Lumped impedance treats pipe cross-sections as single units using volume velocity

Evidence

Acoustical impedance (lumped) Z. The specific acoustical impedance is determined at a single point. Sometimes a lumped impedance is better to work with. For example, when we want to determine the impedance mismatch and reflection upon a sudden change of pipe diameter, it is convenient to have a single lumped impedance for pipes of given diameter. For this, the impedance definition is changed a little, so that all the cells across the pipe are lumped together and the velocity used is the volume velocity—that is, the velocity attained by the little cells times the area S of the pipe. — Eric J. Heller, Why You Hear What You Hear- An…, 103

Categorization

• DEFINE: Introduces lumped vs specific impedance distinction, key for pipe and waveguide acoustics.
• XREF: connects to: Snow grounds require a layered porous-medium impedance model. — Donald G. Albert and John A. Orcutt, Acoustic Pulse Propagation Above…, 26