AUDIOCLINIC (Jun. 1989)

Softened Water and Room Humidifiers

In the September 1988 issue, a suggestion was made in "More About Humidifiers" to use hot, softened water in ultrasonic room humidifiers to eliminate calcium-carbonate dust problems common to these units. This is a dangerous suggestion.

The manual which accompanied my humidifier warns in bold print not to use softened water. Because there was no explanation in the manual as to why this caution need be taken, I called a local plumber who told me that, because salt is used in the regeneration cycle of all water softeners, sodium molecules replace calcium molecules in the process. The net result is that your readers will be depositing a much more corrosive dust of salt on their electronics if they use softened water in humidifiers.

-Bill Caucutt, Madison, Wisc.

Loss of Bass

Q. I have a problem with two separate systems, and wonder if you've ever heard of this problem. I have a receiver and the usual assortment of accessories. I also have a good boom box. I have used these very satisfactorily over a period of a year or so, and loved both systems. Now, both have much less bass than they did when I first purchased them! Could the power line be responsible for this lack of bass? It's about the only item common to both systems. I often use the boombox connected via the power line, rather than operating it from batteries.

-Jimmy Edwards, Greenville, N.C.

A. I have not run into a situation like yours. There are situations in which bass has been lost, but I can't recall a time when two independent sound systems suffered from the same condition at the same time! My first thought is that the loss of bass could stem from an increasing familiarity with the sound of your components. On top of that, you may be more critical of sound than you were at the time you purchased your two systems. You may, therefore, expect more than what can be obtained from the equipment you now own.

I really don't think the power line could be responsible. If it were, rather than producing a diminished bass output, it would probably have caused your equipment to fail completely. This would have occurred because of surges on the power lines which happened often enough and which were severe enough to produce damage to the power supplies in the two systems you own--or perhaps to produce damage to other components fed by those power supplies.

In the case of the portable equipment, could it be that you often stored it in your auto for long periods while the car stood in the hot sun? This could gradually result in deterioration of electrolytic capacitors. If these are in tone control circuits, emitter bypasses, or perhaps interstage-coupling circuits, bass could be reduced with decreasing capacitance.

Assuming that this theory is right, It could be that you have two different reasons why bass was lost. In the case of the boombox, it may have been due to excessive heat. In the case of the main receiver, the loss of bass could be due to any number of causes not related to the portable's problems. The most logical reason has to do with decreased capacitance values. However, it could also be nothing more than that you have relocated your loudspeakers, and that the room acoustics don't support good bass in these new locations.

Amplifier Channel Imbalance

Q. I've read much in your column about channel imbalance. My system appears to suffer from this condition. At low volume levels, the left channel of my power amplifier shows higher power output than the right channel (on its LED display). As I increase the setting of the volume control on my preamplifier, however, the balance between the left- and right-channel displays evens out. I can't hear any imbalance, despite the discrepancy between the two displays. Do I have a problem?

-Matt Farley, Shreveport, La.

A. Since the balance you hear does not change when you change volume settings, the cause may be differences in the linearity of the two power-level indicator circuits; I have heard of such a circumstance occurring. One quick way to check for this is to bridge your amplifier's two inputs with a Y-connector, so that you can feed both with one input signal. Then, using either output from your preamp, raise and lower volume and see if the indicated channel balance still shifts. This will be easier to observe if you use a steady signal such as a test tone from a CD, test record, or audio generator, or interstation noise from an FM tuner with its muting off. Be careful not to overdrive either your amp or your speakers.

Loud test signals can be annoying, so it's best to substitute a dummy load for your speakers, if you can find a suitable one.

You can also check the difference in amplifier output levels by connecting an a.c. voltmeter between the amp's two "hot" output leads. When the same signal is fed to both channels, the meter should indicate 0 V. If you heard the balance changing as you raised the volume, then it would seem probable that the two channels of your preamp's volume control did not track together at low volume settings. To double-check, interchange the right and left cables from your preamp to your amp at one end; if the trouble was in the preamp, the amp channel which formerly read low would now read high, and vice versa.

