Horizontal MTM Center channel: avoiding off-axis nulls - diyAudio
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Old 12th June 2009, 01:44 AM   #1
DcibeL is offline DcibeL  Canada
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Default Horizontal MTM Center channel: avoiding off-axis nulls

For the past while I have been researching different speaker arrangements for an affordable HT center channel that overcomes the major drawback in most commercially available center channels: the horizontal MTM.

When an MTM is vertical, off-axis nulls will occur at at +45 degrees, and -45 degrees on the vertical plane, at the crossover frequency. In most listening conditions, these nulls would be aimed at the floor and ceiling and so are not a problem. However, when we make a horizontal MTM for a center channel, these nulls are now at 45 degrees left and right, rather than up and down, meaning horizontal off-axis performance suffers greatly, and moreover the crossover frequency where the response suffers is usually the vocal range of the frequency response where we want the center channel to perform best.

So, how can we make a horizontal speaker without this problem? There are a few options:

-A 3-way with drivers oriented such as Zaph's center channel here. Here, the crossover point between the horizontally arranged woofers and the midrange is low enough that off-axis performance doesn't suffer. The midrange and tweeter are located vertically so that off-axis performance suffers only in the vertical plane. However, moving to a 3-way can add a lot of cost to the speaker.

-A WMTMW. Again, not very cost effective, but in this method we have the ability to move the crossover point of the MTM higher to not be in the vocal range, or at least reduce the effects to the vocal range.

-A 3-way with a coaxial mid-tweeter, such as the Kef Uni-Q. There aren't a lot of good coax drivers available to choose from.

-design a horizontal MTM using a wide-range driver instead of a tweeter. Here, we can use a wide-range driver crossed over low enough so that off-axis response is not a problem. This is the same idea as the Zaph 3-way above, but using a wide-range driver in place of the midwoofer and tweeter. Here, we can keep the driver quantity low, and crossover part count low as well.

I chose the last option, as a cost effective solution is what I am after. So, what drivers to choose? The biggest hurdle here is finding a good wide-range driver that works well from about 500Hz to 20kHz, and is sensitive enough to mate with a couple 6.5" woofers. The sensitivity is the biggest one, as most 3" drivers have very low sensitivity of ~80dB@1W/1m, and I can't use multiple without feeling the need for a tweeter to cover the high end. However, I did find the Fostex FF85K which is 88dB@1W/1m and is highly regarded by the full-range crowd.

88dB is great, and leaves a lot of options for the woofers. The woofers will be crossed over low, so only need to be about 88dB sensitive, and perform well up to about 1.2kHz. No problem, right? I originally had started this design around the Fountek FW168 for it's cost/performance ratio, but then moved to the Peerless 830883 when I found a few cheap ones on the meat-swap (formerly known as the Trading Post). I can post the design for the Fountek as well if there is any interest.

So here's the design. I was a little worried about the wonky response of the FF85K in the datasheet, particularly the rising response in the top octave, and the 4-5dB dip in the midrange. I could not find any 3rd party measurements for this driver to confirm the frequency response, so I left a few tweakable options in the crossover:

-looking at the transfer function, there is a hump in the midrange for the FF85K to compensate for the dip in it's response. This hump can be raised and lowered by adjusting the 2.4 Ohm resistor in series with the 0.62mH inductor.

-the dip in the transfer function in the top octave can be decreased by unwinding the 0.1mH inductor in series with the driver a couple turns at a time. Unwinding this inductor will raise the top octave response.

What we have here is a horizontal MTM where a wide-range driver is used in place of the tweeter. Component count has not increased in comparison to a traditional MTM center channel, and we have overcome horizontal off-axis problems!

Peerless 830883 and FF85K Center Channel:
IMAGE: Simulated Frequency response, phase, transfer function, and impedance
IMAGE: Crossover Schematic
IMAGE: Cabinet Drawing

Notes:
-This is a modeled response using the FRD tools, not measured.

-The 830883 frequency response is traced from Zaph's measurements, and the impedance is traced from the datasheet.

-The FF85K frequency response is traced from the datasheet, and the impedance is traced from Planet10's measurements in the Tysen thread.

-All inductors are Solen 16AWG solid wire air core, except the 2.25mH which is a Sledgehammer 15AWG steel laminate from Madisound.

