Is power compression an issue for dome tweeters?

[noah katz]

I want new L/C/R's that can better keep up with four Avalanche 18's in output and lowered power compression.

I'm thinking of one of the MTM's using the RS180 drivers, but doubling up on the RS180's.

Should I double up on the tweeters too?

I guess it comes down to how much of the power in program material is there in the range from 1 - 2kHz on up.

Thanks

[JonMarsh]

Doubling up on tweeters will create problems with mutual interference at high frequencies depending on the angle off axis and wavelength.

A better solution might be a waveguide approach, which can increase the lower range of the tweeter output (where problems usually manifest most measurably and audibly) by 10 dB or more. But dual tweeters with the appropriate filters may be more esthetically pleasing, and easier to implement. Evidently, that's what Dynaudio thinks.



~Jon

[AJINFLA]

A better solution might be a waveguide approach

Might be? :W
I'm tempted to also say that the 2 tweeter approach would create more problems than it solves. OTOH, I don't design towards commercial sales appeal. It certainly appears workable if implemented properly: http://www.soundstage.com/revequip/a...2_followup.htm



conspicuously absent is the vertical off axis. Still not a bad looking set of boxes, if thats your thing

Cheers,

AJ

[Dennis H]

Rather than double the woofers, why not a 3-way? Chris and Brian's 3-way adds two 10" to the two 7". Should be a nice match for an uber-sub.

https://www.htguide.com/forum/showthread.php?t=11873&page=16&pp=35

Scroll down to the finalized design.

Or, if those are too big, maybe dawaro's WMTW/WWMT with two 8" and one 6". Two 8" will move as much air as four 7" and the 6" can cross higher, taking some load off the tweeter.

https://www.htguide.com/forum/showthread.php?t=15323

[noah katz]

This is for HT in a narrowish longish room so there won't be any listeners more than a little bit off axis.

The waveguide idea sounds good, increasing the efficiency that much would reduce power compression the most.

"Two 8" will move as much air as four 7" "

Not unless the 8" have a much higher xmax.

Oh, what about the original question? Is it even a problem that needs solving?

Thanks

[Dennis H]

"Two 8" will move as much air as four 7" "

Not unless the 8" have a much higher xmax.

There you go again, Noah, making me "prove" what most people already know. Check the Sd of typical 7" and 8" drivers (sort of misnamed because the 7" has a 7" frame and the 8" has a 9" frame). Sd of the 8 is often nearly double Sd of the 7. For the RS series, the 8 is a bit smaller in than normal but it makes up for it with more stroke:

RS180: Sd = 123 cm^2, Xmax = 6 mm --> 74 cm^3
RS225: Sd = 206 cm^2, Xmax = 7 mm --> 144 cm^3

Oh, what about the original question? Is it even a problem that needs solving?

It's not a problem but, if it were, crossing one tweeter 1/2 octave higher is the same as using two tweeters. Excursion on the low end is the limiting factor, not power compression.

[ThomasW]

Oh, what about the original question? Is it even a problem that needs solving?

Jon answered it

Doubling up on tweeters will create problems with mutual interference at high frequencies depending on the angle off axis and wavelength.

This is also referred to as comb filtering

And it would be better to add a bass bin (pair of 10"s) to take the workload off the 6" midwoofers. Doing that could be done easily with a generic active XO. Your idea (doubling the number of 6"s) would require a redesign of the crossover.

[noah katz]

Dennis,

Sorry, my mistake, I just took 7" and 8" at face value.

Actually 225 mm = 8.9", so calling it an 8" driver is questionable.

"It's not a problem but, if it were, crossing one tweeter 1/2 octave higher is the same as using two tweeters. Excursion on the low end is the limiting factor, not power compression."

Do you mean LP'ing the 2nd tweeter 1/2 oct above XO? That would do it.

Forgot to address this:

"Rather than double the woofers, why not a 3-way?"

Requires another XO and doesn't package well for the center channel in my setup space.

