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CHAPTER 13: Loudspeaker Cabinets > ABR SYSTEMS - Pg. 393

CHAPTER 13 Loudspeaker Cabinets excursion is reduced, providing higher SPLs and lower distortion levels compared with reflex and sealed box designs. The calculation of the length of the line required for a certain bass extension appears to be straightfor- ward, based on a simple formula: where: f is the quarter wavelength frequency 344 ms is the speed of sound in air at 20°C is the length of the transmission line However the introduction of the absorption materials reduces the velocity of sound through the line, as discovered by Bailey in his original work. Bradbury published his extensive tests to determine this effect in an AES Journal in 1976 (Bradbury, 1976) and his results agreed that heavily damped lines could reduce the velocity of sound by as much as 50%, although 35% is typical in medium damped lines. Bradbury's tests were carried out using fibrous materials, typically longhaired wool and glass fiber. These kinds of materials however produce highly variable effects that are not consistently repeat- able for production purposes. They are also liable to produce inconsistencies due to movement, cli- matic factors and effects over time. High specification acoustic foams, developed by PMC with similar characteristics to longhaired wool, provide repeatable results for consistent production. The density of the polymer, the diameter of the pores and the sculptured profiling are all specified to provide the cor- rect absorption for each speaker model. Quantity and position of the foam is critical to engineer a low- 344 4 f (13.4)