Pros & Cons of Various Center Channel Designs

[Stanton]

The question is are there compromises (in addition to cost) with an acoustically transparent screen? If holes let sound through, surely they let light through. Secondly some sound gets reflected off the screen, requiring equalization of LCR speakers. I've been leery of acoustically "transparent" screens though I would love to use them with line array speakers.

I agree with you regarding acoustically transparent screens; I see 2 factors:
1) They are more $$
2) The introduction of "dialog lift" in modern AVRs mitigates the issue of placing center channel speakers under the screen.

 

[alex30]

Hi,
I am in a smallish room and use Q Acoustics Concept 20 standmounts as my fronts. They are a two way.
I could use another Concept 20 my display but am not sure if just one speaker is required or two.
Any advice by anyone in the know would be much appreciated.

 

[Swerd]

I am in a smallish room and use Q Acoustics Concept 20 standmounts as my fronts. They are a two way. I could use another Concept 20 my display but am not sure if just one speaker is required or two.

There is no reason why you cannot use another Concept 20 as a center channel speaker.

 

[alex30]

There is no reason why you cannot use another Concept 20 as a center channel speaker.

Hi, Thanks for the input.
I presume that I would be best to put the center up off the floor or maybe even above the TV.
Have you any idea what would be best and how high I would need to put it off the floor if it was under the TV ?
Regards Alex.

 

[Swerd]

I presume that I would be best to put the center up off the floor or maybe even above the TV.
Have you any idea what would be best and how high I would need to put it off the floor if it was under the TV ?

If your left and right speakers are mounted on stands, aim to put the center speaker at a similar height, if possible. Above or below the TV also can work. If the center speaker's location is much higher or lower than the left and right speakers, find a way to tilt it so it points toward the listeners.

As always, practical considerations come before theoretical considerations in these matters. If your location does not meet some theoretical ideal location, it will still work well enough for you to enjoy it.

 

[alex30]

If your left and right speakers are mounted on stands, aim to put the center speaker at a similar height, if possible. Above or below the TV also can work. If the center speaker's location is much higher or lower than the left and right speakers, find a way to tilt it so it points toward the listeners.

As always, practical considerations come before theoretical considerations in these matters. If your location does not meet some theoretical ideal location, it will still work well enough for you to enjoy it.

Hi,
Thanks for the simple to understand but very useful information.
At present I use a 55 inch screen (always considering going bigger but am equipping my first home theater room at present so TV upgrade must come third to an AVR and more speakers including Atmos). I sit about six and a half feet from the screen. Consequently I don't want to put the TV too high.
I will probably put the center under the TV and, as you say, tilt it up towards my head.
Once again, thanks for the input. You have told me all I need to know.

 

[massacre]

what about emp E56Ci center would this perform better than rc 35 klipsch?

 

[Roen]

Youtube

http://forum.ascendacoustics.com/showthread.php?6529-3-RAAL-Towers-or-3-RAAL-Horizon-for-front-LCR&p=57773#post57773

Any response to this as it relates to the Center Channel article?

 

[panteragstk]

http://forum.ascendacoustics.com/showthread.php?6529-3-RAAL-Towers-or-3-RAAL-Horizon-for-front-LCR&p=57773#post57773

Any response to this as it relates to the Center Channel article?

It looks to me that what he is saying is that their center was designed to have wider dispersion than the towers because of what it's job is. Makes sense and I can see why he though the AH article was too general.

We all know though that most companies don't put as much effort into their center designs. It really seems most of them rotate the drivers and make a horizontal box and are done at that point.

Where's @TLS Guy when you need him?

 

[shadyJ]

http://forum.ascendacoustics.com/showthread.php?6529-3-RAAL-Towers-or-3-RAAL-Horizon-for-front-LCR&p=57773#post57773

Any response to this as it relates to the Center Channel article?

He is correct. While there might be some slight lobing with the woofers, that is crossed over at such a low (180 Hz he claims) point that the standing modes below the Schroeder frequency will have a far more detrimental effect on the bass response than lobing. There might be some diffraction issues by placing the mid and tweeter on such a wide baffle, but I would want to measure it before criticizing it on that point. As for it being better than the Sierra tower as a center, I can't say without measuring. I would say if it is better than the Sierra tower as a center, there is no reason why it wouldn't also be better as the front left and right aw well.

