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Sub Sub-Bass System Dear Kindly Editor: Because there has been so much interest in the 12 Hz cutoff system mentioned at the end of my article, "Switched-On Bass," I'd like to briefly describe it. The system uses the same basic enclosure as the 20 Hz unit with one modification. The vent dimensions are changed so that the vent area is 645 cm2 (100 in^2). The vent length is the same as for the system with the 15W38C. A 5.0 Ohm resistor is used in series with the 18W54C to adjust its Q. The efficiency is about 0.2 percent. Personally, I see no need to build such a system. There is no musical content below 40 Hz on most recordings. I know. I spent over 8 years as a recording engineer in Nashville. The musical instruments just don't have any output in that region which is useful. Also, very, very few studio tape recorders are capable of accurately recording signals below 40 Hz. Furthermore, most recent psychoacoustic work tends to point to a 20 to 25 Hz cutoff in the ear. Nature seems to have included this in the ear's design to protect us from wind noise, etc. Therefore, I recommend the 20 Hz system. Whichever you build, I wish good luck with your system! One final note. There has been some confusion about which drivers are used in the 20 Hz system. This was caused by typographical errors in my manuscript which somehow slipped past the Erudite Copyreader. The correct model numbers are Altec 416-8A, CTS 15W38C, and JBL 2205A. I am indebted to one of the Gentle Readers, Mr. Jack L. Boyle of Philadelphia, for pointing out an error in the schematic of the crossover. The 0.33 µF capacitors in the low-pass section, which feeds the subwoofer, should be 0.033pF. W.J.J. Hoge CTS of Paducah Paducah, Ky. 42001 Wants AM Sections Reviewed Dear Sir: It is my strong belief that is inexcusable for manufacturers of high fidelity tuners, if they elect to include an AM band on their equipment, not to make a bona fide effort to provide quality reproduction. Despite the excellent article you published by George McKay Jr. on the subject a while back, there prevails the general notion that AM radio is hopelessly lo fi with little to be expected from it, and I suspect that view is largely the result of audiophiles' familiarity with the performance of the AM section of supposedly high fidelity equipment. Since your "letters," column shows a continuing interest in this subject, and since, apparently, only Audio is open minded enough to publish material on AM reception, may I cast my vote for a specific (even if brief) review of AM sections of tuners and receivers under test. It seems to me that only when the often despicable inadequacies of these AM sections are regularly made the subject of public examination are we likely to see an attempt by the manufacturers to bring about improvements. By the way, our Chief Engineer at KEX, Paul Matthew, and his assistant, Donn Werrbach, have recently upgraded the radio station's audio to essentially flat response over the entire audio spectrum, to 15 kHz, with extremely low distortion. On the Miller Crystal Tuner (how I wish they still made those), we are clearly the best fidelity (and lowest distortion) AM station receivable in Portland, and our fidelity beats all the regional FMs too. We're proud of this and wish more people were able to hear this. -Eric G. Norberg, Program Director, Radio Station KEX, Portland, Ore. (Source: Audio magazine.) Also see: Syndication of Quadraphonic Radio Programs (Nov. 1976) = = = = |
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