Sloped baffle

Hi Roy, thank you for your response.
At the risk of appearing argumentative; here on Audiogon another speaker designer has suggested that placing multiple woofers in a room at different distances could be beneficial towards evening out standing waves, if that were the case; wouldn't W M T M W speaker arrays have some advantages? I'm not aware of any speakers that are touted be time coherent having more than one tweeter per channel, are there any?
As for sound bouncing differently above and below ones head, wouldn't that be typical during live musical performances? Wouldn't the wave size from midranges and woofers (and live performers) be large enough to extend above and below a typically seated listener's head?
Thiel's concentric drivers appear to be flat, so reflections should be minimized, no?
I have no direct experience, but I seem to recall that DEQX suggests that speaker correction should be first done close to the speakers and then followed with room correction at the listening positions.
Another question if I may; could horn loaded speakers be time coherent?
Thanks again!

Hi Unsound,

Thank you for your thoughts. The use of multiple subs does smooth out standing-wave issues. The math used for the theory behind that is formed from adding together the simple sinewave/wavelength equations for standing waves you have seen for bass tones and room modes before.

That is fine for long-running test tones, for movie sound effects, and certainly for a pipe organ. The test tones used to adjust those multiple subs are long-running, and not found in music.

When the time-arrivals at the ear between multiple subs are 'excessively different', you would think we'd hear stumbling or mumbling on string bass, drum kits and perhaps even cello. But if those subs are not allowed to go above ~40Hz, those issues are bypassed.

===

WMTMW bass problems arise from both woofers being close to the bottom and top surfaces of our room. This is a 'very symmetrical' situation, which always produces the strongest standing waves. Another 'very symmetrical' layout would be subs placed in every corner.

Have a look at this drawing: Reflections

Also, do note that WMTMW woofers operate to 150 or even up to 300Hz, which is above middle 'C' on the piano. In these upper ranges, changes are very audible standing vs. sitting vs. walking into the kitchen.

===

You ask about the over/under head effect of an image jumping when hearing live sound from vertically-large concert speakers. Good question. I can say I've never heard that problem, including from long line-source speakers. Remember, most concert sound systems are mixed close to mono, so everyone hears everything. And in most live situations, sound from a tall concert speaker comes to you from a narrower vertical angle than when at home listening to a six-foot tall speaker ten feet away.

Also, I probably did not make it clear enough before that the over/under head leakage of sound to the opposite ear is caused by the WMTMW use of double mids, not double woofers, because of those shorter wavelengths vs. the size of our skulls.

===

We get reflections off any hard surface-- it matters little that a Thiel's mid surface might be flat or corrugated around its coax tweeter. This is because any 1" tweeter, without a several-inch deep horn around it, is omnidirectional below 5kHz. That means it pushes waves between ~1kHz and ~5kHz across the face of the cabinet, since they cannot escape to the rear.
So those pressures escape to the front as they move across the face of the cabinet.
Hence, reflections.

===

Putting the measuring mic for DEQX up close to a speaker is pointless (except for fixing up a subwoofer), as what the mic would then be hearing is coming from drivers at much different path-length-differences to the mic compared to the path-lengths to an ear ten feet away. We all know how walking up to a speaker changes everything we hear. Perhaps they are suggesting this for fixing one driver at a time. That has problems too, because any driver's tone balance is different at ten feet away vs. ten inches away.

===

Horn speakers can be made time coherent, but our best technology leads to that speaker being at least a four-way if not a five-way design, to stay far enough away from horn cutoff points on the low-end of each driver, and the high-frequency breakups which come from running a large mid high into the upper voice range, and a compression driver with a 4-inch diaphragm into the high treble. Also, with 4 to 5 horns stacked up, their vertical height would make for very strong changes as one stood up or even just sat higher.

