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early 1970's speaker design
Bartholomew MacG...
post Apr 5 2012, 06:18
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Here's a quote from the engineer Siegried Linkwitz that was in an interview that was posted in Stereophile:

QUOTE
Dickson: You're most widely known as the developer of the Linkwitz-Riley crossover. Could you explain a few of the characteristics of this crossover?

Linkwitz: To answer your question, we need to go back to when I started out exploring the whole speaker issue in the early '70s. Then you could take the grille cloth off many of the available speakers and see a strange, almost haphazard arrangement of the drivers on the baffle. It really puzzled me and I wondered what was going on. So I asked some of the designers why they were doing this and they said; "Because we've found it sounds better."

As I looked further into this issue, I realized that two principal things were not well-understood. First, very little was known at that time about the effects of diffraction from the cabinet edges. Second, and more importantly, very little was understood about how phase-shift with respect to the current passing through the voice-coils of different drivers affected the polar radiation pattern of a speaker. In other words, the interaction between the electrical side of a driver and the acoustical response was not clear at the time. For example, the phase-shift between the current in the tweeter and midrange voice-coils, relative to the placement of these drivers on the baffle, affects the speaker's radiation pattern.


http://www.stereophile.com/content/siegfried-linkwitz-page-4



I was wondering if anyone knows if this is really true - that they just didn't know what they were doing and were in some way desinging the speakers by trial and error.


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julf
post Apr 5 2012, 09:40
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QUOTE (Bartholomew MacGruber @ Apr 5 2012, 07:18) *
I was wondering if anyone knows if this is really true - that they just didn't know what they were doing and were in some way desinging the speakers by trial and error.


Well, loudspeaker "design" in the early 70's was mostly by copying and tweaking "well-known designs". There was some serious research at places like BBC, and KEF was pioneering use of computer analysis, but most "designers" rehashed some basic designs and design rules. My favourite example is the original Linn Isobarik (I still love my modded early examples). Linn bought the design from the original designer/developer, but didn't understand it - as a result, pretty much every "improvement" Linn introduced during the production lifetime of the speaker was a backwards step...
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Bartholomew MacG...
post Apr 5 2012, 10:53
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QUOTE (julf @ Apr 5 2012, 09:40) *
QUOTE (Bartholomew MacGruber @ Apr 5 2012, 07:18) *
I was wondering if anyone knows if this is really true - that they just didn't know what they were doing and were in some way desinging the speakers by trial and error.


Well, loudspeaker "design" in the early 70's was mostly by copying and tweaking "well-known designs". There was some serious research at places like BBC, and KEF was pioneering use of computer analysis, but most "designers" rehashed some basic designs and design rules. My favourite example is the original Linn Isobarik (I still love my modded early examples). Linn bought the design from the original designer/developer, but didn't understand it - as a result, pretty much every "improvement" Linn introduced during the production lifetime of the speaker was a backwards step...



That seems strange to me. You'd think they'd be engineers who knew something about acoustics and would try to use some of that. The other thing that's weird is the phrase "sounds better" which sort of seems to imply they listened to music on them and made their decisions based on that instead of using measurements.
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julf
post Apr 5 2012, 11:42
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QUOTE (Bartholomew MacGruber @ Apr 5 2012, 11:53) *
That seems strange to me. You'd think they'd be engineers who knew something about acoustics and would try to use some of that.

There was a fair bit of academic work going on, but it only trickled into the industry during the 70's - remember that stuff like the Thiele & Small work was only published in the early 70's, and took a while to be accepted.. High-end hifi was also a very marginal business.
QUOTE
The other thing that's weird is the phrase "sounds better" which sort of seems to imply they listened to music on them and made their decisions based on that instead of using measurements.

