Fibrous absorbers become ineffective below roughly a quarter-wavelength thickness.

Evidence

Above the transition frequency fibrous (resistive) absorbers are widely used to reduce the energy in reflected sounds. Their performance is dominated by their thickness, becoming rapidly less effective when the thickness is less than about 1/4 wavelength because particle velocity diminishes to zero at the reflecting surface. For now, it is sufficient to know that resistive absorbers are, in effect, low-pass filters for reflected sounds; they attenuate the higher frequencies. Impulsive measurement methods tend to respond visually to the elimination of high frequencies, and this is often regarded as a measure of success. — Toole, Floyd E, Sound reproduction- the acoustics and…, 648

Categorization

• DEFINE: Pins down the quarter-wavelength rule that governs why thin fiber panels only absorb high frequencies.
• XREF: Connects to the reader’s deep interest in measurement methodology: the visual seduction of impulse responses showing only high-frequency removal.
• QUESTION: The author flags that impulse measurement tends to reward high-frequency absorption visually — worth investigating how this biases practitioners’ choices of absorber thickness.