-if using a different 0.62mH inductor, note the change in DCR and adjust the series resistor accordingly.

-Cabinet is ~30 Litres, tuned to 42Hz. A 3" straight port fits nicely in the middle on the back of the cabinet. If you wish to build this speaker, I recommend downloading Unibox, and choosing your desired port tuning.

-I have not built this speaker yet. I will post any changes I make once I have built and listened to it.
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Old 12th June 2009, 04:10 PM   #2
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Great project idea! I have been thinking of building something very similar for a long time. It makes perfect sense to remove the crossover from the vocal range, and what better way to do it than with a full range driver and supporting woofers.

Looking forward to your results
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Old 12th June 2009, 04:56 PM   #3
DcibeL is offline DcibeL  Canada
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As am I.
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Old 19th June 2009, 07:07 PM   #4
DcibeL is offline DcibeL  Canada
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I'm a bit surprised in the lack of interest this has gathered. Total build cost ends up being $250-$300 using FW168, add ~$50 on that for 830883. I can post the Fountek FW168 crossover as well if there is interest. This speaker can obviously be tipped upright and used as a regular speaker too...

My speakers arrived yesterday, great success! However, I am a little disappointed in the FF85K. The one I received had excessive black goop between the aluminum cap and the cone, and was not very evenly distributed. I'll take a photo later tonight when I'm not at work.
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Old 20th June 2009, 02:39 AM   #5
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Click the image to open in full size.
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Old 20th June 2009, 04:22 AM   #6
CLS is offline CLS  Taiwan
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That looks quite different from the published one:

http://www.fostexinternational.com/d.../pdf/ff85k.pdf
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Old 25th June 2009, 07:22 AM   #7
tresch is offline tresch  United States
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I'm actually working on an identical project, using a Markaudio Alpair5 for the fullrange, and two Dayton DA175s as the woofers.

Click the image to open in full size.

I have it assembled in a prototype box and have done some initial listening using the variable crossovers in my bench amp. Later this week I'll be taking acoustic and impedance measurements in the box to finalize the crossover.

So far, however, it sounds pretty good. Hard to tell in the garage, terrible acoustics. The Alpair5's +10k hz rise is pretty harsh, in that environment anyway. Definitely helps to stand well back from the speaker, which will be the case in the home theater these are being designed for, but I might tame it a bit with a notch filter.

Just as you I'm suprised noone else on this board has really tried this, yet. Or at least I haven't stumbled across that particular post
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Old 26th June 2009, 03:44 PM   #8
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Nice to see! Regarding your speaker, you will probably have smoother response by moving the Alpair driver off-center a bit, and round-over the baffle edge. The round-over is probably out of the question for your cabinet, as you have screws lining the edge...A cheap metal cone driver is a good option for bass duty, since the crossover should be low enough that you won't have to worry about the cone breakup.

If all goes as planned I'll be building my cabinet early August during my vacation time.

I think that for the most part this type of speaker has limited options for wide-range driver choice, as the sensitivity has to be high enough to work with the bass drivers. A regular 2-way might have some more flexibility as the sensitivity will be lower. Another option would be to use an active crossover, in which case sensitivity matching is not a concern, just need a large amp to make loud happen.
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Old 27th June 2009, 08:40 AM   #9
tresch is offline tresch  United States
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this is just a prototype box, with no concerns put into aesthetics, only build speed

Attached is a pic of a previous finished single-driver I did. (the blemish on the edge is just a little glue, it came off). These will be of a similar design, when finished.

On these I'm going to experiment with a "staged" roundover, with a truncated roundover like on the speaker in the attached picture around most of the speaker, and then transition to a full flush half-round near the mid-tweet.

As far as sensitivity goes, the DA175 and the Alpair5 both have a rated sensitivity of 85 SPL 1W/1M, but the DA175 is an 8 ohm speaker where the Alpair5 is a 4 ohm, so the amp delivers more power to the Alpair, relatively acting as if it were more sensitive. The overall speaker won't, unfortunately, be really sensitive, but it should be able to handle a decent amount of juice.

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Old 27th June 2009, 06:45 PM   #10
DcibeL is offline DcibeL  Canada
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85dB@1W/1m is a little low for an MTM, but really isn't that bad as a lot of 2-way designs end up around this level.

Sounds like you've got things well underway, good luck with your project!
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