bullet ribbon tweeters

[Ang]

I was wondering if anyone had experimented with essentialy perching a ribbon tweeter on top of a cabinet to avoid diffraction effects. I was thinking of trying this with a couple aurum cantus's (canti?).

Thanks.

[454Casull]

What about diffraction from the frame?

[454Casull]

What about diffraction from the frame?

[Ang]

I was envisioning that there would be some diffraction from the casing of the ribbon, but hoping that by essentially separating it from the rest of the baffle the overall diffraction would be minimized. Isn't mfr. freq response measured this way anyway?

[Patrick Bateman]

Originally posted by Ang
I was wondering if anyone had experimented with essentialy perching a ribbon tweeter on top of a cabinet to avoid diffraction effects. I was thinking of trying this with a couple aurum cantus's (canti?).

Thanks.

Actually that's a worst-case scenario.

The best way to reduce diffraction is with a waveguide, and ribbons are not waveguide-friendly.

A good compromise is to use a large baffle. If you feel like experimenting, why don't you put a ribbon into a spherical enclosure? That would reduce diffraction nicely. Have you seen B&Ws top-of-the-line?

Putting the ribbon in free-space is the worst thing you could do. Do you see why? There will be loads of diffraction at the edge of the driver's frame.

:: PB ::

[LineSource]

Originally posted by Ang
I was wondering if anyone had experimented with essentialy perching a ribbon tweeter on top of a cabinet to avoid diffraction effects. I was thinking of trying this with a couple aurum cantus's (canti?).

Thanks.

There were several designs similar to this Audio Consulting link which added a large radius baffle to the ribbon, likely about 2x wider than the crossover frequency. Baffle diffraction simulation programs show a large improvement in smoothness with about a 4" radius.

http://www.audio-consulting.ch/DescriptRefSpeakSyst.htm

[LineSource]

Originally posted by Patrick Bateman


The best way to reduce diffraction is with a waveguide, and ribbons are not waveguide-friendly.
:: PB ::

There are several commercial true ribbon speakers which use a very shallow waveguide to control dispersion, lower the crossover frequency point, and increase the efficiency of ceramic magnet ribbon motors. Dipole ribbons put the same waveguide on both sides.

Some speakers use a deep waveguide on a robust planar magnetic transducer to better match the dispersion pattern of a wide bandwidth woofer, and also to help lower the crossover frequency. -attached-

[JohanH]

Like this?

http://www.newformresearch.com/r58.htm

Cheers,
Johan

[LineSource]

Constant Directivity Planars from VMPS

For 2006 VMPS introduces a new breed of loudspeakers: planar hybrids featuring Constant Directivity with Frequency full-range.

Why is Constant Directivity (CD) important? What effect does it have on listening quality?
Both questions are equally important. Loudspeakers without horns radiate in a narrowing pattern as they transverse higher frequency ranges—this is a function of their effective radiating width. In most multiways, and planars in particular, each driver is much larger in diameter than the wavelength of the top frequency it must reproduce. As drivers approach this cutoff, response suffers from an undesirable phenomenon called “roughness” and their directivity narrows to the point that, once their width/diameter is larger than the wavelength of the upper frequencies, they more or less radiate in a straight forward beam, like a headlight. This reduces the “listening window” to on-axis and makes it difficult for listeners sitting away from the “sweet spot” to hear all of the music.

The problem worsens in the crossover regions of a multiway where a large diameter driver and the smaller one above it differ in phase and dispersion causing frequency response protrusions and suckouts collectively known as “lobing”. While this effect can be reduced with increasingly higher-order crossover filter slopes, there are practical limits to implementing passive high-order networks due to poor transient response, group delay, and reduced dynamics caused by losses through the filter elements. Speakers like ours with first order filters (chosen for their optimum listening quality, not for smooth dispersion) suffer from the “lobing” phenomenon more than their steep-sloped cousins.

What is needed for CD is for all drivers in a multiway to be the same width, and that width needs to be smaller than the wavelength of the frequency you want to hear with good or constant directivity. If you want to hear 20kHz well off axis, that means a speaker no more than 2/3” wide, down to as low in frequency as possible, where it can mate with larger diameter woofers which are already working into 2 pi (180 degree) space and are about to transition to omnidirectional or 4 pi space. In the past speakers have been made with long and narrow drivers. These tend to be tweeters, however, and virtually all exceed 1” in width, which means they will beam at around 13 kHz or lower.

All our planar speakers use a 2.5” wide midrange panel working from about 280Hz up to 7 kHz, handing off to a 7/16” wide tweeter operating to above 20 kHz. While their horizontal dispersion is very good, there is lobing in the crossover region and some wave interference in the vertical plane (vertical dispersion of long ribbons is poor, usually not much beyond the length of the driver itself). If we want Constant Directivity type dispersion then whole speaker would have to be made drastically skinnier, impossible to do without sacrificing LF extension and sensitivity. For this reason older designs have not attempted to achieve CD coverage without horn loading.

http://www.vmpsaudio.com/d-cdwg.htm

[Ang]

thanks - see what y'all are getting at now

- the newform model was pretty much what I had in mind, come to think about it