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CHAPTER 13: Loudspeaker Cabinets > CABINET CONSTRUCTIONS - Pg. 397

CHAPTER 13 Loudspeaker Cabinets Partly for this reason, but also because fibrous materials are better absorbers where the particle veloci- ties of the air movement are at their highest (they must be zero at rigid boundaries, so they are at their lowest close to the boundaries), the absorbent materials are best placed in the volume of the box, lightly packed, and not only against the sides of the box. Reticulated (open cell) foams, glass fiber, mineral wool, bonded acetate fibers, polyester fibers and cotton-waste felt are all common lining materials. The cut-away view of the "transmission-line" cabinet shown in Figure 13.9 illustrates the use of a synthetic foam lining which the manufactures specifically chose for its ability to maximally damp the line. Material types and densities are normally chosen with care for specific applications. The KEF loudspeaker company, in the 1980s, noticed some differences in their loudspeaker frequency responses depending upon whether the excitation signal was of a steady state or transient nature. The dis- crepancy turned out to be due to non-uniform movement of some of the internal lining materials, which was somewhat uncontrolled after the shock excitation of a transient signal. The lining materials should therefore not be in panels which can vibrate en masse, or non-linear effects may be sufficient to be noticeable in the sound from the loudspeakers. Vibrating lining materials may settle into regular patterns on relatively steady signals, but can be excited rather unpredictably by transient shocks. At high SPLs, the linings can move in rather erratic manners, but the effects are usually swamped by the higher SPLs of the radiation direct from the driver. Nevertheless the ideal lining would be relatively inert. Colloms (2005) claims that unstable lining materials can impair the sense of "rhythm" from a loudspeaker system. At low frequencies, the thicknesses of the lining materials are far too little to provide much absorption, but the cabinets are normally also far too small to support any resonant modes (100 Hz would require an internal dimension of at least 1.6 meters) so the lack of absorption rarely becomes a practical prob- lem. However, at higher frequencies, the absorption is important in order to reduce internal resonances which could powerfully excite structural resonances in the cabinet walls, and which would then radiate