 

[TLS Guy]

It looks to me that what he is saying is that their center was designed to have wider dispersion than the towers because of what it's job is. Makes sense and I can see why he though the AH article was too general.

We all know though that most companies don't put as much effort into their center designs. It really seems most of them rotate the drivers and make a horizontal box and are done at that point.

Where's @TLS Guy when you need him?

Center channels are very difficult and should be the most expensive speaker probably. For me they are a difficult challenge.

You want them relatively small and the driver as close as possible to the screen. The side MTM is a mess and I do not like it. So options are a three way with tweeter above mid, and side to side woofers with a low crossover. I don't like passive crossover below 400 Hz. I note B & W have upped their woofer to mid crossovers to 500 Hz. So to make a good job of a three way demands an active design or at least a hybrid. The other options are a coaxial driver or full range driver.

I opted for a coaxial solution with active BSC.

This gives good dispersion over the listening area with good dispersion to 60 degrees off axis +. So the listening area is well covered without exciting too much reflection.

That is my center response on axis and out to 60 degrees. I suspect the slightly lumpy response below 500 Hz is due to reflections from the screen. That is another problem with centers you have a hard reflective surface from the screen just where you don't want it.

One of the big issues is power handling. If you don't have a center, then the center dialog or singer is shared between two speakers. In a discrete multi channel recording then the center has to produce the entire spl. This of course places a big power load on the center speaker. I have had to replace one tweeter in my center, even though the crossover is 2.9 KHz and an electrical roll off of 18 db per octave.

The BBC have been doing some experimental multi channel broadcasts in the last 12 months. They do not have a center channel signal, just right left and surrounds. They are of the view that a center channel in the domestic environment is a detriment. I have to say they may well have a point

My observation of other rigs I listen to is the center integration is sub optimal far more often than not.

 

[yepimonfire]

Center channels are very difficult and should be the most expensive speaker probably. For me they are a difficult challenge.

You want them relatively small and the driver as close as possible to the screen. The side MTM is a mess and I do not like it. So options are a three way with tweeter above mid, and side to side woofers with a low crossover. I don't like passive crossover below 400 Hz. I note B & W have upped their woofer to mid crossovers to 500 Hz. So to make a good job of a three way demands an active design or at least a hybrid. The other options are a coaxial driver or full range driver.

I opted for a coaxial solution with active BSC.

This gives good dispersion over the listening area with good dispersion to 60 degrees off axis +. So the listening area is well covered without exciting too much reflection.

That is my center response on axis and out to 60 degrees. I suspect the slightly lumpy response below 500 Hz is due to reflections from the screen. That is another problem with centers you have a hard reflective surface from the screen just where you don't want it.

One of the big issues is power handling. If you don't have a center, then the center dialog or singer is shared between two speakers. In a discrete multi channel recording then the center has to produce the entire spl. This of course places a big power load on the center speaker. I have had to replace one tweeter in my center, even though the crossover is 2.9 KHz and an electrical roll off of 18 db per octave.

The BBC have been doing some experimental multi channel broadcasts in the last 12 months. They do not have a center channel signal, just right left and surrounds. They are of the view that a center channel in the domestic environment is a detriment. I have to say they may well have a point

My observation of other rigs I listen to is the center integration is sub optimal far more often than not.

I 100% agree on the point that it’s the most important speaker. I cringe when I see setups with monster left and right speakers and a wimpy center channel.

I do not agree that a center channel is a detriment, in fact the opposite. Phantom centers can never match the real thing, and come with response and comb filtering problems that I can perceive easily when switching back and forth between stereo.

I would also agree with the lower crossover point, you want an xover that matches the dispersion of your tweeter, and you want the woofers to be as close to 1/2-1/4 wavelength distance to that tweeter for even off axis coverage.

The center I currently have has an xover of 1800hz. Both woofers are well within the distance previously mentioned, and the tweeter being horn loaded matches the dispersion to the woofers. Horizontally oriented, I have measured decent off axis response out to 14khz @+-45 degrees, with no lobing or notches in response.

Coaxial designs offer near perfect dispersion, and avoid the issue of off axis problems, but they also increase inter modulation distortion, something I find incredibly objectionable.

Why are you of the opinion it should be small (other than practically reasons)?


