Membrane/panel absorbers

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. 

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. 

Figure 3 Light-induced absorbance change measurements (Left panels) Kinetics of P700 photooxidation(AA7oo) with thylakoid membranes of control and a Chi Mess mutant of C. reinhardtii. Upper trace, control lower trace, Chi Mess mutant. (Right panels) Corresponding semilogarithmic plot of theAA70o kinetics.

Materials cost reduction has been achieved through the use of thln-fllm polymeric materials In both the absorber and glazing portions of the collector. The films, attached to a lightweight bent-metal frame, form a set of stressed membranes that contribute to the overall strength of the panel. 

The absorption spectrum of the acceptor is shown in the bottom panel of Figure 15.19. A favorable aspect of retinal is that it can be photobkached. thus decreasing the acceptor concentration in the disk membranes. Rrior to pho-tobleaching, the absorption spectrum of retirtal overlaps with that of the Tb donor. The emission spectrum of terbium is typical of the lanthanides. The emission is from /orbitals of the atom, and the emission spectra are typically highly structured line spectra. Also, it is cormnon to use lanthanide chelates, rather than lanthanides alone. This is because the lanthanides are extremely weak absorbers, with extinction coefricients near 0.1 cm. Fortu-... 

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