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CrossTalk -- Practical answers to your audio questions, by Michael Riggs--Foiled Reception; Beat the Band; Left, Right, Wrong; On the Level. Foiled Reception When we built our cabin in the woods, we experimented with builder's paper, which is coated on one or both sides with aluminum foil, and we put glass-fiber insulation with an aluminum vapor barrier below the floor. In effect, our house is an aluminum box as far as radio reception is concerned. After experiencing numerous difficulties and even allowing for our "boonies" location, far from the stations we enjoy--I'm beginning to wonder whether all that aluminum is acting as a super-nondirectional antenna. The tip-off is that when we open and close doors, which we built our selves and which (naturally) include foil, the static is horrendous. Have I analyzed the problem correctly? If so, what's the solution? -R. W. Pugh, Wolf Creek, Ore. First, let readers other than Mr. Pugh rest assured that if this letter is a put-on, it fooled all of us at HF, and that similar effects can occur with aluminum siding. So the problem is real, and Mr. Pugh's analysis is essentially correct (though the foil must be acting more like an RF screen than an antenna), however outlandish it may seem to urbanites. The first line of attack is an outdoor antenna on as high a mast as is practical. High gain is desirable; in commercial FM antennas, this implies narrow directivity and, therefore, a rotator (unless all your preferred stations are in the same direction). If you're not electrified, you could rig the mast so that it could be rotated by hand. And unless the mast is tall enough to hoist the antenna well above your "aluminum box." it should be placed on the side of the cabin that's toward the stations you are most eager to receive well. Beat the Band I am confused about the difference be tween the rated power bandwidth of an amplifier and its frequency response. Could you please explain? Is there any audible loss in an amplifier rated at 40 Hz to 20 kHz instead of the presumably more acceptable 20 Hz to 20 kHz? -John Potter, Oakdale, N. Y. Frequency response is normally measured at the 0-dBW (1-watt) output level. and power bandwidth at full rated out put. By FTC fiat, power bandwidth is the frequency range over which the rated power can be maintained without exceeding the rated distortion. In contrast, the old IHF standard required only that rated power be maintained within 3 dB, a more realistic figure for music reproduction, where power requirements are not as severe toward the frequency extremes as they are at midband. The answer to your second question depends. in part, on which of the three ratings-frequency response, FTC power bandwidth, or IHF power bandwidth you're quoting. Let's say, for example, that an amplifier is flat within a fraction of a dB from 10 Hz to 100 kHz, will put out 16 dBW (40 watts) between 20 Hz and 50 kHz. and is capable of producing 19 dBW (80 watts) between 40 Hz and 20 kHz without exceeding its THD spec. Once upon a time, such an amplifier would have been rated at 80 watts, with a power bandwidth of 20 Hz to 50 kHz and a frequency response of 10 Hz to 100 kHz. The frequency-response spec would remain the same today, but the manufacturer would either have to re duce the claimed power bandwidth (making it 40 Hz to 20 kHz) or cut the power rating to 40 watts and leave the power bandwidth unchanged (at 20 Hz to 50 kHz). Other options. though they're beside the point here, would be to split the difference (making the power band width, say, 30 Hz to 35 kHz), to raise the distortion spec until the old power band width could be maintained, or to redesign the amp for a better-looking FTC rating, though no net audible gain would necessarily result. If, as it seems, you're quoting the FTC rating, the key point is that no mu sic is ever likely to require as much power below 40 Hz as it does above, and no audible difference is to be expected between an amp rated down to 40 Hz and one rated to 20. On the other hand, if you're quoting frequency response and you like to listen to organ music through a really wide-range speaker system, the 40-Hz spec would be downright unacceptable. Left, Right, Wrong When I make cassettes from records, using a Nakamichi 350 deck and a B&O 1700 turntable, going through an old Marantz 1060 amp, the right channel plays back dull and muted unless I press the mono switch for that side. This doesn't happen when I'm listening to the record directly-only when it has been taped. What's wrong? -Don Peterson, Thermopolis, Wyo. It sounds as though your Nakamichi needs servicing. The likeliest cause of consistently muffled sound in one channel of an old deck such as yours is un even head wear. First clean the head thoroughly, in case the problem is merely unusually persistent dirt. Also check the tape connections to the Marantz. The mono button for the left channel--not the right, as you say should clear things up by putting the good (left) signal into both speakers. We've found that when head wear affects one channel more than the other, it's most often the outer (left) one. On the Level I recently upgraded from an Aiwa AD-6300 cassette deck to an Aiwa A D-L40U. In taping on the L40, I find that I get a VU reading 2 dB lower on playback than it was during recording (3 dB lower in the peak mode). When I play on the L40 a tape I recorded on the 6300, the meter shows the same level as it did during recording. What gives? -Tom McCannon, Pittsburgh, Pa. You can go bananas trying to account for discrepancies of a dB or two in consumer cassette decks. The meters and controls built into most models simply aren't ac curate enough to justify being ultra-persnickety with them. And since you make no complaint about the sound. it appears that nothing is seriously amiss here. If, in spite of this, you want to pursue the matter further, check the tape you're using against the manual's tape lists. The L40 may be pre-adjusted for a tape that's 2 or 3 dB more sensitive than the 6300's "standard" formulation; if you use the latter in both decks, you should have the experience you describe even if all metering and controls are exceptionally accurate. --- We regret that, due to the volume of reader mail we get, we cannot give individual answers to all questions. ---- Also see: Sound Views by Robert Long--What's causing the storm over CBS's new CX dynamic-range expansion system.
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