Vertical vs Horizontal Center Channel Speaker Designs An Alternate Perspective

A

admin

Audioholics Robot
Staff member
We've noticed lately that horizontally placed MTM speakers used as center channels have been getting a bad rap mostly because of their limited off-axis performance. But just how far off axis does one have to get until this does become problematic? Some consumers have alternatively chosen two-way bookshelf speakers in-lieu of using an MTM to get around this alleged issue while others chose W(T/M)W dedicated center channel designs or placing an identically matched speaker from their front channels behind a perforated screen. Not everyone has the luxury of the later option so this article discusses the tradeoffs of different center channel designs and their applications.


Discuss "Vertical vs Horizontal Center Channel Speaker Designs “ An Alternate Perspective" here. Read the article.
 
jliedeka

jliedeka

Audioholic General
Thanks, Gene. It was cool to see some real world measurements and a reality check for off axis angles.

Part of my current speaker building project includes 3 Natalie Ps for the front. I'm building the center as a horizontal MTM. I was discouraged from that but decided to try it anyway. Now I feel better about that decision.

Jim
 
J

jostenmeat

Audioholic Spartan
Thanks for the article. I have a few questions.

How much is that RBH 661 speaker? I see a figure cited at the top of the AH review, but I assume that is for a package of 5 speakers.

That is interesting that a -10db dip at 450hz might possibly be deemed inaudible due to the narrow bandwidth, if only for real world program.

Can it be assumed that the further offaxis one is to a horiz MTM, the worse it will sound?
The reason I ask is because one fellow told me that my own PSB Image C60, based upon the xover point of 2.5khz and driver spacing, I would essentially be 180 degrees out of phase at 20 degrees off axis listening. To clarify the question above, It couldn't possibly sound better at 27 degrees than 20 degrees for my speaker, could it? (I suppose not).

Unfortunately, the extreme edges of my seating are pretty much at 20 degrees on the dot. Fortunately, I rarely sit there, probably only once for every twenty viewings.

Do you believe that with a popular speaker, such as my Image, or say a Paradigm Studio, or Ascend 340, etc, that the results would be pretty similar?

How dramatic, if at all, could one predict the differences could be with offaxis listening based upon xover point and driver spacing?

Thank you very much. I have now decided to italicize the questions above for everyone's convenience.

Ok, I tracked down the post where someone told me about my PSB design. This was April of last year.

josten,

I looked at the specs of the PSB image C40 and C60. A horizontal MTM speaker's lobing errors depend on 1) CTC spacing of two woofers and 2) crossover frequency. The C40 uses smaller woofers and as a result shorter CTC spacing. It also uses a lower xover freq, 2.2 kHz, than the C60's 2.5 kHz.

But even C40 has unacceptable horizontal lobing patterns in your situation. According to my calculation, 20 degree off axis is the exact position where one woofer is about 180 degrees out of phase with the other woofer, resulting a deep null at 2.2 kHz, at which two woofers' SPL sum is supposed to be only 6 dB down from the system SPL.

A well designed MTM speaker that can be used horizontally as CC should use 1.5 k to 1.8 kHz xover frequency and/or much closer woofers.

-jAy
Thanks again for any input!
 
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gene

gene

Audioholics Master Chief
Administrator
How much is that RBH 661 speaker? I see a figure cited at the top of the AH review, but I assume that is for a package of 5 speakers.

That is interesting that a -10db dip at 450hz might possibly be deemed inaudible due to the narrow bandwidth, if only for real world program.
Not sure about their current prices but I would guess its around $900-1500/ea depending on finish.

The -10dB dip is likely a room anomaly. After XMAS I plan on retaking these measurements outdoors to eliminate the room as much as possible. I suspect the lobing will show up more in this environment since there will be no wall reflections to help reinforce those frequencies. Luckily we don't listen to our home theater speakers outdoors or in anechoic chambers so our rooms play a large role in how well a speaker will integrate in them.

Regarding your questions on your center channel, something doesn't sound quite right to me, but I've had a few glasses of wine tonight so my head isn't too clear. You don't want to use too low of a crossover frequency else you will sacrifice system dynamic range. Good MTM's are usually crossed over between 2-3kHz from what I've seen.

Cheers.
 
