In very small rooms the air mass loads sources and reduces their radiated output.

Evidence

In this region, the room is smaller than a half wavelength in all dimensions. However, this does not mean that the room does not support sound propagation; it does, although it behaves more like the air in a bicycle pump when the end is blocked. This means that the environment “loads” any sources of sound in the room differently (such as loudspeakers or musical instruments), and often, but not always, the effect of this loading is to reduce the ability of the source to radiate sound into the room, and so results in reduced sound levels at these frequencies. This is because the acoustic impedance at the source is altered, making it harder for it to radiate sound. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2021

Categorization

• DEFINE: Renames the ‘room cutoff region’ as the pressure zone and clarifies it does not block propagation but impedes radiation.
• XREF: Links back to acoustic impedance and radiation-loading concepts introduced in earlier chapters.