Tweeter placement?

[NcHalfrican]

I often see tweeters placed off-center. Why is this?

[matevana]

A possible means of lessening the diffraction effects, particularly off-axis.

[godfrey]

Two reasons I can think of:
1) Diffraction/reflection from cabinet edges: Having the various edges different distances away from the tweeter spreads the bumps and dips in the frequency response around.
2) Apparently you get better stereo imaging if the left and right tweeters are closer together than the woofers. Hence a lot of panel speakers e.g. Apogee and Maggies have the treble ribbon on the side rather than down the center of the speaker, with the suggestion that the speakers should be placed with the ribbons on the "inside".

[DBMandrake]

Originally Posted by matevana

A possible means of lessening the diffraction effects, particularly off-axis.

Offsetting a tweeter to one side a few inches might reduce the amplitude response variations caused by diffraction (especially directly on-axis, where its usually the worst) however it doesn't lessen the total amount of diffraction, it just redistributes it more in time. (something often overlooked or misunderstood) Re-radiated signal from the baffle edges is still just as bad as before.

Offsetting the tweeter horizontally on a baffle also introduces asymmetry in the horizontal polar pattern at treble frequencies - something to be avoided IMHO.

Better solutions for controlling diffraction are one or more of the following:

* Radius the edges of the baffle with a sufficiently large radius for the frequencies concerned (and avoid any bumps on the surface of the baffle as much as possible so diffraction doesn't happen before reaching the curved baffle edge)

* Add some sort of soft absorption on the baffle surface so the wave traveling along the baffle surface is mostly absorbed before reaching the edge (only practical above several Khz)

* Use a tweeter with some sort of wide band directivity control, such as a wave-guide. This will reduce the amount of signal traveling along the baffle to the edge by as much as 10-20dB and drastically reduce diffraction problems, provided the wave-guide is smoothly terminated to the baffle itself.

I know a lot of speakers over the years have used offset tweeters, and some of them may be very good, but I think it's now generally accepted that a speaker that is axially symmetric down the middle (drivers in a vertical line, left and right side of the speaker mirror images of each other) is a fundamentally better approach, and that diffraction should be treated at the source rather than offsetting the tweeter in an attempt to make the on-axis response look flatter.

[sreten]

Hi,

You can do all the diffraction reducing ideas, and offsetting can still
be a good idea. Assymetric left and right responses are not bad per
se, its all about the drivers responses, and what suits the c/o used.

rgds, sreten.