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Multi-Way Conventional loudspeakers with crossovers

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Old 7th June 2007, 01:03 PM   #981
MBK is offline MBK  Singapore
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IIRC in line sources there is also a falloff of SPL with frequency at -3dB/decade, which would overlay with the proposed changed room reflection pattern, and of course the line source character depends on the length of the line vs. frequency, so I am not sure if this effect would be so clearcut. In any case line sources seem to be one of those terrible to measure but pleasant to listen to things...

One possibility that I see in this design is that the source size would vary more strongly with frequency than in the usual speaker. If a ribbon is not used, and the transitions between the drivers are kept at somehow conventional frequencies, quite possibly it would not act as a line source in any frequency band (line too short for line source character at any frequency, by eyeball). It would, however, gradually increase in source size as frequency goes down.

Now say from 100 to 10000 Hz the wavelength varies 100-fold. Assuming a 1" CD as the smallest element and all 12" acting in unison at the largest, we'd have a 30 to 40-fold variation in source size, quite a bit better then the usual 1" to 8" or 10" change and close enough to 100x. This might end up sounding more natural, since low frequency sources in nature are usually physically large. Incidentally humans judge apparent source width through the envelope of the LF part of the sound, that's how we can localize LF sources and ascribe a size to them.

Michael btw that graph of attenuation with humidity was illuminating - I thought I was hallucinating but I feel a strong change in sound character depending on the weather. Some days things just sound noticeably duller than others.

Finally, a suggestion re: cosmetics: Another of my thought experiment speakers (), never built, was an OB with a trapezoid baffle shape, but folded back around a square cross-section "wireframe" column (using wood of course). The areas of the column that should be left open would be covered with black speakercloth. That would give the appearance of column speakers while preserving the OB character, and with a trapezoid foldback you'd hardly have much cavities in the back to watch out for.

Ugly sketch attached.
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Old 7th June 2007, 01:48 PM   #982
JoshK is offline JoshK  Canada
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re: aesthetics. I was thinking of making a more visually appealing shape in wood or colored baffle and using clear lexan for the rest of the baffle, or in front of the solid baffle. This way you have acoustically the better shape while visually a more pleasing shape.

Its not my idea really as I have seen this done by someone on another forum and the results were quite nice.
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Old 7th June 2007, 05:20 PM   #983
Variac is offline Variac  United States
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I LIKE the looks of trapezoidal open baffles myself....
The luxon, acrylic, or glass wings have been used in various projects and DO seem to minimize the size.
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Old 7th June 2007, 11:23 PM   #984
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Default Geometry of Sound Source

This section of the Lenard Audio page discusses the subjective impression of large sound sources - and matching emission areas to the wavelengths involved. The author specifically mentions the perception of a flat wavefront midway in the theatre that matches the image size of the screen.

With an interesting and personal view of the pro audio side of things, there's a lot of information here. There's a good section on theater horns and the history of their development in the cinema.

The decade, or three-octave, rule for horns makes sense. If the crossover from the 12" driver is at 1.2 kHz (woofer cone is one wavelength across), the horn should ideally cover a range from 1.2 ~ 12 kHz (1.2" wavelength), with a small supertweeter helping out above that.

Below 240~200 Hz (five to six foot wavelength), the bass array grows 2~4X larger, and is close-coupled to the floor reflection. Below 80 Hz, stereo servo subwoofers are located at the floor/sidewall boundary.

Contrast that to the typical audiophile speaker with its single 7" woofer and 1" tweeter, and the very large range of wavelengths they emit - no wonder the size and scale of instruments doesn't come across as realistic.
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Old 8th June 2007, 12:43 AM   #985
Pano is offline Pano  United States
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Default Re: Geometry of Sound Source

Quote:
Originally posted by Lynn Olson
Contrast that to the typical audiophile speaker with its single 7" woofer and 1" tweeter, and the very large range of wavelengths they emit - no wonder the size and scale of instruments doesn't come across as realistic.

You ain't kiddin'!


But it does start to get hard to squeeeeeze a symphony orchestra into the living room. Large shoehorn needed.
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Old 8th June 2007, 02:19 AM   #986
MBK is offline MBK  Singapore
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Hi Lynn,

excellent links. Somehow it all comes together in a set of emerging design principles, with a few trade-offs at builder's discretion.
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Old 8th June 2007, 03:37 AM   #987
CLS is offline CLS  Taiwan
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About the serial R for attenuation, I found the same dull effect on various speakers. So I avoid them at all costs.

One of my friend claimed that he got excellent results from autotransformer on his Klipschorn. For the 3way passive xover in such system, there's almost invetible serial R for attenuation. So does the loss in liveness.

He found that in the very old version of Klipschorn, there're the autotransformers for mid-high attenuations, and the vividness in sound is much better than the newer one with normal resistors.

