"Sound" normally has variations in local density, velocity, and pressure.

In this way individual cells can sense the local density of bacteria, so that the population as a whole can make a coordinated response.

If that ratio is greater than the local density of water, you sink.

The local density is estimated by the typical distance at which a point can be "reached" from its neighbors.

This reduces the effective area for current and increases the local current density.

Migration can regulate the local density of animals.

Within that range, dispersion is variation in local density.

Channel interactions might occur if the local density of channels places them in contact.

Furthermore, while party size varies, the local density of animals does not.

The sources are appropriately taken to be as local densities of charge and current.