AUDIOCLINIC (Feb. 1988)

SPL and Loudness

Q. My original speakers produced 88 dB SPL, and my new ones produce 102 dB SPL. How much louder are the new ones, given the same amount of power feeding them? Both loudspeakers are of the same impedance.

-Hiram Garcia; McAllen, Tex.

A. Your new loudspeakers are far more efficient than the original ones.

Assuming that the outputs of both sets of speakers are measured in the same way, the new models will produce a bit more than four times as much acoustical power as the old ones did when driven with the same electrical power.

Subjectively, I am not sure this information tells you much. Our ears don't measure quantities of sound as one would measure weight. Subjectively, however, you will definitely know that the new speakers are louder than the original pair. Keep in mind that other factors contribute to how loud a sound seems to be. Suppose the original speakers had some strong peaks in the midrange and treble. Such peaks would make the sound seem louder than if those speakers were flat. A speaker's location in the listening room can also make a difference as to loudness, as can the addition or subtraction of furniture.

X-ray Damage to CDs

Q. Is there any damage to a CD when it runs through the X-ray machine at an airport?

-Carl Huff, Dallas, Tex.

A. To the best of my knowledge, no damage to CDs will occur when they are passed through airport security equipment. Certainly none of my readers have written me to that effect. X-ray equipment can damage tape recordings, because the X-rays partially demagnetize them. No physical damage to the tape other than demagnetization takes place. Inasmuch as CDs do not in any way involve magnetic fields, I cannot conceive of a way by which X-rays would damage them.

Flickering Stereo Indicator

Q. When I tune to a certain FM station, sometimes the stereo indicator flickers. Why?

-Richard Harding; Peabody, Mass.

A. The most likely explanation is that, for whatever reason, the tuner is switching to mono for brief intervals. Of course, the light goes out during these intervals but relights when the tuner switches to stereo once again.

It is reasonable to suspect that the strength of the signal in question is weak. A passing airplane or a change in atmospheric conditions may further weaken the signal to the point where it would be too noisy for good stereo. Thus, the tuner switches to mono.

Getting a Clean Sound

I was interested to read Allen R. Meals' letter in your November 1987 issue. He traced a channel-imbalance problem to his hearing loss in one ear. I would like to relate my own experience, which may be of interest to your readers.

About six years ago, some dirt be came lodged in my ear, and a trip to the doctor was necessary to remove it. Upon my return home, I noticed that my stereo system sounded like new! Since that time I have made it my practice to cleanse my ears two or three times a year, using the Murine earwax removal system. By doing the same thing, or simply by using warm water and a syringe (following your doctor's instructions!), you too can reap the benefits of a whole new audio system.

I do not exaggerate when I tell you that this low-dollar process will pro duce results similar to buying a new pair of speakers. Don't think you'll notice the difference only if you work in a sawmill or in some other dusty environment. Even an office worker who com mutes in city traffic will notice a remarkable improvement.

-Bob Hovey; Columbus. Ga.

Equalizing Loudspeakers

Q. What effect does an equalizer have on loudspeakers? If a loudspeaker has a response dip at 30 to 40 Hz and I boost this range to restore a flat response, will the speakers really pro duce that flat response at low frequencies? If equalizers do work that way, why don't loudspeaker manufacturers build in electronic boosts?

-Steven Di Carlo; Lanham, Md.

A. If you have a loudspeaker which is deficient in bass but whose bass frequencies can be heard to some ex tent, perhaps the equalizer will enhance that region. Whether or not flat response can be brought about depends on whether the bass boost results in so much power fed to the loud speakers at low frequencies that they are overdriven and perhaps damaged.

On the other hand, if you have a speaker system in which the lows are virtually nonexistent and where we really hear harmonics of the bass frequencies rather than the fundamentals, no equalization can compensate.

Some loudspeaker manufacturers do in fact supply special equalizers, to be placed either in the tape loop or between the preamplifier and power amplifier. The purpose of such equalizers is to compensate for any dips or peaks in response-not just those in the bass.

Interfering with FM Reception

Q. Sometimes when I walk by my receiver when it is tuned to FM, I get a lot of interference. When I walk away, the signal clears up. What is the cause of this? How can I fix it?

