First theatre- Ascend seems to be for me

[Quinn]

Thanks. In the course of my search, I found the answers to a couple of questions that I raised earlier. First, What has been done to the CMT-340 center from an engineering standpoint to minimize the problems inherent to using an MTM speaker on its side as a center Dave F. explains and specifically, would the 340s or the 170s be a better choice for the HT under discussion Dave F.s recommends 170s for a 15x15 room

I haven't found the answer to whether, in a small room, it would be better to use a 170 center (that avoids the whole off-axis MTM problem) or a 340 center (optimized to minimize the MTM problem). Can anyone direct me to a thread where that specific question was addressed?

Kurt- this may be what you are looking for-

"Just the other night I was watching SWAT with my wife and we both commented on just how natural the *full range* of Sam' Jackson’s voice was reproduced through our CMT-340c. I am most proud of this speaker

I was hoping to avoid a technical discussion regarding MTM designs and “lobing” but a few public negative postings from perhaps 1 or 2 non-Ascend customers are creating quite a stir. Business as usual I guess…

In as simple as I can explain terminology, “lobing” is used to describe an acoustic effect created by two identical sound waves reaching the listener’s ear at different times. If you are to the extreme right of the speaker, the right woofer’s sound waves will reach you before the left woofer. Imagine two identical sine waves drawn on top of one another but the second one is offset a bit on the horizontal axis. Instead of lying on top of one another, these waves will now intersect at certain points. At these intersection points, the two waves will either be somewhat in phase or somewhat out of phase. When the intersection point is in phase, the second wave will support the first causing an increase in amplitude. Conversely, when they are out of phase there will be a decrease in amplitude. This can have the audible acoustic effect of diminished clarity.

Fact # 1: Unless the loudspeaker is a single point source, ALL loudspeakers will exhibit lobing.

Fact #2: The MTM design is well proven in the audio industry; it is a design choice, NOT a design flaw.

Fact #3: All loudspeaker designs are a compromise; each specific design offers advantages combined with certain disadvantages over another design.

Regarding our CMT-340c:

Like all our loudspeakers, this speaker was designed to be extremely accurate. We specifically chose the woofer – tweeter – woofer configuration for several reasons that I will later address. First of all, the lobing effects of MTM configurations are only evidenced when listening off-axis, usually at extreme angles. Several design features have been incorporated to specifically minimize off-axis lobing

1. We used a high performance tweeter with a low resonance frequency. This allows for a lower crossover point which in turn allows more of the speaker’s critical midrange response to be reproduced by the tweeter. This increases the horizontal off-axis angle at which lobing first becomes evident.

2. The woofers use a steep 4th order slope in the crossover. This further reduces off-axis lobing while also increasing the off-axis listening angle by greatly reducing the directional high frequency energy reproduced by the woofers.

3. Notice how closely spaced the woofers are to each other? Spacing the woofers closer together further reduces off-axis lobing.

What are the results of these optimizations? Off-axis lobing only becomes evident by measurement at a horizontal angle of 20 degrees or greater. That is by measurement, not what is actually audible as our measurement equipment is far more sensitive then human hearing. To my experienced ear, off-axis lobing only begins to become audible at an angle of 25 degrees. So, what does that mean exactly? Quite simply, if you are 12’ back from the center channel, the accurate listening window for the speaker is approx 9’ wide. Off-axis lobing will not be audible unless you are sitting outside this listening window. Lobing effects with this speaker are simply not a factor unless you enjoy watching your TV and listening to your home theater system considerably off-center. And in this case, far greater acoustic problems will become evident (because you are most likely sitting directly across from your left or right speaker )

Why did we choose this specific design as a center? Our design goals are to build speakers that reproduce the source material as accurately as possible. Here are some advantages of the MTM design over a design with a woofer – midrange- tweeter atop midrange – woofer design. I will call this the M-TM-M design.

