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Distance between compression driver / horn and mid

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Teunos View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Teunos Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 8:25am
Originally posted by pfly pfly wrote:

I am not quite sure what will happen when we'll get near edge of cone or horn mouth, so on following text I focus only on center to center distance, which can be assumed to be worst case scenario, also I assume both drivers have same phase response:

In an ideal world you would want two sources to be at least within 1/3 or even 1/4 of wavelength apart. Wavelength in this question would be the crossover frequency, assuming the acoustics crossover to be same as electronic, 1400Hz. That is the frequency where two sources will have most overlap going on. Moving away from that point in either direction, other source will get quieter and thus won't affect the summation that much.

Full 1400Hz wavelength is 24cm. Having center to center spacing of 8cm (1/3 wavelength) will be impossible with typical arragnment of say, 12" cone with 32cm driver diameter and let's say 20cm HF horn. If those two sources were within 1/3 wavelength away from each other, they would sum positively on full 180 degree vertical front field of that loudspeaker. Of course cone beaming and horn dispersion characteristics come to play here but that's not the point right now.

If these sources were put right next to each other, center to center spacing would be 26cm (32/2+20/2). This means that measured at 90 degree off axis vertical, these two sources would emit sound almost exactly wavelength apart and would show some kind of summation behaviour, depending on signal. I don't bother drawing or calculating triangles right now but I assume that somewhere around 40-70 degree off axis vertical they would be half a wavelength off, so that's where you would get phase cancellation at the crossover frequency.

This is somewhat academic with typcal PA loudspeakers that have something like 40-60 degree nominal total vertical dispersion anyway.

Actually I just bothered to inspect your example

12" chassis radius = 16cm
Space between 12" and horn = 8,5cm
Horn height = 24cm, center to lowest point = 12cm
12" to HF center to center distance = 16+8,5+12 = 36,5

With these measurements you will get cancellation at crossover frequency around 20-30 degrees off axis vertical. Time difference between HF and 12" will affect this and so do dispersion behaviour of both sources. it will also be different going down or up the vertical axis.

There might be few brain farts somewhere, I should be sleeping right now and not thinking about these things.
Think you have got the main points listed very clearly.
One sidenote i would make on this post, is that when you get slightly above or below the crossover frequency, especially above the actual crossoverpoint, with typical x-over slopes, the problem will be there as well.
It is not just a single frequency that is affected, but actually a band of frequencies  around the crossoverpoint.
If you make the slopes steeper, the phase changes around the xover point will become much bigger off axis then when the slopes are less steep. The suckout in amplitude off axis where the primary null is located will then become deeper, but over a smaller interval since the slopes are steeper, and indeed the source that is outside of its passband will reproduce less sound already.

So steep slopes, you will have a suckout that is pretty intense, but along a narrow frequency content. With verry slow slopes, you will have suckout that is less intense just outside crossover, but the notch will be much wider and comb filtering can also occur way outside the xover region.
Therefore, both very steep slopes and very slow slopes are not that great. 
18dB slopes is generally sufficient to crossover any driver imaginable.

If you increase the distance between the drivers, the angle at which the cancellation will occur will decrease. With the calculated example of Pfly at 20-30 degrees, this may not sound like a bad thing, but consider that this means that at this angle, the xover frequency and the region around it, will be completely NOT reproduced. (well, with directivity and all, some sound will still be reproduced). If you thus put your tops on your subs and angle them down about 10 degrees to give the front row better coverage, the people halfway around the room will experience a nearly completely missing band of frequencies, deadsmack in the center of the important hearing rangeShocked.

Increasing the distance simply makes the problem bigger as the null shifts closer towards the forward axis. Increasing the xover point again makes it worse as the phase change around crossover will grow faster due to the shorter wavelengths involved.

Now this whole theoretical derivation that Pfly has given, and i support, basically means;
  • cross the lowest possible ( considering electrical, mechanical and distortion limits).
  • keep the distance between drivers as small as possible
  • use around 18dB slopes, and definitely not 48dB.
So yeah, if you can, move the logo.
Best regards,
Teun.
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AndyWave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AndyWave Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 8:25pm
Why don't you put the logo on the middle of the 12" grill. It would act as a high range diffuser. That would help having more identical radiation patterns between drivers trough crossover region.

How many times I have used gaffatape on my guitar cabinets to achieve the same. LOL
torturing electrons since ......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cheif_Stringer_ Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 9:05pm
Thanks for the advice everyone, ive decided im gonna build the seperate hf and low boxes as meantioned before, and test them close together and with abit of distance and see what i make of it, listening to the sound in various areas, if i do add the logo between them i will keep the distance less than i said but yeah im gonna test it, it means buying an amp and processor before finishing these cabs but itll be worth the wait since there is some concern there, if i think theres a big enough difference in sound ill not add the logo, if i notice it but its not a massive deal ill use the logo with the less distance (i could make it 70mm rather than 85), ill be building two cabs at the same time two so i can play with the angles on them too etc.

