The volume velocity of a gas flow is defined as particle velocity times the cross-sectional area of the flow, or
where denotes position along the flow, and denotes time in seconds. Volume velocity is thus in physical units of volume per second (m /s).
When a flow is confined within an enclosed channel, as it is in an acoustic tube, volume velocity is conserved when the tube changes cross-sectional area, assuming the density remains constant. This follows directly from conservation of mass in a flow: The total mass passing a given point along the flow is given by the mass density times the integral of the volume volume velocity at that point, or
As a simple example, consider a constant flow through two cylindrical acoustic tube sections having cross-sectional areas and , respectively. If the particle velocity in cylinder 1 is , then the particle velocity in cylinder 2 may be found by solving
It is common in the field of acoustics to denote volume velocity by an upper-case . Thus, for the two-cylinder acoustic tube example above, we would define and , so that
would express the conservation of volume velocity from one tube segment to the next.