Coaxial Distortion?

[Stocker]

While reading XOtothelight's HT Speakers? thread just now, a thought occurred to me:

You have a coaxial driver like the Seas H1333 suggested by AJinFLA, with a tweeter actually inside a woofer, with the woofer's cone for something like a horn for the tweeter.

The woofer moves. So the horn-load moves while the tweeter does not, yes?

Doesn't it play crazy havoc with the HF to have the horn moving all over the place?

[inertial]

IMHO,yes it is a problem,expecially with large movements of the cone.
Cheers,

[angel]

Has anyone considered using electrical compensation of some sort to deal with this?

The movement of the cone, and the effect this movement has on the dome in the center of the cone, should be an essentially known quantity. As such, it should be possible to cancel it electronically by biamping.

Any takers?

[Stocker]

Hoooo boy,

I think anybody who goes to *that* sort of effort is working too hard. By the time you get the stuff built/programmed/Bought, you might be better off getting some Jordan fullrange drivers!

[gugi100]

The most important part of the horn, the little throat direct around the dome, is not connected with the voice coil and hence does not move.

Gr GJ

[inertial]

(sorry for my english)
Yes ok,but the dome "see" anyway the 15cm cone, and if it moves for example +/- 5mm at a very low frequency ............

[janneman]

Originally posted by Stocker
While reading XOtothelight's HT Speakers? thread just now, a thought occurred to me:

You have a coaxial driver like the Seas H1333 suggested by AJinFLA, with a tweeter actually inside a woofer, with the woofer's cone for something like a horn for the tweeter.

The woofer moves. So the horn-load moves while the tweeter does not, yes?

Doesn't it play crazy havoc with the HF to have the horn moving all over the place?

This is well know effect called FM distortion or doppler distortion. The legendary Paul Klipsch has written about it, I'll see if I can find anything.

What happens is that the hf signal is FM modulated by the moving lf cone. This generates two sidebands frequencies above and below the hf frequency. The modulation depth (the distance between the original signal called the carrier in FM speak and the sidebands in Hz) depends on the lf excursion, IIRC.


Jan Didden

edit: google, 12 seconds:
http://stereophile.com/reference/1104red/index1.html


"Frequency-Modulation Distortion in Loudspeakers," reprinted in the Journal of the Audio Engineering Society, Vol.29 No.5, May 1981.

"Doppler Distortion in Loudspeakers," Hi-Fi News, January 1967. See also Moir's paper of the same title, presented at the AES 46th Convention, September 1973.

"Modulation Distortion in Loudspeakers," Journal of the Audio Engineering Society, Vol.17 No.4, April 1969.

"Simulation and Investigation of Doppler Distortion," AES 56th Convention, March 1977.

"The Audibility of Doppler Distortion in Loudspeakers," AES 70th Convention, October 1981.

[inertial]

Exact answer!!!!
(perdone me my sense of humor, janneman)
cheers,

[Feyz]

I am afraid it is not doppler distortion here. The cause of the doppler distortion is the movement of the source. Here the source, the tweeter, is not moving but the waveguide that loads it is moving, causing the shape of the waveguide to be modulated. I believe this would cause some amplitude modulation distortion, but I have no idea how audible and how much it would be to be a concern or not.

[Bare]

It's a "how many Angels on the head of a Pin' question. In actual practice with several Dual concentric Driver manufacturers JBL Altec Tannoy (all seriously prized drivers decades after production ceased)
this a Non issue.
No Idea if the current cheaply designed/made dual concentrics have this problem or not.. but I suspect they lack in so many other more ifundamental aspects that I doubt this 'problem' would surface as being noticable above the others.