Low-frequency sound passes through ordinary walls because impedance shifts with frequency

Evidence

The reflection of sound from surfaces also depends on frequency. Massive objects like brick walls naturally tend to yield mass-dominated impedance, but at low enough frequency the force is acting in the same direction for longer periods of time, and the impedance can become resistance-dominated or stiffness-dominated. Thus a thick, heavy pane of glass reflects about 97% of the energy at 1000 Hz, but only 70% at 100 Hz. This portends a problem for concert halls. The usual construction of rebar-reinforced concrete block and brick is wonderfully reflecting (too reflecting without further treatment) above 150 Hz, but between 20 and 80 Hz it becomes quite absorptive, resulting in poor low-frequency reflection and response. The solution is to make the walls concrete, 3 to 6 feet thick! — Eric J. Heller, Why You Hear What You Hear- An…, 2441

Categorization

• XREF: The impedance regime shift (mass- vs stiffness-dominated) ties into the earlier chapter discussion of friction in pipes and impedance analysis. — connects to: Ground impedance, not foliage, governs low-band attenuation by vegetation. — Donald Aylor, Noise Reduction by Vegetation and Ground, 2
• SEED: The 3-to-6-foot concrete-walls fix for low-frequency reflection is a vivid detail for an essay on why concert hall bass response is so hard to engineer.