In Installment 2 of this continuing series, I wrote about horn-type speakers being the first best solution for the problem of getting sound - whether music or speech - loud enough to be heard by a large group of people; in a movie theater, at a concert, political rally, or sporting event; or at any other large or even huge venue. Horns had one potential problem, though: In order for them to be able to produce even moderately deep bass, they had to be BIG. For public use, two things made that okay: Public venues usually have plenty of room for large speakers, and, in many instances, LOUD is all that matters and bass can be sacrificed without problem. Other applications, however, are not always that forgiving.
When the first audiophiles came along, in the late 1940s, they wanted music at home and, for the first time, they wanted it to be of good quality and had neither the space for huge speakers nor the willingness to do without music's bass foundation. This led to (as I described in that installment), smaller speakers that could make better and deeper bass than even the best horns and, partly because those new speakers were less efficient than horn designs, the development of new, more powerful amplifiers to drive them.
I closed Installment #2 by saying that, even as good as the new speakers were, and even with the new more powerful amplifiers, the coming of stereo brought with it new requirements and new problems that made most of even the best then-current speaker designs obsolete.
In Installment #3, I told how stereo, especially in the form of the stereo LP, had created a vast new market for speakers and amplifiers: Not only did newcomers have to buy a whole new stereo system, but even people who already had a monophonic ("mono") system still needed, if they were going to "go stereo", one more speaker and one more channel of amplification in order to listen to their new stereo (either tape or LP) playback gear.
As new or greatly expanded markets always do, this resulted in strong competition for the new business, both from existing manufacturers and from new ones attracted by possible profits. The result was not just the more normal forms of competitive behavior, but a great deal of research and innovation, all directed toward building the "better mousetrap" that would, hopefully, have customers "beating a path to their door."
In the speaker area, the first thing that manufacturers tried to do was to get people to buy a second speaker identical to the one to the one they already had or, for newcomers or even those who wanted the very best of the new stereophonic sound, to buy a whole new matched pair. The original logic to this had to do with frequency response, and correctly assumed that the stereo effect would be better if both speakers were the same. (Frequency response differences between right- and left-channel speakers would produce differences in amplitude for the same frequency that could be interpreted by our ears as false locational information.)
Even more important than amplitude in identifying the location of a sound source, though, is phase and arrival time information. Which of our ears ear hears it first, and a sound's relative phase at both of our ears helps us to accurately "triangulate" on and pinpoint its source. In the days when monophonic ("mono") sound was all there was, this location information was of little importance - all of whatever we were listening to came from a single location, the speaker, and it was easy to hear where the speaker was. For stereo, though, the whole new appeal was based on location. That's what people were buying it for, and because of that, all of the ways that speakers could affect our ability to localize a sound source became major subjects for research and product development.
One of the very first things to be looked into was the physical placement of a speaker's drivers. As far back as the late 1930s and early 1940s, companies like Tannoy (with its "dual concentric" speakers) and Altec (with the Model 604) or, a little later, RCA (with the LC-1a) made coaxial ("coax") speakers that placed the drivers of a "two-way" system concentrically, with the tweeter centered within the cone of the woofer. I'm guessing that this was originally done for some reason other than to provide more correct high-and low frequency arrival times, but whether intentionally or not, it helped to do just that. Later, specifically arrival-time-oriented designs, though perhaps not fully concentric, made it a point to position all of their drivers along a common vertical line instead of, as had been the ordinary case for multi-driver systems, just plunking then down all over the speaker's mounting face anywhere that was convenient.
Another new development that came along, also to correct the differing arrival times that would normally be expected from more than a single driver, were "time-aligned" systems which staggered the forward and back placement of the drivers so that, although their physical positions were all different, (but still usually vertically-aligned) their voice coils all lined-up on a single plane. (That was, incidentally, NOT always or even usually the case with coax speakers, and it continues not to be the case even today.)
Any number of arrival-time correct (panels, for example) or corrected ("line source" columns, for example, or electronically time aligned, like the Quad electrostatics) speakers have been developed and marketed over time. The one kind of speakers that, simply because of the different lengths required for the propagation of different frequencies, have steadfastly resisted time alignment are the horns. If the horn mouths are set in the same plane, the voice coils (and hence the arrival times of the different frequencies) are nowhere near proper alignment, and if the voice coils are aligned, the mouths (and thus the off-axis arrival times) can be spectacularly different.
It's not all time-alignment, though, correct phase and a number other factors also contribute to good stereo sound. I'll write about some of them next time.
See you then.