The idea i have had for the logo will make the midtops look really special imo, im not gonna try too hard to describe it with words but itll be a bold logo thats cnc'd not just like a little one what would look fine anywhere, so again im willing to make a compromise to sound for it, just not a one that defeats the purpose of using such quality components, so yeah thanks to all the comments ill do something i should have wanted to do but wasnt going to and thats do some testing first,

thanks people :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cheif_Stringer_ Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 9:11pm
Btw just while were on the topic am i right in saying that the most ideal place for a hf among to mids would be in the middle of them?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve20131 Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 10:20pm
Lol no 2x12 together is so they couple and help dispersion,  always hf above or to the side if it's a array
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 25 September 2016 at 11:45pm
Originally posted by Steve20131 Steve20131 wrote:

Lol no 2x12 together is so they couple and help dispersion,  always hf above or to the side if it's a array


Well, it depends. Plenty of mid / top / mid boxes out there - both in vertical and horizontal planes. As has been said, dual drivers means they're different distance from HF so cancellations and such will be made worse than having drivers either side of HF and at equal distance.

If HF is crossed sufficiently low, then they will still couple in varying amounts up to crossover frequency so long as they are workig within 1/4 wavelength distance.

Edited by toastyghost - 25 September 2016 at 11:47pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve20131 Quote  Post ReplyReply Direct Link To This Post Posted: 26 September 2016 at 8:54am
But surely for the 12's to be more than 1/2 wave apart means a crossover too low for a 1.4" cd.  If they're chassis to chassis you can squeak 1.5-1.8k without too much honky sound.  

Personally I'd rather wideband mids and crossover distortion than polar problems or going 4 way.  But yes toasty ghost is correct it depends lol
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 26 September 2016 at 2:25pm
Originally posted by Cheif_Stringer_ Cheif_Stringer_ wrote:

Btw just while were on the topic am i right in saying that the most ideal place for a hf among to mids would be in the middle of them?


http://https://en.wikipedia.org/wiki/Midwoofer-tweeter-midwoofer
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Claer Quote  Post ReplyReply Direct Link To This Post Posted: 04 October 2016 at 9:06pm
Originally posted by Cheif_Stringer_ Cheif_Stringer_ wrote:

Again, im willing to make a little compromise to the sound, just not too big of a one since im investing what for me is alot of time and money into this


Please stop saying you're willing to compromise sound for looks, especially if you are investing a lot of time and money.

Sound first

Sound second

Sound third

Then looks

An awesome logo will just make them more memorable as poor sounding speakers
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cheif_Stringer_ Quote  Post ReplyReply Direct Link To This Post Posted: 04 October 2016 at 9:38pm
Well i am willing to compromise 'some' sound quality for looks to 'some' extent, im not willing to make something that sounds bad because i think it looks good either, thats why im going to do some tests before deciding, to find out how much of a difference there is and go from there, im not trying to build the very best sounding system possible within a certain price bracket, and ill no doubt be on here asking about different testing methods or whatever when i have to cross that bridge.

Im not trying to copy their design just have a similar style / placement logo with abit of a different approach but the system i heard that made me wanna build a system with front radiating midtops was the Electrikal Sound System at a festival this year using thier smaller front radiators, and im limited in expereince but i think they sounded pretty f**ken good, would there sound better if the horn was closer? according to answers here yes, but the system wasnt anywhere near bad, at all.


http://www.thebongoclub.co.uk/wp-content/gallery/electrikal-end-of-fringe-13-party-white-belly/Electrikal-48-of-117.jpg

I am investing what for me is alot into it and i appreicate your concern buddy, again, im going to do some tests and that will tell me what happens next, its not something i was gonna do before (stupidly), but until then im just waiting, thanks mate
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 05 October 2016 at 8:23am
Originally posted by Cheif_Stringer_ Cheif_Stringer_ wrote:

Well i am willing to compromise 'some' sound quality for looks to 'some' extent, im not willing to make something that sounds bad


i don't want to sound condescending, but for two pages now people have tried to explain the physics of why your idea is acoustically not viable.
Some of those people are sound reinforcement professionals with years/decades of experience.

THIS:

Originally posted by Claer Claer wrote:


Please stop saying you're willing to compromise sound for looks, especially if you are investing a lot of time and money.

Sound first

Sound second

Sound third

Then looks

An awesome logo will just make them more memorable as poor sounding speakers
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 05 October 2016 at 12:18pm
The Electrikal tops are a rear ported dual 10" box using high end Faital drivers and horn - I did their settings and acoustic centre was lower than the logo and gain / filter placement was adjusted to compensate

Edited by toastyghost - 05 October 2016 at 12:20pm
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