.

Reactions to our comments on this page in the December 1958 issue are grouped at two opposite ends of the scale - very few being deseribable as mild. By and large, industry was unanimous in agreeing with us -  not about the fact that someone had engaged in tests, but rather that the methods and the conclusions drawn therefrom were, perhaps, unreal.

Similarly, many of our regular readers who are not in the industry agree that no listening test by a given group of people is valid with respect to the likes or dislikes of any particular individual.

On the other hand, some readers disagreed with us most heartily. They base their opinions on the idea that what sounds good to a group of presumably well trained listeners should therefore sound good to everyone.

One objection - and we must consider this one valid - is that not everyone is so situated that he can drop into an audio showroom and listen for himself and that furthermore - and here we have to agree also, though we deplore the fact - even when he can, the demonstration facilities are often not ideal, to say the least.

To this objection, we might suggest a cure: tell your dealer you will buy a speaker in a specified price range, but that you must be satisfied with the quality; try to persuade him to let you try out a few in your own home with your own specific listening habits - one at a time if necessary - tout that you will keep one of those you try out.

Naturally the ones you select for a home trial would be those you liked best in the store, with only the final decision coming from a more leisurely listening. Some dealers might go for it, others might not.

But for a bona fide sale, we think it would be good business. At least, there is no harm in asking, and then you would end up with a speaker you liked and were sure you liked it.

von ARTHUR W. SEAR in AUDIO 1959 Heft 2
In proposing a hypothetical system, the author has described one which is used in several forms commercially - but which has not yet been adopted by the home listener. Maybe it should be.

At Audio Shows and similar demonstrations I have been intrigued by the music and audio effects that come from the new stereophonic sound reproducing systems. Then, when I look at the price tags, I am almost completely disenchanted.

It is not just the cost of the equipment that frightens me: where in my home can I find space for two sound systems that will give me the full audio range that I have been enjoying with my present monophonic system. A medium priced stereophonic system would probably fit into my home satisfactorily but the medium spatial effects of these and their somewhat limited bass response leaves much to be desired.
.

Over the years my monophonic system has grown by easy stages from a simple beginning to a four-way system with a dividing network between the preamplifier and four output amplifiers, one for each speaker. I like the big clean audio that I can get with this set, with nice round bass notes at any reasonable energy level.

In fact, my present system is good enough so that I hesitate to discard it for a lesser system even though it would have the advantage of stereo "spaciousness." I have also toyed with the idea of converting my monophonic system to stereo but even if I could afford another Klipsch horn, where in an ordinary room can one find place for two!

There may be many other hi fi fans who are in the same predicament that I find myself and maybe we can find a way out of our dilemma if we can benefit from some of the characteristics of sound and the methods we use for reproducing it.
.

• First, we cannot sense the direction of sound sources of frequencies below about 300 cps, except for the transients that may accompany the low frequencies.
• Second, loudspeakers that can radiate low frequencies are generally larger and more costly than speakers that only have to radiate mid-range and high-frequency sounds.
• Third, most of the sound energy is concentrated in the low-frequency end of the sound spectrum so that amplifiers for producing high-frequency sounds are comparatively smaller and cost less.

.
These three facts point out the desirability of using a single low-frequency amplifier and speaker for a stereophonic sound system if it is possible to do so and still keep the over-all system symetrical.
.

In addition, there are characteristics inherent in the recording process that tend to simplify the problem of designing a system having a single low-frequency channel. Low-frequency sounds have long wave-lengths in air so that low-frequency tones picked up by the microphones are likely to be nearly in phase.

Placing the low-frequency instruments of an orchestra near the center of the stage, so that the distances to the stereophonic microphones are nearly equal, aids in keeping the low frequency energy in the two channels in phase. In the Westrex 45/45 recording system it is desirable to keep the low-frequency energy in the twTo channels in phase since in-phase components result in lateral motion of the cutter.

It is these same low-frequency components that require a large amplitude of the cutter and when voltages at these frequencies are in phase the vertical excursion of the cutter, or hill-and-dale effect, is likewise kept desirably small. The low-frequency signals for the two channels could be kept completely in phase if this part of the pick-up were taken from a third microphone placed midway between the two side microphones, and added equally to both channels. Since it is already standard practice to use three microphones when making a stereophonic recording, this proposed method of recording would not require a great change in procedure. A block diagram of how this could be done is shown in Fig. 1.

A stereophonic reproducing system that makes use of the in-phase low-frequency sound energy would have two mid-range and high-frequency amplifiers and speakers. A single amplifier and woofer would supply the nondirectional low-frequency sound energy for both channels.

Since electric wave filters are not completely effective in eliminating transient voltages, some provision must be made for removing any directional effect that may come from the transient response of the low-frequency speaker. This can be done by taking advantage of the Haas effect.

A simplified statement of the Haas effect is that if a sound reaches a listener over more than one path, the source of sound seems to be in the direction indicated by the path over which the sound energy arrived first.

Thus if we excite two speakers spaced some distance apart with the same signal, but delay the signal going to one speaker, all of the sound energy seems to emanate from the speaker which does not have the delay.

This can be further illustrated by the fact that we are not confused in our directional sense when standing in front of a wall and can tell exactly where a sound is coming from even when a good share of the sound energy we receive bounces off the wall. This is because we sense direction by means of the direct radiation which arrives just a few milliseconds before the echo comes from the wall.

Therefore if we put a little delay in the channel driving the woofer we will not suspect it of being a source for transient sounds and it doesn't make any difference where the low-frequency energy comes from.

This is where my big Klipsch horn comes in handy because it has a built-in air column that is the equivalent of a five or six millisecond delay line.

The twisting five- or six-foot path from the speaker cone to the mouth of the horn does very well as an acoustical delay line. Since sound travels at a speed of approximately 1100 feet per second, it takes nearly 6 milliseconds for a sound wave to travel from the speaker cone to the mouth of the horn.

This is ample delay to eliminate the directional effect of the Klipsch horn and leave creating of the spatial pattern to the two mid-range and high-frequency speakers.

Incidentally, the low-frequency horn must be of the type that includes a closed chamber around the back of the speaker so that no radiation comes directly from the speaker cone.

The phrase "mid-range and high-frequency" seems a clumsy expression to identify that portion of the audio spectrum above the low frequencies. I would like to suggest the word "alto" for this range. Alto is a short euphonious word that would designate the audio frequency range above the bass, which would probably be the range upward from about 250 to 350 cycles per second whether this range was covered by one, two, or more speakers. In this usage, alto would mean minus-bass, similar to the color designation of magenta to specify a color that is minus-green.

As shown in Fig. 2, my proposed system will consist of a stereo pickup, two preamplifiers, two 300-cps crossover networks, two small power amplifiers for the two alto speakers, and one power amplifier to drive the low-frequency speaker. Five watts should be ample to drive the small speakers and twenty watts should be enough for the woofer.

For me, the big advantage of this arrangement is that I can use my present low-frequency speaker to get those nice round bass notes and the two alto speakers with their two amplifiers will not be too expensive.

I suppose that I should finish the project and try it out before writing about it. But the project is moving along rather slowly so if anyone can point out why this system will not be satisfactory, maybe I won't have to finish the job.

Actually, I am still debating with myself whether the stereo project is worth while. When I go to a concert I usually hit so far back that all the sound seems to come from one direction. In fact, the only way that I can tell that the string section is on the left is because I can see them there with my opera glasses. But in the interest of progress, I shall continue with the current project.

Bildunterschriften
Fig. 1. Block diagram of recording system.
Fig. 2. Block diagram of reproduction system.