B

BTT917

Audioholic Intern
Any reason 1/4th of an octave resolution was chosen for the measurements, instead of, say, 1/12th octave? Seems too coarse to me to accurately show what is really going on.
 
Wayde Robson

Wayde Robson

Audioholics Anchorman
Amazingly well reserach documentation on center channel speakers, Gene. Thanks a lot for all the work.

I always heard MTM wasn't the way to go, now I know it's not that simple. Like you say - there is always a trade off when it comes to speaker design.
 
gene

gene

Audioholics Master Chief
Administrator
Any reason 1/4th of an octave resolution was chosen for the measurements, instead of, say, 1/12th octave? Seems too coarse to me to accurately show what is really going on.
B/C these were in-room measurements at the listening position which would look terrible if 1/2th oct res was chosen. 1/4th is more like how we hear as we are mostly concerned with low Q dips and bumps. After XMAS I will redo these measurements outdoors and in closer proximity with more precision.

Cheers.
 
W

westcott

Audioholic General
B/C these were in-room measurements at the listening position which would look terrible if 1/2th oct res was chosen. 1/4th is more like how we hear as we are mostly concerned with low Q dips and bumps. After XMAS I will redo these measurements outdoors and in closer proximity with more precision.

Cheers.
I really liked this article. It points out how real world room interaction and seating position are far more important than actual basic speaker design.

I would be interested to see other variables like basic speaker design compared. Speakers like Martin Logan, KEF, Klipsch, and other mfg's that use technologies other than direct radiation.

I also would like to throw in phase questions and what seems to have more real world affect in one's own home theater. Phase distortion (multiple drivers) Vs. MTM arrangment (2 way design).

Once again, thanks for a great article. I hope others find it educating.
 
Swerd

Swerd

Audioholic Warlord
Gene - Thanks for doing all that. It puts things into proper perspective. Like so many things where people don't fully understand the details, they interpret advice in a black or white fashion. Yes, it would be somewhat better if an MTM speaker stood vertically. But a horizontal MTM is much better than no center channel speaker, and as you showed, better (for different reasons) than a vertical MT speaker.

The best part was where you showed just how far off-axis you would be while sitting in a typical 3 cushion-wide sofa 12 feet away from the speakers. It isn't far enough off-axis to be real trouble.

Again, reality trumps conventional wisdom.

I would find it helpful if you could show those measured SPL levels in a polar plot such as below. Would that be possible?

 
B

buzzy

Audioholic Intern
In discussions like the Technical Notes about dispersion of sound from loudspeaker systems by and the section after that, it would be great to have some examples with frequencies and driver sizes in them ... "so at a frequency of x, a y inch speaker ..." It would be good to convey at what frequencies beaming intensifies, at what point in the lower frequencies the benefits mentioned are meaningful, etc.
 
AJinFLA

AJinFLA

Banned
I would suggest 1/6th octave as most representative of what we would hear. Even so, the smoothed data displayed should result in clearly audible differences due to positioning.
Perhaps not so distracting when watching a movie, as one's position will be stationary, so the variance won't be obvious. But it will be there.
Our brains are pretty good at filtering/ignoring things when it isn't essential to hear them, just like it's good at adding things that aren't really there :).

cheers,

AJ
 
highfigh

highfigh

Audioholic Slumlord
I haven't done any testing of MTM speakers but the benefit I have read and heard about that configuration is wider dispersion when orientated vertically. The other characteristic I heard and read about was that when speaker height was varied WRT the listening position, some issues occurred with phase cancellations. If the MTM center channel speaker is centered and the mic or listener moves 20° to one side, the distance to the mid-bass drivers will be different and this will cause comb filtering. The wavelength at 450Hz is about 4.8" and if the difference is half of that, one driver will cancel much of what the other produces. The far mid-bass driver of a MTM center channel speaker of that size, laid horizontally, could easily be 2.4" farther from the mic that the near one.

Our hearing may be more geared for 1/4 octave resolution but if a note's fundamental or first harmonic is at 450Hz, it will be noticed. Phase cancellations are easily seen, even at 1/3 octave resolution and if it's not caused by two drivers that produce the same sound having different distance to the mic or listener's ears, it's either a crossover or first reflection issue.
 
K

Karl W.

Audiophyte
I use Mcintosh centers. Multiple drivers and very wide coverage.
 
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