Large waveguides?

[Elbert]

Time to get back to the forum again to sound out some ideas and get some input from all the gurus out there!

Waveguides... A shallow "horn" used to controll the directivity and off-axis response of tweeters as exemplified by SEAS in their DXT tweeter H1499-06 27TBCD/GB-DXT and various experimental applications such as: Zaph|Audio

Obviously great stuff for matching directivity with mids/ midwoofers.

Apart from that, I believe a controlled, or "narrow" dispersion is most usefull managing unwanted reflections in a less than optimum, i.e. most domestic environments, listening rooms.

Just bought a pair of Fountek FE85 on sale at my local DIY pusher, and that got me thinking.....

The small diameter of these fullranges will translate in to a very wide dispersion well up in frequency, (compared to say a 6,5" midwoofer) but at some point it will start beaming and end up with a dispersion which is probably narrower than a dome tweeter in the treble range.

From my initial reasoning, the narrow high frequency dispersion is not necessarily an evil in it self, on the contrary, but the wide dispersion in the lower frequencies will be a disadvantage.. varying dispersion pattern and a lot of room reflections at lower frequencies..

Now, how about concidering that little FE85 as a slightly large tweeter with a very low resonance frequency?

Not entirely unlike a 2" horn-driver?

Ok, placing a 2-300Hz cut-off horn is an obvious solution. but what about a large wave guide that could be effective down to 2-300 Hz rather than 1-2000Hz for a tweeter?

EQ'ed bakc flat again, that should give a fullrange source with controlled directivity covering the entire critical midrange area with no x-overs ot tweeter.

So, where is the flaw in my thinking??

Bring it on!!

[AllenB]

One of the things I like about full range drivers is the narrowing directivity. Though you'll change that about it if you use it as a compression driver and build a phase plug for it, making it no different to other compression drivers.

[sreten]

Hi,

Its doable but requires an extremely large / wide baffle.

Its fairly obvious 200Hz needs to be 10 times bigger than 2KHz,
and that is 100 times the frontal baflle area, 100 times bigger.

rgds, sreten.

Its been done with larger drivers, these are waveguides not horns :

[Elbert]

Yes, well, perhaps my ramblings were not too clear.. if I wanted a compression driver and horn, I'd go for that in the first place..

What i was thinking about was simply using a wave-guide on the FE85 as has been done with dome-tweeters

All in the hope of narrowing the lower frequency dispersion down to better match the inherently narrow high frequency dispersion of such a driver.

[Elbert]

Sreten,

you are in deed right, I would hardly anticipate such a wave guide to be small!

looking at "conventional" horns, a horn with a cut-off of, say 200 Hz will have a large-ish mouth area and be relatively long.. but what about a wave guide?

Wave guides for tweeters are undeniably shallower/shorter than a theoretically correct "conventional" horn...

I guess the question really is how does one determine the dimensions/ profile of a waveguide vs a horn??

[weltersys]

Originally Posted by Elbert

looking at "conventional" horns, a horn with a cut-off of, say 200 Hz will have a large-ish mouth area and be relatively long.. but what about a wave guide?

Wave guides for tweeters are undeniably shallower/shorter than a theoretically correct "conventional" horn...

I guess the question really is how does one determine the dimensions/ profile of a waveguide vs a horn??

A low horn cut-off does not mean it has pattern control to that frequency.

“Waveguides” are usually conical other than the throat and mouth transition areas, the clear devices sreten posted in #3 look like horns to me.

Without getting caught up in the semantics of “horns” vs “waveguides” a simple conical expansion (in which the sidewall angles determine the dispersion angle up to the point where cone piston diameter takes over) requires a mouth about 1 wavelength wide to provide pattern control to that wavelength.
Speed of sound is 1130 ft per second, 1130 /300 Hz =3.77 feet.

Big.

The transition is not on/off at that frequency, even a conical “waveguide” of 1 foot diameter will provide some control much lower, but will raise sensitivity above that point, requiring compensating EQ if running the HF driver lower than around a 1130 Hz crossover point.

A 2” exit driver also beams like your little FE85 drivers unless the throat is pinched to a narrow width.

[picowallspeaker]

the clear devices Sreten posted in #3 look like horns to me.

I saw the advertisement on a magazine some years ago , and they cost a lot...
much ,much more than a normal person would like to spend on some passive device . I guess that , if you have a big piece of thick vinyl and a big oven , you can put a load in the center ...et voilà ! you can have your homemade waveguide . Also ,some unused / broken big woofers might be used for the purpose . It's just a matter of sensibility between the ways of a loudspeaker system ,so if a woofer has ,say ,92 dB sens. and you want to use a little 2" in a sort of FAST arrangement , using the WG will raise its sensitivity .
Just plot through the frequency and deciBel graphic table at which frequencies the wg+fr combo is gaining respect to the fr alone .

[sreten]

Hi,

Typically a waveguide is much shorter than a horn and does not
go as low as the mouth circumference would indicate the cut-off
frequency for proper horn loading. Also the efficiency boost of
a waveguide is a lot less than that given by full horn loading.

Its semantics as to whether its horn loading or a waveguide,
as waveguides untilise "horn loading", but it seems clear to me.

This is a waveguide :



This is a (long throw) horn :



At some point the two blur.

rgds, sreten.

Considering it further the picture I posted #3 does not have a typical
waveguides profile / expansion rate so I guess you could call it a horn.

"All waveguides are horns, but not all horns are waveguides" (Earl Geddes)

[Elbert]

Interresting... I was sort of assuming that waveguides would need to have some sort of curvature but if it is only a matter of smoothing out the transition to the baffle.. hmmm.. that would require some sort of profile/ radius? And what would determine that transitional profile??

So, assuming a conical wave-guide, with a mouth area given by the wavelenght of the cut-off, the depth would then be a function of the cone-angle desired for directivity-controll, i.e. the dispersion at the frequency the driver starts becomming directional.. well, this all makes sense.

But if it makes sense from a practical and hi-fi point of view is perhaps another issue...

[Elbert]

Obviously, a true horn is best if increased efficiency is a criteria, but for my case, I'm solely interrested in the directivity controll.

Winder if there are other means of narrowing dispersion at lower frequencies, e.g. absorbing "baffles" on the sides of the driver?