-Joseph S. Barbesa, Buffalo, N.Y.

A. This interference is the result of your body's coming between the receiving antenna and the transmitting antenna. To some extent, your body can block out the desired signal. This will happen only if you use an indoor antenna. The cure is to move the antenna to a location where your physical presence cannot affect it. Mount it near the ceiling, if possible.

As you move the antenna around, take note of the signal strength of the most desired stations. You may not only find an antenna location which will eliminate the interference caused by people walking in and out of the signal path, but you may also obtain greater overall signal strength-which will make reception better.

Static from the Volume Control

Q I am having a problem with static electricity. When I turn on my music system, I hear static from the speakers when I adjust the volume control. I grounded my system to a water pipe, but the problem is still present. How-ever, if I turn the equipment on and let it stand for a few minutes before making any volume adjustments, the static is no longer present.

- Joe Pudlo, New York, N.Y.

A. The "static" that you hear when turning the volume control on your sound system is not the result of static electricity or of bad grounding practice. The offending sound results from the way in which the equipment works during startup. Capacitors associated with the volume-control circuit charge up, and this takes time. During this process, the voltage across the capacitor changes, and hence d.c. flows through the volume control. Whenever d.c. is present in such a control, adjusting it will produce a scratching noise. There is less of a tendency for this sound to be produced when the control is clean (free of oxide). What you have described may be an early sign that the control is dirty. It should be cleaned with a suitable contact cleaner. Failure to do so will ultimately result in noise every time the volume control is adjusted.

Where to Begin?

Q. When shopping for an audio sys tem, how can I choose components in which I can be confident? Why does one choose one brand over another reviews, reliability, recommendation? Among literally hundreds of brands, what should be important to me?

-Dr. David Blank; Cleveland, Ohio

A. Most audio equipment is very good these days. In fact, I have to look in order to find really bad gear. The reliability of today's equipment has, for the most part, been surprising; I myself don't consider that aspect when I purchase a component.

To get started, one thing you can do is to look at the advertisements of high-end manufacturers. By and large, these firms have been around for some time, and they would not have remained in business had they produced consistently poor-quality goods.

Another good way to buy equipment is by recommendation, but only if the person doing the recommending has some knowledge.

Product reviews and spec sheets will tell you about the features included in any given piece of equipment. If you have a good knowledge of a particular reviewer's likes and dislikes, you may find his opinions useful for you.

Personally, I would not want to buy something based solely on what a re viewer had to say. I would want to examine it and--most important--listen to it. An examination will tell you how easy or hard it is to operate and will help you learn about the features it contains. You may find that there are features you do not need or want but which could result in a higher price.

That alone could be a reason for you not to buy a piece of equipment.

Though it is important to listen to a product before you buy it, it's difficult for me to explain how to do that, especially if you are inexperienced in auditioning audio components. An under standing can come about only by doing a great deal of listening. Even so, this listening won't have much meaning unless you have heard live music and have a reasonably good "ear memory" of what the music sounds like. Then you can listen to a system and determine how close it comes to your impression of how a given kind of music should sound.

In any event, when auditioning equipment, you may want to have with you someone whose opinion you respect. Further, you should use unflawed recordings with which you are thoroughly familiar.

Rising Cartridge Output with Rising Frequency

Q. Why does the output of a moving-coil or moving-magnet cartridge rise at the rate of 6 dB per octave?

-O. O. Callaway; Carlsbad, Cal.

A. A magnetic phonograph cartridge is really a tiny electrical generator. If we move the magnet in a generator with respect to the coil, or vice versa, a voltage will be generated. As the relative motion between coil and magnet increases in velocity, the voltage rises.

This is exactly what happens when a phonograph record is played using a magnetic cartridge. The stylus is coupled to the moving magnet or the moving coil. Because of the nature of disc recording, the stylus velocity increases with increasing frequency. If the stylus moves faster, so will the magnet or coil to which it is attached. This increase in speed accounts for the voltage rise of 6 dB per octave.

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(Source: Audio magazine, Feb. 1988, JOSEPH GIOVANELLI)

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