Maximum sound output requires balancing radiation loss against internal friction

Evidence

Clearly the larger the surface area available for interaction, the higher the friction and therefore the absorption. This means that porous materials, which consist of a large number of fibers per unit volume, such as high-density rock-wool or fiberglass and plush carpets, will tend to have a high-level absorption. This also explains why curtains that are draped to a fraction of their cloth area absorb more strongly than ones that are flat. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2046 (#exemplifies, via linked claim)

Another form of energy loss occurs when the sound wave hits or travels through some porous material. In this case there is a very large surface area of interaction in the material due to all the fibers and holes. There are frictional losses at the surface of any material due to the interaction of the velocity component of the sound wave with the surface. A larger surface area will have a higher loss, which is why porous materials such as cloth or rock wool absorb sound waves strongly. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 344 (#exemplifies, via linked claim)

Categorization

• XREF: connects to: Sound instruments must balance confinement against letting sound escape; In-pressure-zone rooms sources radiate less sound due to altered acoustic impedance; Porous materials absorb sound because their large surface area creates frictional losses. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 344 (+3 more)

Broader Connections

• Porous materials absorb sound because their large surface area creates frictional losses. #exemplifies (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 344
• Porous absorbers damp sound through velocity-induced frictional losses #exemplifies (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2046
• In-pressure-zone rooms sources radiate less sound due to altered acoustic impedance #expands (0.80)
• Adjacent opposite-phase regions on a vibrating surface cancel their radiated sound #complicates (0.75)