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[yepimonfire]

I would also add that using a TM design (i.e. identical lcr) is not always the best solution either. Outside of coaxial designs, nearly all two way plus speakers exhibit some issues at the xover point vertically off axis. Since the center must often be placed above or below the screen, which is usually not exactly at ear level, this becomes an issue. Angling the speaker can help, but not all seats will be on axis vertically, that’s one thing a horizontal mtm does help with.


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[Roen]

I would also add that using a TM design (i.e. identical lcr) is not always the best solution either. Outside of coaxial designs, nearly all two way plus speakers exhibit some issues at the xover point vertically off axis. Since the center must often be placed above or below the screen, which is usually not exactly at ear level, this becomes an issue. Angling the speaker can help, but not all seats will be on axis vertically, that’s one thing a horizontal mtm does help with.


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You are saying that a horizonal MTM has better vertical dispersion than vertical TM?

 

[TheWarrior]

You are saying that a horizonal MTM has better vertical dispersion than vertical TM?

The separation between the mid drivers in a 2 way MTM means there will be cancellations of the sounds at or near the frequencies with wavelengths matching the distance between mid range drivers, down to and including, half and quarter wavelengths. This is true of horizontal MTM's as well as vertical, only the dispersion narrows in relation to the driver orientation. Meaning a horizontal MTM will have narrow dispersion on the horizontal axis, which is kind of a problem considering the responsibilities of a center channel. Getting everyone on the same couch to hear the same sounds could be a problem.

 

[yepimonfire]

The separation between the mid drivers in a 2 way MTM means there will be cancellations of the sounds at or near the frequencies with wavelengths matching the distance between mid range drivers, down to and including, half and quarter wavelengths. This is true of horizontal MTM's as well as vertical, only the dispersion narrows in relation to the driver orientation. Meaning a horizontal MTM will have narrow dispersion on the horizontal axis, which is kind of a problem considering the responsibilities of a center channel. Getting everyone on the same couch to hear the same sounds could be a problem.

A low crossover solves much of this issue, no? I’ve measured two center channels off axis, one was crossed over at 2400hz, the other at 1500hz. The one with the lower xover point somehow manages to maintain uniform coverage off axis, while the one crossed at 2400hz show some rough nulls outside of 30 degrees.

I wonder if the use of a horn/waveguide is also responsible for the better off axis coverage. Since the horn controls the the radiation pattern at 1500hz, and effectively increases the area of the radiating surface (the mouth of the horn is only about 2.5” from the center of the woofers), it may reduce some of the issues.
http://www.acousticfrontiers.com/20131129controlled-directivity-speakers-open-up-your-acoustic-treatment-options/

This is a good article explaining the relation of driver diameter, wavelength, and xover frequencies and how they effect off axis response and the interaction between two drivers playing the same frequency.

While you want the distance between the drivers as close as possible, you also want to maintain similar directivity between the two drivers. Ka refers to the circumference divided by wavelength, ideally you want a Ka=2 or less to avoid beaming, since the biggest cause of lobing vertically (or horizontally in the case of a center channel) is narrowing directivity of the woofer in regards to the tweeter. a narrowing radiation from a woofer essentially makes the tweeter and woofer behave as two separate sound sources, with a wide directivity that is similar to the tweeters directivity, the drivers behave as a single radiating source at the xover frequency, avoiding the issues mentioned. A 5.25” driver crossed over at 1500hz has a ka=1.8, perhaps this explains the good off axis performance horizontally.





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[TheWarrior]

A low crossover solves much of this issue, no? I’ve measured two center channels off axis, one was crossed over at 2400hz, the other at 1500hz. The one with the lower xover point somehow manages to maintain uniform coverage off axis, while the one crossed at 2400hz show some rough nulls outside of 30 degrees.

I wonder if the use of a horn/waveguide is also responsible for the better off axis coverage. Since the horn controls the the radiation pattern at 1500hz, and effectively increases the area of the radiating surface (the mouth of the horn is only about 2.5” from the center of the woofers), it may reduce some of the issues.
http://www.acousticfrontiers.com/20131129controlled-directivity-speakers-open-up-your-acoustic-treatment-options/

This is a good article explaining the relation of driver diameter, wavelength, and xover frequencies and how they effect off axis response and the interaction between two drivers playing the same frequency.

