Cyclic acetal compounds of formula wherein k0 or 1 and n is an integer of 0 to 6 are novel. The non-woven fabric forms elongated cells. Preferably, the substrate comprises a glass doped with rare earth ions. The amplitude modulation frequency control generators are adapted to accurately control the frequency of the amplitude modulations to within an accuracy of at least 1000 ppm, most preferably to within about 1 ppm, relative to one or more determined or predetermined reference amplitude modulation frequencies.