Subsonic Energy
Q. What damage do sub-sonics cause to bass drivers? When will this damage most likely occur? My equalizer's filter cuts off sound at the rate of 18 dB per octave below 20 Hz. Will this be sufficient?
-Bob Connelly, Green Bay, Wisc.
A. There is no real answer to the questions you have asked. Much depends on the amount of subsonic energy which is present and on the power-handling capacity of the bass drivers.
Bass drivers are made to handle power and should "expect" a certain amount of such subsonic energy.
I would think that your 18 dB roll-off below 20 Hz would be sufficient to re move any possibility of damage, except where there is a case of acoustic feedback at frequencies above the 20 Hz we have just discussed.
Laterally Cut Discs
Q. I have read that a mono switch on a preamplifier is best set to the mono position when playing laterally cut mono records, to cancel vertically modulated components. What are "laterally cut" records? And will the mono records now being produced benefit from being played this way?
-Paul Tomatani, Honolulu, Hawaii
A. All of today's mono records, and most of yesterday's, are laterally cut; that is, the groove is modulated from side to side, rather than up and down.
Stereo records are cut with monophonic information common to both channels, modulating the groove laterally, while stereo difference information is a vertical modulation.
Setting the preamp switch to mono will clean up some mono records-and even some noisy stereo ones, if you don't mind hearing them in mono. It will only help, however, if the noise is in the vertical components, such as vertical rumble (in the recording or the play back turntable) or stylus motions due to "pinch effect." The mono switch will also clean up noisy FM stereo broad casts, though it's better to switch to mono at the tuner, where possible.
Old Edison, discs and cylinders (and, I think, Pathe discs) were cut with vertical modulation, as were some radio transcriptions on 16-inch discs. If you collect these, you can play them with a modern stereo cartridge by wiring its two channels together out of phase-the exact opposite of the connection made by a preamp's mono switch.
Outer Groove Record Noise
Q. I notice that, when playing phonograph records, a "roaring" noise occurs somewhat before the start of the program. I am puzzled because, when I hear recordings over my tuner, there is absolute quiet.
-Victor Ogorodnick, Narrowsburg, N.Y.
A. Phonograph records tend to be noisy at their unmodulated outer edges. Most of this noise has to do with problems in molding the raised area at the outer edge, sometimes known as the "groove guard." If you could start playing your discs at the point just prior to the start of the pro gram, this noise would not usually be heard (except where modulation be gins too early or where the raised area extends too far into the disc).
That's how the broadcasters do it (though they do it primarily to have the selection start right after it's announced, without "dead air"). And some stations play tape recordings of their records rather than the records themselves; this allows the edge noise to be edited out and prevents subsequent wear on the records.
"Bad Vibrations" Again
I recently moved from the comforts of a semi-soundproof home to a very "hard," reflective apartment. My Thorens TD160 MKIIB turntable does not "like" its new location. After setting up my SME Type II tonearm (fitted with a Shure V15 Type IV cartridge), I got acoustic feedback when the volume control was advanced a very small amount. The frequency of the feed back was in the range of 30 to 40 Hz.
My turntable sat on a base, suspended from the ceiling, to prevent "bumps" from heavy footsteps and direct vibrations from stands, tables, etc.
There was quite a pronounced bass resonance.
I attempted to stop the "bad vibes" by adding weight to the base, by surrounding the base with soft materials, and even by putting cord and foam under the feet of the table. No luck! In a last-ditch effort, I bought a tube of silicon rubber and proceeded to line the entire inside of the table, with the exception of wires and moving parts.
Success at last!!! No feedback!!! Sound is great, with the volume control well advanced. The turntable now sits 3 1/2 feet from a loudspeaker. The undesired bass resonance is also completely gone.
-Victor Wasend, Vancouver, B.C., Canada
I try to answer all questions, but it is also very nice to receive comments of this kind. Information of this sort broadens our knowledge. I wish, therefore, to thank all of you who have taken the time and effort to share your problem-solving experiences.
Mixing Buss
Q. Please explain what a buss is, as applied to recording gear.
- Christopher Berry, New York, N.Y.
A. A buss is any electrical line used by several different circuits at once.
Recording mixers usually have several signal busses. At a minimum, there will be one such mixing buss for each out put channel, and all inputs assigned to that channel will feed that buss. There may also be effects busses (feeding signals to echo or delay systems), equalizers and other signal processors whose outputs are fed into the output buss. Monitor busses control which signals within the console feed each monitor speaker. Busses are also used for ground and power.
Load Impedance and Amplifier Power Output
Q. Why does an amplifier put out more power into a lower impedance?
-Bob Robinson, Warrensville Heights, Ohio
A. The lower the impedance of a load, the more power that can be sup plied by an amplifier feeding that load.
This is true because the lower the load impedance, the closer to an impedance match that load becomes. We never match loads to the output stage of an amplifier. If we did, the amplifier would attempt to supply more power than could be dissipated as heat, and the amplifier would be destroyed.
(adapted from Audio magazine, JOSEPH GIOVANELLI)
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