While you want the distance between the drivers as close as possible, you also want to maintain similar directivity between the two drivers. Ka refers to the circumference divided by wavelength, ideally you want a Ka=2 or less to avoid beaming, since the biggest cause of lobing vertically (or horizontally in the case of a center channel) is narrowing directivity of the woofer in regards to the tweeter. a narrowing radiation from a woofer essentially makes the tweeter and woofer behave as two separate sound sources, with a wide directivity that is similar to the tweeters directivity, the drivers behave as a single radiating source at the xover frequency, avoiding the issues mentioned. A 5.25” driver crossed over at 1500hz has a ka=1.8, perhaps this explains the good off axis performance horizontally.





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The lower crossover helps for sure. But 1500 hz has a 9 inch wavelength, which an average MTM array could easily fit in to. You'll still have cancellations. An MTM is really just a waste of a mid woofer.

And to show my lack of bias, my RBH WM-30's are MTM's, and one is used as a center. It works well enough in my space but is definitely the weakest element of my whole setup. Which is why I am designing a new

 

[Verdinut]

IMO, the best setup is to use 3 identical speaker cabinets, not too high and wide enough to take the TV. That goes along the suggestion of SONY when they released their first SACDs.

My setup consists of three identical DIY cabinets. Originally, they contained Altec A7 VOTT components. I later replaced the 511B horns with two 5¼ inch mid-woofers and a air motion ribbon tweeter. The 416-8A woofers were replaced by Dayton RSS390HF-4 subs. With bi-amping of the three channels, now I get the full bass actively crossing over at 190 Hz going to the three subs with a response down to below 20 Hz.

 

[yepimonfire]

The lower crossover helps for sure. But 1500 hz has a 9 inch wavelength, which an average MTM array could easily fit in to. You'll still have cancellations. An MTM is really just a waste of a mid woofer.

And to show my lack of bias, my RBH WM-30's are MTM's, and one is used as a center. It works well enough in my space but is definitely the weakest element of my whole setup. Which is why I am designing a new

I wouldn’t say it’s a waste of a midwoofer, maybe just a compromised implementation of it. 60% of the entire soundtrack is anchored to the center, therefore greater demand is placed on the center than any other speaker in your setup. A second woofer gives an extra 3dB of headroom, ensuring it can better handle those demands. My setup consists of two rp 150ms for the left and right, and a rp 250c for the center, because of the extra woofer, my left and right speakers are calibrated at 3dB louder than the center to match. Since the horn loaded tweeter can already play stupidly loud in comparison to the direct radiating woofers and has likely been turned way down with an L-pad, it’s the woofers that have greater demand placed on them. Even with a low xover point, the range from 100hz-1khz is the most demanding for two reasons, for 1, a majority of content falls in this range, and two, it’s generally the point where woofers see their lowest impedance, requiring as much as two to three times the power as the upper midrange and treble. Adding a second woofer ensures the center can cope with those greater demands without distortion.

It wouldn’t necessarily have to be an mtm, a tmm with woofers arranged vertically or horizontally would actually work better. If you’ve got a set of tmm towers, a 2.5/3way mmtmm or a similar is needed to keep up with those towers in a home theater setting. The center channel should be the biggest, baddest speaker in your setup in my opinion, and should have more headroom than even the left and right speakers.

Unfortunately, all speaker designs involve a compromise. The only way to completely avoid off axis issues is to use a coaxial alignment, but then you have to deal with greater inter modulation distortion, most home theaters use LCD displays vs perforated projection screens, so the center has to fit below the tv. This either requires a compromise in size, for example, a small bookshelf paired with large towers, or a compromise in driver orientation. It also depends on the behavior of the mtm speaker off axis, and how far off axis your seating goes. For example, I don’t experience any nulls or cancellations up to 45 degrees off axis with the center I’m using now. All of my main seats are within 25 degrees of center axis, with the worst seat being about 40 degrees off axis, so it would make no difference if I were to use a tm design. An mtm design that only managed to remain uniform out to 30 degrees in a larger room with seats further off axis would be an issue however.


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[shadyJ]

Unfortunately, all speaker designs involve a compromise. The only way to completely avoid off axis issues is to use a coaxial alignment, but then you have to deal with greater inter modulation distortion, most home theaters use LCD displays vs perforated projection screens, so the center has to fit below the tv.

Andrew Jones has written that coaxial speakers do not have substantially more IMD than conventional designs.