I myself have no experiences on this. Anyone?
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Old 8th June 2007, 04:34 AM   #988
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The Klipsch autoformers are easily available as replacement parts and only cost about $20 or so. Remarkably, PWK started using them back in 1959, partly as a way to use much smaller values of caps (somewhat different crossover than I sketched out).

Altec and JBL, of course, were too proud to ever use autoformers, and stuck with textbook 12 dB/octave crossovers made from mylar caps and Radio Shack-grade L-Pads. I've looked inside the crossover boxes for Altecs and was dismayed they'd spent more on the beautiful precision-cast aluminum box than the parts inside - and this was in late-Sixies production for a professional-model A5 theater speaker.

I know this is going to enrage the Church of Altec devotees, but that was the difference between Altec+JBL vs Klipsch. PWK would use junky-looking PA drivers with phenolic diaphragms and surprisingly refined crossovers - and this goes back to the late Fifties. Back then, Altec+JBL made drivers that were Museum of Modern Art-quality works of art, but with generic crossovers with no driver compensation or correction at all. Out of sight, out of mind.

When a company doesn't bother to optimize the crossover to the drivers, builds it from the cheapest parts they can find, and spends a fortune on elegant machining of the drivers, cabinetry, crossover boxes, and advertising, that tells you a lot about priorities.

Altec+JBL continue to make good, even superb, drivers, with some of them true classics by any standard. But if PWK - and the BBC in England - were doing crossover/driver optimization in 1959, and Altec+JBL weren't doing it 10~15 years later, something ain't right. Maybe the engineers at Altec+JBL felt that system optimization wasn't worth the trouble, and all the glamour and high-tech went into the drivers. The difference could be summed up in two speakers: the JBL Ranger-Paragon vs any BBC monitor. Completely different set of esthetics, both visually and sonically.

I am sure the Altec+JBL bias against crossover optimization is why the US industry was ten years late to the table, when the Brits had settled on BBC/KEF optimization throughout the Seventies. JBL finally started making mirror-imaged speakers by the early Eighties, ten years after the Brits did.

Maybe part of the reason is that Los Angeles is a company town, with a strong Not Invented Here attitude towards non-Los Angeles technology. I went to college in LA during the late Sixties, and Altec+JBL pretty much owned the place, with a firmly established duopoly in the broadcast, recording, and movie business. Back then, Brit or Euro-design speakers of any type were very hard to find in Los Angeles, and the Altec 604 Duplex and the studio version of the JBL Century 100 ruled the roost. I was very glad to see competition start to appear in the Seventies - it was long overdue.
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Old 8th June 2007, 11:13 AM   #989
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Default Re: Geometry of Sound Source

Quote:
Originally posted by Lynn Olson
This section of the Lenard Audio page discusses the subjective impression of large sound sources
Thanks for the link, Lynn. A very interesting site.
Though I understand what he is trying to convey about sound, and agree with him, I have to take a technical quibble with his inverse square law.

Click the image to open in full size.

Once passed thru a lens, light no longer falls off in the inverse square. Not at all. If it did, you'd lose the major benefit of using a lens! Surprisingly, most technicians don't understand this.

Does the same apply to sound passed thru a "lens" such as a horn? Maybe. From what I've read and the diagrams I've seen of wavefronts in horns - it seems to have some bearing.

Light coming from a lens does not radiate the same way as light coming from a point source, nor does sound coming from a horn radiate in the same way as sound coming from a point source.
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Old 8th June 2007, 04:16 PM   #990
MBK is offline MBK  Singapore
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Quote:
Does the same apply to sound passed thru a "lens" such as a horn? Maybe. From what I've read and the diagrams I've seen of wavefronts in horns - it seems to have some bearing.
I don't think horns act as lenses do. A horn acts as an impedance transformer (increasing efficiency, not relevant in this context) and a waveguide (restricting radiation into a fraction of full 4 PI space, possibly relevant).

The inverse square law comes from the idea that the sound intensity will spread out spherically and pass through ever increasing surface areas:

- At a distance r from the source, the sphere area the sound will pass through will be 4 PI r^2.
- At a distance 2r from the source, the sphere area will be 4 PI (2r)^2 = 4 PI 4r^2, or 4 times larger, but receiving the same initial power.
- Hence, 1/4 the power (-6 dB) for twice the distance.

If the point source is not radiating spherically, but into an axially symmetrical waveguide AKA horn, the area receiving the initial sound power will be smaller by a fraction f. So the source will radiate through a surface of area (4 PI / f) r^2. Since the r^2 term has not changed, the inverse square law should also not change, whether in a horn or a waveguide.

Special case line sources, here one assumes two parallel walls (floor and ceiling), so the surface of the wavefront is no longer proportional to r^2 but to r * h (h: room height). While r varies with distance from the line source, h does not, so the surface the sound radiates into is proportional to r, and a doubling of r gives you a halving of power per surface, in other words, -3 dB per distance doubling.
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