutually exclusive $n$ Voronoi polytopes $W=\{ W_1,..,W_n \}$. A function $\phi :Q \to \mathbb{R}_+$ assigns probability density that a certain event (here sound source) has happened over $Q$. There are $n$ sensors to be distributed $P=(p_1,..p_n)$ over each Voronoi partition that satisfies the following equation:
$$H(P,W)=\min\sum_{i=1}^n \int_{W_i} f(\| q-p_i\|)\,d \phi(q)$$
The quality of observation at point $q$ from sensor $p_i$ is the distance of the point $q$ from the sensor $f(\| q-p_i\|)$ which makes sense.
Can anyone explain why are we multiplying $(q)$ after $d\phi$ in the objective formula?