A wall’s edge acts as a point source of diffracted sound

Evidence

The diffraction from the edge is understandable from the cell model as follows: a cell just above the wall, but more than about half a wavelength from the end point of the wall, won’t “feel” the end point, since it is too far away to send and receive information (within one period) from the end point at the speed of sound. The impedance of such a cell is therefore not reduced because it does not sense the lower resistance to pressure changes near the wall. More than half a wavelength beyond the end of the wall also acts like any other cell in free space, again because it is too far away to send and receive the information that the wall is present. There is one special cell acting strangely, straddling the end of the wall. This cell is busy scattering sound because its impedance is different than any adjacent cell. It scatters whatever impinges on it. The scattered (diffracted) wave may be understood as coming from this small region, which therefore acts like a small, or point source of sound. — Eric J. Heller, Why You Hear What You Hear- An…, 187

Categorization

• DEFINE: Gives an intuitive, mechanistic explanation of why an edge behaves like a point source — the edge cell has odd impedance.
• QUESTION: The ‘information within one period’ framing suggests limits on how fast pressure signals propagate locally — worth chasing the original cell-model derivation.
• XREF: connects to: A source at a wood’s edge creates large boundary effects on forest sound attenuation. — T. F. W. Embleton, Sound Propagation in Homogeneous…, 14