Piezoelectric Tweeters

Q. What are piezoelectric speakers (tweeters), and what are their advantages and disadvantages? Why are these tweeters not used by most loudspeaker makers?

-Roosevelt A. Anderson, Jr., Las Vegas, Nev.

A. Rather than using a magnet and a voice-coil, a piezoelectric speaker employs a special ceramic material which is used to drive its cone or other diaphragm.

You doubtless have heard of ceramic phonograph pickups. These work by virtue of a thin slab of a special ceramic fitted to the stylus. When the stylus moves, this motion twists the slab.

When this kind of ceramic is twisted, an electrical voltage appears between its two faces, and this voltage will be in proportion to the modulation on the record grooves.

If, on the other hand, voltage is fed into such a pickup, the ceramic will twist in proportion to the strength and polarity of the voltage, and the stylus will move. This system was used as the driver for some recording heads many years ago.

The ceramic could be attached to a diaphragm or cone rather than to a stylus. If sound waves strike the diaphragm, it will move, twisting the ceramic. As you can guess, a voltage will appear across the ceramic slab.

(Many microphones have been constructed using this principle.) Now, put voltage across the ceramic, and it will again twist, causing the diaphragm to vibrate in accordance with that voltage. This is your piezoelectric tweeter.

Because these elements cannot be twisted very much without breaking, they do not lend themselves for use as drivers for woofers or even midrange speakers. But the system works well for tweeters, as you know. The advantages are that, because of mechanical and electrical considerations, piezo tweeters usually can be operated without need for a crossover network. They are voltage-operated devices and thus consume virtually no power from the amp. These tweeters produce a surprising amount of acoustical output and so can be matched with almost any woofer/midrange system.

The disadvantages have to do with a lack of smoothness in their frequency response. To date, I have not heard one which is smooth enough for me to choose as my tweeter. Perhaps this is why most manufacturers don't use these tweeters for true high-fidelity applications. They are very common in inexpensive equipment and in speaker systems which were designed for sound-reinforcement applications.

Crossover Networks and Bass

Q. I bought a rack system a while ago and was not satisfied with the speakers. I bought new drivers. I noticed that the stock speakers did not employ crossover networks just capacitors for the midrange and tweeters. The new drivers were supplied with capacitors, except for the woofer--still no crossover networks! I connected the drivers, and the overall sound of my system improved, except for bass response. I bought a crossover network and added it to my setup, and the bass response was just as poor as before.

Could the lack of bass have to do with the enclosures rather than the drivers or crossover networks? I would like to know of a solution because I'm left with a handful of expensive drivers which are now useless!

-Jeff Nachshon, Brooklyn, N.Y.

A. A crossover network won't result in increased bass output from a loudspeaker system. It may smooth out peaks in the region of the crossover frequency, but this is another subject.

Its use also assures that low-frequency energy does not enter mid- and high frequency drivers. Removing these frequencies is essential in many instances because drivers can be ruined by excessive lows.

It is very likely that the enclosure plays a part in the lack of bass response. You can't just "marry" a woofer to an enclosure and get the bass response you expect. You might be lucky, but most of the time, luck will evade you. The enclosure's internal volume, amount of sound-absorbing material, and other critical items will be determined by various parameters of the woofer-such as cone area, cone resonance, and cone compliance. If you are to rescue your woofers, check with their manufacturer for suggestions as to woofer parameters.

If you are unwilling to experiment or to take risks when working with loudspeakers, you should consider purchasing ready-made loudspeaker systems. Of course, even commercially made loudspeaker systems vary considerably in sonic quality from one make and model to the next. It is therefore important to listen to any loudspeaker system before buying it.

Listen only to loudspeakers which fit your budget or other requirements, and listen to them using your own music sources. You will be familiar with this music, which will aid you in selecting a "good" loudspeaker. If a pair sounds good, purchase that make and model. They may sound somewhat different in your home than they did in the dealer's showroom; still, you have a better chance of liking the speakers than you would if you walked in and bought them without an audition.

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(adapted from Audio magazine, Jun. 1989, JOSEPH GIOVANELLI)

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