Resonators are designed to absorb a narrow frequency range of sound. They are made of materials that are perforated and have either slots or holes in them. For example, the resonator Hemholtz is in the design of a bottle. The amount of air in the bottle, the length of the bottle's neck, as well as the size of its mouth all control the resonant frequency. Slots, as opposed to holes, generally absorb mid-frequency sound and longer narrows slots will absorb low frequencies.
Panel absorbers are designed to resonate in response to sound. They are generally made of non-porous, non-rigid materials. Examples include, lightweight wood paneling over a frame, lightweight floors and ceilings, glazing, as well as other large surfaces that will be able to resonate in response to noise. Panel absorbers trap low frequencies.
More commonly used, porous absorbers convert sound into heat energy. Examples of porous absorbers include, draperies, carpet, open-cell foam, cast-porous or felted ceiling tiles, fibrous mineral wool or glass fiber, aerated plaster, and sprayed-on cellulose. In general, the sound absorption will be better, when the material is applied to a hard surface if it's thicker.