Thomas,

"Your idea (doubling the number of 6"s) would require a redesign of the crossover."

That's another reason for doubling on the tweeters; I could just halve/double the component values capacitor/inductor values.

I also might use active XO's, either Rane AC22 analog (L-R24) or DCX2496.

Thanks

Oh, and I do like the waveguide idea a lot, been reading up on that.

[cjd]

That's another reason for doubling on the tweeters; I could just halve/double the component values capacitor/inductor values.

If you think that's going to be sufficient, it shouldn't take you much more thought to figure out how to double the woofers and not the tweeters.

What are your space limitations? I think that a 2.5 way MMTMM is going to do better than a straight-up 2-way, but at that point you're pretty much playing with a 3-way, just using the same driver sizes.

C

[Dennis H]

Do you mean LP'ing the 2nd tweeter 1/2 oct above XO?

No, I mean only use one tweeter. You will get comb filtering and bad polar response once the C-C distance gets above one wavelegth. Actually 1/2 wavelength is a better goal. So, for most tweeters with a 4" faceplate, you will start having problems above 3kHz or so.

By the same reasoning, a 2-way crossover with 4 woofers is a bad idea. The distance to the outside drivers is too big for a point source (you'll get comb filtering) and too small to be a line array. The off-axis response will be poor. Ignoring the off-axis response is a mistake, even if you only plan on listening in the sweet spot. The off-axis stuff contributes to the room's reverberant field and affects the perceived timbre of the speaker. The only way 4 midwoofers will work is to do a 2.5 way where the highs are rolled off for the outside pair. Now you've basically got a simple 3-way so you might as well build a "real" 3-way.

[ThomasW]

By the same reasoning, a 2-way crossover with 4 woofers is a bad idea

Which is why the bass bin with an active XO was recommended...

[noah katz]

"If you think that's going to be sufficient, it shouldn't take you much more thought to figure out how to double the woofers and not the tweeters."

I almost said that, but some of the designs have these confusing series XO's.

"crossing one tweeter 1/2 octave higher is the same as using two tweeters."

"No, I mean only use one tweeter."

Ah, reducing the excursion requirements by crossing higher.
Today's thought is the 2 1/2 way waveguide design with another set of woofers above and below.

Thomas,

"Which is why the bass bin with an active XO was recommended... "

Do you mean in addition to a passive XO 2-way?

That's getting too complicated/expensive for me.

Thanks

[ThomasW]

Do you mean in addition to a passive XO 2-way?

Yep, and sorry but no free lunch. The cost of the components for a passive 3-way, will be very close the $90 or so for something like a Behringer CX-2310.

My advise is build the MTM and see if it has adequate output for your room. If not, save up and add the bass bins later.

[Tommythecat]

But this is where I get caught up in my thoughts ThomasW. You'll say that 90 bucks in passives in near an active unit, but what of the additional cost of amplifiers?

I haven't been able to crack the home DIY world because of this (but i'm young so im busy trying to fit two Avalanche 15's IB in my Accord - not sure its possible ). Even though I'm considering picking up a Behringer DCX unit I immediatly cringe at the thought of picking up a series of two channel, or even multichannel, amplifiers.

Any thoughts?

[ThomasW]

It doesn't take big power to drive 4 woofers, and in this day and age, watts are indeed cheep...

[Tommythecat]

Oh, are you suggesting just an active setup for the "bass bin"? What I'm thinking about is a fully active system, 3/4 way requiring 6-8 channels of amplification.

[ThomasW]

Oh, are you suggesting just an active setup for the "bass bin"?

The fundamental topic of this thread has been to use Jon's Modula MTM design. The crossover in that design can't be replicated with generic active XO's. But a bass bin can be added to the MTM using a generic active XO.

[Tommythecat]

I was kinda stealing a little thread space to ask my own question. Im thinking I might make my own space (to stop stealing from this) to ask my question. My bad.

Anyways, I second - or wherever I am in supporting the idea - to build the MTM's and add the mid-bass if necessary.