Why Is a Room With Curtains Quieter? Turning Sound Into Heat

Sound arriving at a surface is partly reflected, partly transmitted through and partly absorbed, meaning converted into heat. Absorption in porous materials happens because sound is a movement of air, and forcing air back and forth through the narrow passages in a fibrous or open-celled material makes it rub against the fibres, which dissipates energy as friction. The quantity of heat is negligible, and the reduction in reflected sound is not. That mechanism requires the material to be porous rather than merely soft, which is why an open-cell foam absorbs and a closed-cell one does not, and why a heavy solid curtain absorbs less than a lighter porous one of the same weight.

Different mechanisms handle different frequencies and no single material covers the range:

• •Porous absorbers including mineral wool, foam and heavy fabric, effective at middle and high frequencies
• •Thickness determines the lowest frequency a porous absorber handles, since it must be a substantial fraction of the wavelength
• •Mounting a porous absorber away from the wall improves its low frequency performance considerably
• •Panel absorbers, where a sheet vibrates and dissipates energy, tuned to low frequencies
• •Resonant absorbers with a cavity and an opening, which absorb a narrow band around their resonance
• •Bass frequencies are the hardest and most expensive to treat, since the wavelengths are metres long

The most consequential confusion in the whole subject is between absorbing sound within a room and preventing it passing to another, which are different problems with opposite solutions. Absorption needs light porous material that lets air move through it and dissipates energy. Blocking transmission needs mass, since a heavy dense barrier resists being driven by the sound, and needs airtightness, since any gap passes sound freely, and needs decoupling so that vibration is not carried structurally. Lining a wall with foam therefore reduces echo within the room and does essentially nothing for a neighbour, which is why people who fit it for that purpose are disappointed, and why studio construction uses heavy isolated shells with absorption applied inside them.

Absorbing sound is not the only treatment and over-absorbing produces its own problem, namely a room that sounds dead and unpleasant to be in. The alternative is scattering, where a surface shaped with irregularities of varying depth reflects sound in many directions rather than back along its arrival path, which removes the distinct echoes and the standing waves without removing energy from the room. Diffusers designed on mathematical sequences scatter across a defined frequency range predictably, and a well-proportioned room with bookshelves, mouldings and furniture achieves something similar by accident, which is why old rooms full of objects frequently sound better than modern empty ones. Good acoustic design balances the two rather than treating absorption as the whole answer.

Treating a space means deciding how long sound should persist, which differs by purpose. Reverberation time, meaning how long a sound takes to decay by a specified amount, is the standard measure and has target ranges for each use, with speech requiring short times so that syllables do not blur, orchestral music requiring long ones for blend and fullness, and recording studios requiring very short ones so that the room contributes as little as possible. Achieving a target means calculating the total absorption in the room from the area of each surface and its absorption at each frequency, then adding material until the result matches. Uneven treatment produces uneven decay across frequencies, which sounds wrong even when the average is correct.

Sound moves air, and forcing air through the passages of a porous material dissipates that energy as friction, which is why porosity rather than softness is what matters. Thickness sets the lowest frequency a porous absorber handles, so bass is hardest and most expensive to treat. Absorbing sound in a room and stopping it reaching a neighbour need opposite materials, which is the commonest confusion in the field.