Sound absorbers resonant

Resonant Sound Absorbers. Two other types of sound-absorbing treatments, resonant panel absorbers and resonant cavity absorbers (Helmholtz resonators), are used in special appHcations, usually to absorb low frequency sounds in a narrow range of frequencies. Resonant panel absorbers consist of thin plywood or other membrane-like materials installed over a sealed airspace. These absorbers are tuned to specific frequencies, which are a function of the mass of the membrane and the depth of the airspace behind it. Resonant cavity absorbers consist of a volume of air with a restricted aperture to the sound field. They are tuned to specific frequencies, which are a function of the volume of the cavity and the size and geometry of the aperture. 

It is often difficult to obtain absorption at low frequencies with porous textile absorbers because the required thickness of the material is large and the sound absorbing layers are often placed at room boundaries where the absorbers are inefficient due to the low particle velocity. Resonant absorbers might be a solution. There are two common forms of resonant absorbers manbrane/panel absorbers and Helmholtz absorbers. A membrane/panel absorber is a sheet of vinyl or plywood, which is free to vibrate while for a Helmholtz absorber, the mass is a plug of air in the opening of a perforated sheet. The spring in both cases is provided by air enclosed in the cavity. Best performance can be obtained by placing a porous textile absorbent in the neck of the Helmholtz resonator or just behind the membrane in the panel absorber. The resonant frequency of this type of absorber can be tuned to the frequency of interest. 

The degree of attenuation at the critical frequency can be very large, but this type of silencer has a very narrow bandwidth. This device may be suitable when the machine being dealt with emits sound predominantly of a single wavelength. Lining the chamber with absorbers can expand the absorber bandwidth of a Helmholtz resonator, but this has the effect of reducing the efficiency. The perforated absorber, which forms the basis of many acoustic enclosures and silencers, is a development of the resonator principle. 

Respirators muffle and distort speech because the facepiece material absorbs sound, the face-piece moulding forms resonant cavities and the faceseal restricts movement of the lower jaw (Mozo and Peters, 1984 Johnson and Sleeper, 1985 Nelson andMozo, 1985 Fine and Kobrick, 1987 Taylor and Orlansky, 1987 Wetherell and Gwyther, 1993). Voice modules help, but do not resolve the problem completely. 

Schematic representation of nuciear resonance based on the recoiiiess emission and absorption of a 7 photon. The story that goes with the figure is this. A nucieus in excited state emits a y photon with energy (Eol equai to the difference between the energies of the excited-and ground-state ieveis, and, subsequentiy, the same y photon is absorbed resonantiy by another nucieus in the ground state by transforming it to the excited state. This sounds rather triviai but in order to happen both the emission and the absorption of the photon must be a recoiiiess/ recoii-free event (this is caiied the Mossbauer effect], because recoii of either nucieus wouid render the energy of the photon insufficient for the resonance to occur...

Warburton G (1982) Optimum absorber parameters for various combinations of response and excitation parameters. Earthquake Eng Struct Dyn 10(3) 381 01 Warminski J, Kecik K (2009) Instabilities in the main parametric resonance area of a mechanical system with a pendulum. J Sound Vib 322(3) 612-628 Wiesner KB (1979) Tuned mass dampers to reduce building wmd motion. In ASCE convention and exposition, vol 3510. ASCE, Boston... 

Other properties which have provided lead with several new applications in recent years are its capacity, when used in either metallic or chemical form, to absorb radiation, sound and vibration. In this respect, the most important of lead s characteristics are its density, high atomic number and formability. Lead (and its alloys) has no natural resonant frequencies and for this reason is considered to be a silent metal. ... 

---