# CTC spacing/comb filtering



## egoaudio (Feb 26, 2008)

Hi guys,

Quick question as I have been searching for the answer for a bit but cannot find it. 

I'm building some three-way towers, this configuration, WmmtmmW. Actively processed and I need to know about CTC spacing. So you take the CTC distance and find the wavelength of that distance. Do you want to HP vs. LP x-over above or below this? What's safe? Do I only take CTC spacing into consideration when dealing with tweets and mids (I'm guessing no)?

This seems like info I need to know and many people talk about it like it's second nature but it's not to me yet. 

I appreciate any help. 

Best regards,

Erik


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## cshaw (Oct 7, 2008)

i tried to post but it had to be reviewed for some reason. i guess spam ftl


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## MiniVanMan (Jun 28, 2005)

This will hopefully address cshaw's thread as well.

Okay, yeah, comb filtering will occur when the wavelength is smaller than the distance between the two speakers. So, you want to cross below that point. 

Now here's the barb, and a very challenging aspect of loudspeaker design....

How do you get high sensitivity, while also avoiding comb filtering? Since I see you're trying to do a WmmtmmW configuration, I'm going to assume that you're looking to match sensitivities. Let's say that you go with a 2" driver that measures about 2.5" in diameter. That would give you 7.5" CtC between the outer two midranges, WITHOUT a tweeter. So, you're crossover point would need to be under 1800 hz to avoid any combing. Now add in a tweeter and you have an even lower number. Obviously, the larger the driver, the more combing you're going to get. 

Dome midranges are nice because of their sensitivity. They're utilization is high sensitivity systems is much easier. However, there are many, many viable Line Arrays out there. So, there's some debate on how much impact comb filtering has on your sound field. 

It's a good question though. Welcome to loudspeaker design.


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## cshaw (Oct 7, 2008)

this is what i was going to post earlier. something i found related to line arrays. might help along with minivanmans response.

Center-to-center Driver Separation (Circular drivers). We want our discrete driver 
array to approximate a continuous line source. This spacing is the separation between 
the centers of the adjacent drivers in the line and includes any mounting allowances and the flanges surrounding the drivers. In the limit the closest spacing would be dictated by the flange diameters of the drivers although some drivers have truncated flanges that would allow closer spacing. Two different solutions (Table I) for the driver separation guidelines are presented in the literature for circular drivers. These cases are: 

1. Far Field. Ureda [3] uses driver directivity to determine that circular drivers need 
to be positioned within one wavelength center-to-center at their highest operating 
frequency. Wavelength is equal to the velocity of sound (344 m/s or 1130 feet/s) 
divided by the frequency. Directivity of the multiple drivers in the line increases 
until one wavelength spacing is reached and starts to decrease beyond this 
spacing. Figure 7 illustrates how the sound wavefront is created by a line array. 
Spacing less than one wavelength creates a constant phase front but comb lines 
start to form beyond one wavelength separation. At two wavelengths separation 
the first cancellation occurs. Directivity continues to decrease with more severe 
comb line effects as the spacing increases beyond two wavelengths. 

2. Near field. Urban, et al [1] derives a more restrictive criterion of no more than a 
half wavelength separation between drivers at their highest operating frequency. 
Fresnel analysis is used and a disruption grid is used to shutter a continuous line 
source in their work. This analysis is based upon their desire to place any far 
field dips (nulls) in the angle off axis response of the array beyond π/2 (90 
degrees). This assures that secondary (off-axis) lobes in the sound field are 
greater than 12 dB down from the on-axis response (main lobe). 

found this half way down at-
http://www.audioroundtable.com/misc/nflawp.pdf


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## egoaudio (Feb 26, 2008)

Much thanks to all who have replied. I was looking for those “white papers” from Griffin but couldn’t find them last night… dead links and such. Thanks for that. I quickly scanned the paper and it looks like great info. Here’s a quick drawing of what I want to build. So……I’ve determined that the crossover point between the mids and tweet should be in the 3k range due the CTC spacing. Is this ok? I was hoping to cross these higher (around 4k-6k) so i could get more extension from the 4 mids. Heck, I’ve got 2 DCX2496’s….. why am I whining? I can just try tons of different crossover points and see where it sounds the best to me. I guess I just wanted to have an educated guess at where the best crossover point is. Any input would be great. 

Best regards,

Erik


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## MiniVanMan (Jun 28, 2005)

Okay, CtC spacing between your woofer, and your midrange is not necessarily the problem. Comb filtering occurs between different drivers playing the same frequency. So, the drivers you need to be concerned with are your midranges. Your midranges are where you're going to get combing. You'll also exerience some significant lobing due to the large format tweeter. CtC spacing between the two inner midranges are going to create lobes, unless you keep the crossover point low. 

In any case, combing and lobing are going to unavoidable. However, I'm not sure it's going to be significant enough, given you find a suitable listening position.


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## egoaudio (Feb 26, 2008)

MiniVanMan said:


> Okay, CtC spacing between your woofer, and your midrange is not necessarily the problem. Comb filtering occurs between different drivers playing the same frequency. So, the drivers you need to be concerned with are your midranges. Your midranges are where you're going to get combing. You'll also exerience some significant lobing due to the large format tweeter. CtC spacing between the two inner midranges are going to create lobes, unless you keep the crossover point low.
> 
> In any case, combing and lobing are going to unavoidable. However, I'm not sure it's going to be significant enough, given you find a suitable listening position.


Thanks for your input Mini. I guess this is one of those times where you just have to say: Build it and find out! Off I go then. I'll post pics when I get'em rockin'.

Best regards,

EGO


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## MiniVanMan (Jun 28, 2005)

In all honesty, it'll probably be fine. However, knowing, before hand, what problems will arise is pretty crucial to a good outcome. So, you're on the right track. Minimize the things you can't avoid, and avoid the things you can avoid. That's the best you can do.

I'm assuming, you're not going for "Reference Level" with this design, but rather, high output. 

Be prepared for some unusual baffle diffraction with that odd shaped baffle. Be prepared to set each set of midranges to different high pass crossover points. When I say "set" I mean, outer, and inner. The woofers might avoid baffle diffraction if you cross low enough, but it may exist there as well.


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## egoaudio (Feb 26, 2008)

MiniVanMan said:


> In all honesty, it'll probably be fine. However, knowing, before hand, what problems will arise is pretty crucial to a good outcome. So, you're on the right track. _Minimize the things you can't avoid, and avoid the things you can avoid_. That's the best you can do.
> 
> I'm assuming, you're not going for "Reference Level" with this design, but rather, high output.
> 
> Be prepared for some unusual baffle diffraction with that odd shaped baffle. Be prepared to set each set of midranges to different high pass crossover points. When I say "set" I mean, outer, and inner. The woofers might avoid baffle diffraction if you cross low enough, but it may exist there as well.


Well said. You’re right; I’m not going for reference level just as nice as I can get with this layout.

I'll keep what you said in mind about the different crossover setting. 

Thanks again. 

EGO


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