MTM design has vertical symmetry and wider vertical dispersion. M-TM-M design places a tweeter on top of a woofer which is obviously not vertically symmetrical. If you are below the speaker, the energy from the middle midrange woofer will reach your ear before the tweeter. If you are above the speaker, the tweeter energy will arrive sooner. Sound familiar? This is vertical lobing and its effects can be quite nasty if the speaker is sitting high on top of or below a TV.

The 2-way design of our 340 center uses a single crossover point. The 3-way design of an M-TM-M design must have 2 crossover points. The fewer the crossover points, the less phase problems and the better the driver integration. This equates to a more linear on and off-axis response, which closely resembles our acclaimed CBM-170s. Remember, a center MUST be tightly timbre matched to the left / right speakers. The frequency response, phase characteristics, and polar response between a 3-way speaker and a 2-way (even if they use the same drivers) will differ dramatically. No way around this.

Typical 3-way speakers are designed so that the midrange driver shares the same cabinet volume as the woofer. Big problems here…. As we all know, the inside of a speaker cabinet is subjected to tremendous pressure changes. When a small midrange woofer is sharing the same air space as a larger woofer (or worse, 2 woofers), the midrange driver will modulate with the pressure changes caused by the two bass woofers. We have all seen what happens in a dual woofer design; manually pushing one woofer inward will force the other outward etc. When the small midrange driver is trying to reproduce its signal, the larger woofers will actually cause unwanted compliance changes, thus producing high levels of intermodulation distortion. This is easily measurable and audible, regardless of where the listener is seated. It is for this very reason that in true “high end” loudspeakers, the midrange drivers in a 3-way design are almost always in their own dedicated cabinet section.

It is true that the M-TM-M design (if done properly) can have less off-axis horizontal lobing. This is simply due to the woofers of this design rolling off earlier and effectively offloading the midrange response to a single driver. In this case, I am assuming that the crossover point between the woofers and the middle midrange is lower and the slope is at least the same as that of the crossover point and slope between the woofers and the tweeter in the MTM design. Even still, the midrange crossover point must be quite a bit lower in the M-TM-M design because (due to the size of the middle midrange), the woofer spacing will be greater than the MTM design. All in all, we might be talking about only a few degrees difference between one design and another where lobing will become audible…

I would be happy to measure and discuss the lobing effects between our 340c and any M-TM-M center speaker provided one is sent to me

Please don’t assume the technical characteristics of one loudspeaker are “better” than another simply by a choice of design. Remember, I chose our design for a reason. I certainly would not assume my old 8-cylinder Mustang could even compare performance wise to one of the new highly tuned 4 cylinder pocket rockets simply because, technically speaking, with all things being equal, an 8 cylinder engine will have more horsepower than a 4 cylinder…… All things in comparison to our speakers are not equal…. Just look at the quality of our unique woofers and the tweeter we used in our 340c.

I am not surprised at how the most critical aspect of speaker design (the drivers used) is so seldom discussed by the self-proclaimed experts on the various online forums….

Thanks for your time!



Good Sound To You!

David Fabrikant
www.ascendacoustics.com" http://forum.ascendacoustics.com/showthread.php?t=215

[Prairie]

Wow...that's a lot of text.

From what I gathered the 170 may be better for a room of my size..unless I read that completely wrong.

[chas]

Prairie,

You might want to drop Dave F. an email and get his opinion on the best speakers/placement for your room.

[BGHD]

Wow, I didn't expect this to turn into a debate...cool

So, about the surrounds...any opinion on that?

Looks like i'll have to do some reading to figure out what I want to do for the center.

I'd stick with 200s in that tight a space. Looks like the lamp on the left might have to go though. Make sure you get the BT5 wall mount instead of the Omnimount. The BT5 will keep the 200 closer to the wall, which will be important in your tight space. Trust me, from experience, you do not wanna knock you head on the dense MDF of the Ascends.

I'd double check with Dave F, but the 200s mounted 2-3' above ear level on both sides of the couch (but not in front of the listening position), pointed directly towards each other, would be the way to go for HT. It won't be perfect in that tight a listening position, but it oughta work fine, since you're mainly dealing with just ambient surround sounds.

[lmm]

pictures of my setup, with closeup of HTM-200s