Rear mounted ambience drivers, opinions?

Okay, here goes.

I built a pair of Exodus 2641's and after two years they are the best general purpose loudspeakers I have ever heard - they eat KEF 104’s – and even stand up quite nicely to the Wilsons. I think it's hard to get better sound for anywhere near the build cost x 3.


I messed with a 40" Bohlender ribbon last week and got a taste of that neutral *wide* long ribbon sound. Mark my words, not better than the CSS WR-125 of the 2641's, just way *wider* and a boat-load more detailed and listenable.

I wired the Bohlender thru the port of the 2641 to the band-pass outputs of the xo and clamped it against the 2641 cab - it took the place of the CSS WR-125 in the system (which was disconnected). I know, ruff but functional and valid enuff to get the idea. The XO is no problem (impedence aside), look at the specs.

The widening of the soundstage was subtle but noticeable, over time *really* noticeable. Bear in mind, I only had one 40" Bohlender. My impression is that with two, the soundstage and airiness would be really pleasant (bombastic would be more accurate!).

But, that would ruin Kevin’s absolutely fabulous design - my 2641's are the envy of all my audiophile friends - I don't want to f* them up by redesigning the cab to accomodate the ribbons. Won't - and couldn't - do it. Massive redesign.

However, (OMG, here I go...) that ribbon sound added an aesthetic, namely (warning: subjective term follows!) *space*.

The ribbon opened the soundstage immeasurably. It ruined me and I am all f*'d up now. I loved the speaks before the ribbons, now I am thrashing.

I am thinking about adding (ala VonSchweikert) an *ambience* driver (a Bohlender 3" ribbon tweet) on the rear of the cab with its own LPAD to *open* up the soundstage like the 40” did.

It would be crossed over same as the Usher tweet, out of phase and LPAD'ed.

Am I misguided? Wouldn't a little adjustable, delayed, ribbon-floaty, high end, be pleasant?

Okay, wait, before you start, I know about the time alignment issue, etc…, but, real world, wouldn't it sound nice?

Do you think they'd add the *floatiness* I am looking for, or do you think would they just muddy things up?

You could always turn it off...

Thanks for entertaining my eccentric question.



2005-11-04 9:30 pm
was there a back plate on the BGs? i.e. what I'm really getting at is were they acting as dipoles? If so, then maybe that's your answer. That is, that adding a small planar driver in phase on the back of your existing speakers probably won't help you achieve anything like the same thing. Also, if you were using it with the back plate on, the dispersion pattern is that of a (pseudo) linesource which, once again isn't IMO going to be replicated with a second driver. If you have the money to spend, however, it wouldn't hurt to try and I'd be interestedt o know the result because it's always possible that what you liked was a result of a better power response.
In my experience, in those cases where a rear-firing tweeter improves the power response it is generally beneficial, and in those cases where it degrades the power response it is not.

Distance from the rear wall is also a consideration, as in general reverberant energy arriving within 10 milliseconds of the direct sound is detrimental from either in imaging or a coloration standpoint (or both), while reverberant energy arriving later than 10 milliseconds is usually beneficial (assuming its spectral balance is not drastically different from that of the first-arrival sound).

I'd want to look pretty closely at the radiation patterns when bringing in a rear-firing tweeter. Most speakers with direct-radiator drivers have a relative excess of reverberant field energy in the lower end of the tweeter's range (typically 3-4 kHz - right where the ear is most sensitive), and in my opinion you do not want to add even more reverberant energy in this region.

Hi audiokenesis,

I agree. I am looking to *open* the sound up, not *sharpen* it.

The tweeter in my loudspeakers (Usher 9950-C) is crossed over @ 4Kh off a CSS WR-125 @ 500Hz-4Khz. So I anticipate no problem with a harsh upper telephone band.

I will LPAD he ambience tweeter as well.

Do you think it should be in-phase or 180 out? @ 180 out won't the tweeter pair mimic dipole radiation?

Ptwining, note that a direct-radiator that has a flat on-axis response will generally have a power response that falls by 6 dB per octave with increasing frequency. All you will hear of the rear-firing tweeter is its power response, so design your crossover for it with power response rather than on-axis response in mind.

It doesn't matter whether the rear-firing tweeter is in-phase or out-of-phase, as by the time its radiation gets to you subtle timing cues are of no consequence. Also, at those short wavelengths relative to the enclosure size dipoles and bipoles will have the same radiation pattern for all practical purposes.