CTC spacing in ribbon designs

[Dr.EM]

Hello all, there is something I have been wondering that I can't find an answer to. In a design with a ribbon/planar tweeter utilising a long element where can the acoustic centre point be assumed?

In drivers with typical voice coils the centre point is where the voice coil is. With a planar there is no voice coil as such and the "driving force distributed evenly over the whole area of the vibrating element", tweeter in question here (I have these tweeters already):

http://www.swanspeaker.com/product/htm/view.asp?id=16

So where can the acoustic centre be? In the middle of the element, the bottom or the top (ie, furthest from next driver down)? It really needs to be the bottom to achieve good CTC spacing at the frequencies reccomended for such units. I want to integrate it with a Dayton RS52 dome and crossover at 5khz or so.

Any ideas/opinions?

[c2cthomas]

Hello DR.EM, I'm not sure if the white paper by Dr. Jim Griffin answers your question or not - but it is a good read and could provide you with some things to consider. If it does not (the white paper) then you might consider contacting Dr. Griffin as he is a member here and his email is listed.

http://www.audioroundtable.com/misc/nflawp.pdf

Ta Mate - I hope that you find this information of use.

[Dr.EM]

Thanks for the reply! Thats a good paper, lots of info in there. Seems mostly aimed at the pairing of ribbons with other ribbons rather than a ribbon with a cone/dome driver. I may contact him directly, he probably has the answer!

I did come accross this:

http://www.htguide.com/forum/printthread.php4?t=24040

Where one poster states;

"Ribbons emit sound from their full length so the CTC measure starts at the edge of a ribbon vs the center as a dome."

But I am unsure of the accuracy of this as the reasoning does not make sense to me. A dome/cone will also emit accross all of its surface? Dependant on frequency, I would add beyond breakup radiation from the edges is expected to be lower

[c2cthomas]

Hi Dr.EM

It's difficult to answer your question in terms of usage with the dome tweeter because it leaves out discussion of several issues such as 1) what is the intended use of the speaker (floor stander, monitor, 2 way, 3 way, L-C-R position, type of audio to be reproduced, etc.). The 1st thing that comes to my mind is - is there a woofer involved with this speaker? Perhaps something along this type of design? http://www.selahaudio.com/id53.html

So (corny drum roll please) What's Up Doc?

[GM]

Originally posted by Dr.EM

"Ribbons emit sound from their full length so the CTC measure starts at the edge of a ribbon vs the center as a dome."

Works for me since the ends radiate hemispherically and has for all intent and purposes an infinite number of acoustic center points between the two. Look to microwave antenna theory/design for a technical answer.

GM

[thadman]

Originally posted by GM


Works for me since the ends radiate hemispherically and has for all intent and purposes an infinite number of acoustic center points between the two. Look to microwave antenna theory/design for a technical answer.

GM

Wouldn't that principle only apply when the ribbon was functioning closer to a line source and not closer to a point source?

[GM]

??? A true line source is what it is with its acoustic center effectively being anywhere between the acoustic centers of its poles.

GM

[thadman]

Originally posted by GM
??? A true line source is what it is with its acoustic center effectively being anywhere between the acoustic centers of its poles.

GM

A traditional ribbon HF (75-150mm) shouldn't radiate like a linesource around any common crossover point (1-3khz) at any appreciable distance, at least according to Jim Griffins white paper. Assuming they did radiate as a linesource, there would be serious issues with SPL decay between ribbon and dynamic cone drivers since a linesource loses 3dB for every doubling of distance whereas a point source (dynamic cone drivers should behave closer to a point source) loses 6dB.

A ribbon would radiate as a linesource in the nearfield, but that distance is so minute compared to traditional listening distances (1-2m) that it doesn't apply to what we're discussing.

[Dr.EM]

Hmm, I am not well versed in this so all I could add is that the element in my ribbons (planars) is 125mm long and at listening distances of 1.8m or so the treble rapidly dissapears if you move up or down in relation to them.

c2cthomas, It is intended to be a floor standing 3 way with Hi-Vi M8a "bass" in a 1/2 wave closed TL (will be backed up by dipole subs eventually) up to about 900hz where the RS52 takes over 24db LR. The RT2-EA takes over around 5khz (not decided, there is quite a wide possible range here) also 24db LR (have 3 way boards from ESP ). Perhaps a little similar to this:

http://www.lonesaguaro.com/speakers/cyclops/CYC.htm

Except is a floorstander, active (tri-amp), bass driver is loaded differently and tweeter is a bit different. My crossover will naturally incorporate notch filters and phase shift networks as required.

I play a lot of electronic music and some rock/metal. I like to have a fast transient response which this setup should give me, great imaging and a very flat/smooth frequency response, hate any resonant peaks. Voices must also sound excellent which I know has been made potentially difficult by having crossovers at 900hz but any well implemented crossover should not be audible anyhow!

I do plan to route into the RS52 faceplate so the tweeter faceplate sits into it, both for aesthetics and to decrease CTC

[Dr.EM]

I made a mock up RS52 from cardboard to see how close the CTC could be with the RT2E-A with the RS52 faceplate routed as much as possible. It comes to 78mm from the bottom of the tweeter element to the centre of the dome.

I believe reccomended CTC spacing should be half wavelength at the crossover frequency, or even possibly quarter? 78mm will not give me this at 4800Hz (should be just 35mm assuming half wavelengths!) but a lower crossover wouldn't be ideal (tweeter plays better up higher and is further from critical area if that is important).

However I notice this general rule being thrown right out in many reputable designs. Most notably B&W use 4khz crossovers with 150mm spacings AND only 6db rolloff rates (I plan 24db rolloffs) in even the 800 series designs. How is it they are "getting away" with this so to speak? Can I assume that my 78mm spacing will be more than acceptable?