The nicest sound I ever achieved on horns was to use the lowest order of electronic crossover possible (12dB/octave, 'second-order') on a three-way horn system. The tweeter horn was moved far back on top of the mid's horn, and mid horn `way back on top of the woofer's folded horn, to equalize the driver-to-ear distances for people twenty+ feet away. This describes a system I put together for Taj Mahal. I had to add a small amount of EQ to smooth the mids, boost the ultra-highs, and for flat output to 40Hz. Of course I had to reverse the polarity on the mid horn because 12dB/oct. crossovers need that to avoid cancellations at the crossover points.

Since everyone was 20 to 70 feet away from either the left or right speaker (mixed to mono), everyone heard a smooth blend from a speaker whether seated of standing. Sure there was phase shift from those speakers, but it was far less severe than any higher-order crossovers would have been. I received very many compliments on the ease and clarity of the sound.

===

I hope everyone sees my answers are lengthy because I include WHY something is audible or will measure a certain way, so you finally get a proper technical perspective on the VARIABLES that must be considered, and also HOW they must be considered. Magazines and reviews leave out all these variables-- make of that what you will.

Best,
Roy

If you were to try to line up the drivers how would you find the " acoustic center" of a driver. In a midrange or woofer would that be about half way out from the center or would it be further out because the majority of the area of a larger driver would be closer to the perimeter?

Hi Roy, I don't mean to beat a dead horse, but I think that in a W M T M W array, it would be very rare indeed for any of the drivers to be equi-distant to floor and ceiling, and those surfaces would typically be different enough to reflect some frequencies differently as well.
I would have guessed that in an attempt at a staggered driver time coherent horn design that the horns themselves would get in the way of each other with the surrounding horns causing early reflections. Wouldn't a deep throat coincidental multi-driver have more early reflections than a flatter design?

Sounds Real,
For an eyeball estimate, the acoustic center is approximately where the voice coil former meets a cone or dome- the glue joint. But this is true only in the upper-middle range of any driver, whether tweeter, mid or woofer, where each one's frequency response is still flat.

To measure it (within +/- 1/8th inch at best for a tweeter, much more for a woofer), one sends an impulse, a click, to a driver having no crossover.

On a `scope, one examines when that click arrived compared to when the `scope's sweep was triggered by the click electrically.

Now, what we are looking for as markers will not be the top of those two spikes that click generated. We are looking for when each spike just begins to turn upwards from 0.0 at its bottom-- when each just begins to rise up. That is a very difficult transition to judge, which leads to inaccuracy.

Regardless, that time-difference times the speed of sound is your distance from the mic to the acoustic center in a driver's upper-middle range. Compare that to the tape measure distance and you often get close to the eyeball estimate I mentioned above. Of course, the test mic will be expensive, not a $200 special, for those cheaper ones have their own phase shifts in the audible range. Figure $1000 for a proper mic, plus a $1000 wideband mic preamp. Even so, the results will still be rather inaccurate. I was able to find ways around this, fortunately.

===

Unsound,
It is not that the two woofers are equidistant from their surfaces but the fact that we have two (four in stereo) woofers rather near two surfaces with you living in between.

Do have a look at that new drawing I posted to get an idea where "the bass source really is", which is my red dot in that drawing. Imagine what standing waves would then occur in between a red dot on the ceiling and one on the floor when the measuring mic/your ear is placed somewhere in between. Double trouble has been my experience.

No doubt about the outer horn-surfaces making reflections. But those reflections would go mostly upwards, and we can apply wool felt or acoustic foam to minimize most of them. I still think the biggest problem to be getting far enough away from the speakers so stand up/sit down differences would not drive us crazy- a large living room, say 30 x 40 feet is probably enough.

I would like to hear a Klipschorn corner horn triamped with time-delays applied to its mid and tweeter, since the woofer is so far back inside (~4 milliseconds) and the tweeter is so far in front of the mid driver (~2ms). Again, one would be stuck with using second-order crossovers on the drivers with the mid driver in inverted polarity.

Best,
Roy