Ah, but this was the 70's. Measurements were for men in white coats that smoked pipes. The cool crowd *experienced* things (and smoked other kinds of stuff...) smile.gif
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DVDdoug
post Apr 5 2012, 18:30
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QUOTE
The other thing that's weird is the phrase "sounds better" which sort of seems to imply they listened to music on them and made their decisions based on that instead of using measurements.
Today, the final design still depends on listening, testing-measuring, and trial-and-error. You can design an amp on paper and be very confident that it's going to perform as expected. If you try that with speakers you won't get the possible best results.

Thiele-Small was probably the biggest breakthrouh because it allows designers to better predict how a ported speaker is going to perform. But, it's not an exact prediction.

Speakers haven't changed that much... Most speakers are 2 or 3 way systems with dynamic drivers and a passive crossover. Home theater systems with smaller limited-range surround speakers and a powered subwoofer are "new", but you could have built such a sysem in 1970 if you wanted to.

It's easier to build a good speaker with software and modern theories. And, some of the materials used in drivers are probably better. But, with enough time and effort, it was possible to build a very good speaker in the 70's (or 60's). Many good speakers were made, and many of them are still working.
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splice
post Apr 6 2012, 00:49
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There's a lot of documentary and physical evidence that Philips, at least, were aware of the issues that Linkwitz cited. For example, the AD0160T tweeter was designed to be flush mounted in the baffle, and diffraction issues were given as the reason. Philips speakers of the time had several features that indicated attention to phase and diffraction issues. Drivers were spaced on the baffle taking into account phase differences between them. Crossover networks were quite complex, indicating significant work in compensating for issues at crossover points. Grille cloths were mounted on solid boards, with form-fitting cutouts around the drivers to avoid diffraction at the driver edges. The boards were beveled at the edges. Cabinets were deliberately kept shallow to keep the primary front-to-back resonance higher than the crossover from the woofer. Baffle step correction was understood. The cabinets were filled with fibreglass batts instead of having lined walls. If they can be faulted in general, it is because they concentrated more on a flat frequency reponse on-axis to the detriment of other desirable characteristics.

This post has been edited by splice: Apr 6 2012, 00:51


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Don Hills
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Arnold B. Kruege...
post Apr 6 2012, 15:12
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QUOTE (julf @ Apr 5 2012, 06:42) *
QUOTE (Bartholomew MacGruber @ Apr 5 2012, 11:53) *
That seems strange to me. You'd think they'd be engineers who knew something about acoustics and would try to use some of that.

There was a fair bit of academic work going on, but it only trickled into the industry during the 70's - remember that stuff like the Thiele & Small work was only published in the early 70's, and took a while to be accepted.. High-end hifi was also a very marginal business.
QUOTE
The other thing that's weird is the phrase "sounds better" which sort of seems to imply they listened to music on them and made their decisions based on that instead of using measurements.

Ah, but this was the 70's. Measurements were for men in white coats that smoked pipes. The cool crowd *experienced* things (and smoked other kinds of stuff...) smile.gif


The state of the art of acoustics measurements were very different.

(1) The FFT was popularized among engineers as a theoretical topic just a few years before. FFT's required computers, and the computational hardware required to compute even a 2048 point FFT was $10,000s or even $100,000s. Mainframes of the day are dwarfed by the processor in your cellphone!

(2) A really sophisticated lab in that time might be able to create a swept tone using a geared motor to drive the oscillator's frequency controls. There was a chain drive to the plotter to keep it synchronized with the oscillator tuning.

(3) HP came out with a minicomputer based 5427 FFT-based system in the late 1960s early 1970s but if memory serves, it ran more than $100,000. History of RTAs

By the mid-late-1970s Crown's TEF-based portable audio measuring equipment became available but still pretty well destroyed $10,000. Appropriate microphones ran > $1,000 and in those days this was a ton of money!

The state of the art of theoretical analysis was also pretty basic. The basic landmark work by Thiel & Small was published and popularized in the same time frame - early 1970s.

Bottom line. Without predictive theories, everybody was pretty well left to cut and try. Without good measurements, nobody knew for sure what their cutting and trying was actually accomplishing.
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