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$250 MTM Horizontally Oriented Measurements

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250-horizontal-rigging.jpgThe first speaker we’ll look at is a very common MTM design, available from a big box retailer where you can reportedly get some best buys. The picture shows the speaker horizontally placed on a tripod with angle markings so I can accurately measure the speaker’s off-axis response. The horn-loaded tweeter crosses over to the dual 5.25 inch midrange drivers at 2400 Hz. This means that while we will see the performance of the tweeter’s off-axis response, we will focus our analysis in the bandwidth of 80 to 2400 Hz.

In the chart below, the frequency response of this MTM center channel was mapped with 1/24 resolution, from 80 to 20,000 Hz in the x axis and from zero to 40 degrees angle in the z axis (front to back). The initial response on axis was normalized so the measurement you see in the chart is the deviation from the on-axis response as the angle of the speaker was changed. You can see a significant change in frequency response in the top octave, but we’re not here to pick on the off-axis response of this speaker’s horn tweeter, as terrible as it is. What we are here to pick on is the frequencies that the two “Ms” are reproducing, from 80 to 2400 Hz, which is shown with the yellow bracket. In this zone the measurements show peaks of over four dB from the additive effect of the two midranges and wave cancellation of over -10 dB.

250-horizontal-chart1.jpg

The 1/24 octave chart above looks pretty messy. However, the peaks and troughs are less audible the narrower they are because your hearing will naturally smooth out the response. Plus or minus a couple dB also isn’t anything to lose sleep about, but what we can see from the chart is that the two midrange drivers exhibit significant wave cancellation from around 800 Hz to where the crossover kicks in at 2400 Hz. Some of the variation near the crossover point is from all three drivers playing the same frequencies. A higher-order crossover would reduce this problem, but just placing the tweeter above the midrange would fix it completely as we’ll see later on.

If we take a slice of one frequency from the 1/24 octave measurements we can plot out the frequency response of the loudspeaker and get a visual representation of lobing. The lobing effect is from the appearance on a polar map of the peaks and valleys of the frequency response. In a real polar graph, it looks like lobes, or flower petals. This chart is a bit simpler, as I only measured an 80 degree angle (assumed to be symmetrical) and am using a more traditional chart.

250-horizontal-chart2.jpg

Conventional wisdom is that people typically can’t differentiate frequency changes finer than 1/3 octave, but I argue that is a bit too course a guideline to follow. In my experience, 1/6 octave smoothing is a more appropriate method to determine audibility of frequency responses. In order to better chart the audible effects of the wave interference, the chart below shows the same measurements but averaged at a 1/6 octave resolution. This chart indicates the significant, and audible, wave cancellation in the upper midrange. When the variation from on-axis response, from 80 to 2400 Hz is calculated using standard deviation, this speaker gets a score of 1.94. We’ll see later how that compares to other types of center channels.

250-horizontal-chart3.jpg

 

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Recent Forum Posts:

TLS Guy posts on October 20, 2009 23:33
irish, post: 637493
Thanks for the recommendations. The Beta 360 would be too large for my application although it does look nice. The KEF Q series, iQ60c, looks like it might work pretty well as it's less than 7“ tall. The speaker cabinet design is a bit different but that isn't a breaking point. How do co-axials differ from in sound or performance from a more ”traditional" design where the speakers are seperate?

If more speakers had a wider bandwidth, then there would be no crossovers and multiple speakers. Multiple speakers are a workaround for a problem, not an inherent advantage.

The point of a coaxial speaker is to keep the sound coherent. What would be ideal is to have a bass/mid cone that crossed over to the tweeter, 4 kHz, then you would avoid a crossover in the speech discrimination band. However no such animal exists at present and crossover to the tweeter in current units is in the neighborhood of 3 kHz.

In a coaxial, the cone of the woofer acts as a wave guide to the tweeter. Things a re designed such that there is usually time coherence. However because a first order crossover is just about never possible, there are phase anomalies at crossover, just like any other speaker. There is symmetrical lobing and therefore the vertical and horizontal axis response is identical. The coverage is therefore conical.

As far as drivers to choose from the most well known are KEF and Tannoy. Thiel also has a coaxial center. Pioneer also have one in their range.

However, after having auditioned KEF recently the SEAS driver is in my view far superior.

You can buy a LOKI kit that is very good value.

I use these drivers in my center speaker. The tweeter is used only in the lower driver, the upper one is an active fill driver and the tweeter not connected.



In this TL, I could not be more happy with it.
irish posts on October 20, 2009 16:54
lsiberian, post: 637476
For If you are interested in a coaxial accessories4less sells KEF speakers for a pretty cheap clip. Still you'd have to like their other offering. I think the best horizontal center I've heard in the budget range is the Beta 360 treated with rockwool and peel-n-seal

Thanks for the recommendations. The Beta 360 would be too large for my application although it does look nice. The KEF Q series, iQ60c, looks like it might work pretty well as it's less than 7“ tall. The speaker cabinet design is a bit different but that isn't a breaking point. How do co-axials differ from in sound or performance from a more ”traditional" design where the speakers are seperate?
lsiberian posts on October 20, 2009 16:12
irish, post: 637446
Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm

For If you are interested in a coaxial accessories4less sells KEF speakers for a pretty cheap clip. Still you'd have to like their other offering. I think the best horizontal center I've heard in the budget range is the Beta 360 treated with rockwool and peel-n-seal
TLS Guy posts on October 20, 2009 15:30
irish, post: 637446
Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm

The paradigm C190 is on the right lines, but I think you would have to go with an all Paradigm system, as they have a definite voicing about them, that I did not care for when I auditioned them, at least the Studio 100s
irish posts on October 20, 2009 15:13
TLS Guy, post: 637438
Yes they would. To make a good horizontal center, you need either a coaxial driver, or a three way with at least the tweeter above the mid, and preferably the mid band/pass crossover point spread 350 Hz to at least 4 kHz, like the B & W.

Thanks for your response! I was pretty sure that was the case but I may have no choice due to my set up. It's a living room/HT set up and acoustically won't be great but it's what we have. I'm still learning and have no idea what the bolded words mean. If I understand correctly when a center is horizontal the tweeter needs to be raised vertically so that it's not in line with the mids…
The stand I have is like this one http://www.avsforum.com/avs-vb/showpost.php?p=15604297&postcount=105 so there isn't room a for a center due to the units being pushed together. I'm also size limited due to using a plasma on it's stand with a 7" clearance from base to screen so bookshelf speakers won't fit.
The best fit from quality mfgs that I've found would be the Def Techs or Paradigm CC-190 which does have vertically aligned tweeters http://paradigm.com/en/paradigm/speaker_only-specification-6-1-3-4.paradigm. Would that be a better option that the Mythos?
Thanks a bunch for helping me out!

These might work too but they're aligned as well http://paradigm.com/en/reference/speaker_only-specification-65-1-3-20.paradigm
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