AUDIOCLINIC (Sept. 1989)

CDs and Range Expanders

Q. My CD player's outputs are connected to the inputs of my range ex pander, which is calibrated from 1.0 (no expansion) to 1.5 (50% expansion). The sound coming from a CD is about 94 dB; does this mean that by using the maximum expansion of 1.5, I am getting an S/N of 141 dB? The manual for my expander says: "A 1.4 setting means that an input signal having a 40-dB dynamic range will be expanded to 56 dB or that an input having a 50-dB dynamic range will be expanded to 70 dB (1.4, or 40% times the input signal)."

-Don Lacroix, Woon socket, R.I.

A. What you gain by using a range expander is dynamic range. Because the dynamic range available on CDs is very large as it is, I am not convinced of the value of a range expander.

As for the S/N ratio, you can expect only limited improvement. For argument's sake, let's say a recording's noise level is at- 90 dB and that the music is recorded within the range from -10 to -70 dB. In this case, the S/N ratio, when listening to the softest passages, is 20 dB. At full expansion, the noise level will be driven down to -135 dB, and the music's range will now be from -15 to -105 dB. Your S/N ratio, when listening to the softest passages, is now 30 dB.

However, you have also increased the music's dynamic range from 60 to 90 dB. If you now adjust your volume control so that you hear the loudest passages at the same level as you do at present, the softest passages will be 30 dB softer than they were before expansion. If you are listening at a low volume, you might not even hear those soft passages, though you certainly would hear no background noise.

On the other hand, if you set your volume so that you hear the softest passages at the same level as before, the loudest passages will now be 30 dB louder. This may be so loud that it produces overload in your equipment.

You could damage your amplifier or your loudspeakers if your system has insufficient reserve power-handling capacity. Further, and most important, the loudest passages may well be loud enough to cause ear damage. While you can replace an amplifier or a loud speaker, you cannot replace your ears.

To keep the music s dynamic range within reasonable limits, you will have to use only a moderate setting of your range expander. However, you can expect only a modest improvement in S/N. For example, if you set your ex pander to "1.1." the music's 60-dB dynamic range would expand to only 66 dB, and the signal-to-noise ratio at the quietest passages would improve by a mere 2 dB.

More About Filtered Water

In your January 1988 "Audioclinic" column, you may have spread some mistruths. Recommending inexpensive water filters to clean up inorganic water contamination is just about useless unless the problem is gross sediment.

Most water carries suspended silicates, carbonates, salts, etc. that are measured in milli-microns. Most filters, regardless of price, have elements that only filter out particles 1 micron or larger in size. (For comparison, a human hair is about 5 microns in diameter.) The only filters that will work on these dissolved solids are "reverse osmosis" filters, which degrade rather quickly and need a great deal of service. The good of inexpensive filters is that they may remove some of the chlorine com pounds which destroy many benign organisms, such as intestinal flora.

Most filters don't live up to the claims made by their manufacturers regarding removal rates. Distillation is the most consistent source of pure water. It will also give you the best-tasting product over the longest amount of time.

-James Gillette, Berkeley. Cal.

Impulse Noise Eliminators

Q. I recently saw in a catalog a "transient noise eliminator" designed to eliminate small to medium ticks and pops from phonograph records. This sounds intriguing, but how does such a machine work? How can it "know" the difference between noise and the intended content of a recording? The device's frequency response only extended to 10 kHz. How much loss of music will I notice when using a machine with this restricted response?

-Joel Valder, Oroville, Cal.

A. When the stylus encounters a defect in the record groove, the resulting signal builds up to maximum intensity much more quickly than when the stylus encounters music signals, even from percussion instruments. Devices like the one you've described recognize this difference in rise-time.

If the device is limited in frequency response to 10 kHz, some subtle elements of music will be lost-not the fundamental notes produced by musical instruments (the piano's highest note is on the order of 4 kHz) but the harmonics of a tone, the "edge" of strings, the sheen of cymbals. To me, this makes music sound somewhat life less. But how much loss you'll notice will depend on your ears, your system, and the music you're playing. Even if the difference is enough to bother you, it may bother you less than the ticks and pops did.

X-Ray Damage to Recordings

In the February 1988 issue, in response to a question regarding X-ray damage to audio recordings, you said that X-ray equipment can damage tape recordings because the X-rays partially demagnetize the tape. I'm not sure that this is correct.

X-ray damage to magnetic recordings has been discussed off and on for many years, and there has been general agreement that no one knows how the notion arose of possible damage to magnetic recordings. However, it may have been an assumption that grew out of a perceived kinship between magnetic and photographic media.

There is no scientific evidence that X rays, per se, damage magnetic recordings. However, X-ray equipment or, more specifically, transformers associated with it may cause damage through the generation of strong magnetic fields.

The 1970 edition of the 3M Company's Soundtalk (Vol. 3, No. 1) reported that recorded test signals on a commonly used recording tape were not affected by even severe exposure to X rays. The tape was given a 6-S expo sure at a distance of 36 inches from an X-ray machine operated with 200 mA of current and 110 kV.

-Klaus Halm, East Point, Ga.

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(Source: Audio magazine, Sept. 1989, JOSEPH GIOVANELLI)

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