8 Inch Coaixial Build

Mark Oakley

Mark Oakley

Freshman
Apr 12, 2013
30
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Bolton, Ontario
www.oakleyaudio.ca
Hi Folks:

I'm considering building 4 8" coax boxes to use as front fills. I'll be running them off one channel of a Powersoft M50Q-DSP+Eth (1250 w, 4 ohm/ch. ) so I'm looking at making them 16 ohms. Fulcrum Acoustics makes the exact box I'm looking for (the "P"), but the price tag is a little high. I have no doubt it's an excellent box, but I've done a few DIY projects over he years (cases and wedge monitors) and don't mind earning some knowledge by doing it myself.

B&C used to make a 16 ohm version of their 8CXN51; I'm not sure if it's still available. I'm also considering buying a design program like BassBox pro to help with proper cabinet volume and porting, although with a 8" front-fill I'm not too concerned with bass response. Any advice on do's or don'ts?

-Mark
 
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First, here are some older threads with loads of good information and ideas:
https://soundforums.net/forum/low-earth-orbit/diy-audio/3837-front-fill-idea-as-diy-project
https://soundforums.net/forum/low-earth-orbit/diy-audio/6855-s-fill-front-fill-cabinet

Second, some general guidelines for a front fill are:
1. No more than 9" tall at the most.
2. High output above 250Hz or so - the exact point is pretty arbitrary, but the reality is that you're going to high pass them higher than 100Hz, so tuning them for low frequency output or extension is not idea.
3. Like you said, daisy chaining is a must. I see a lot of 2 unit daisy chains, but if you do it right you should definitely be able to do it with three or four, no problem. I just saw d&b E3s used as front fills the other day. Not super high output, but they worked wonderfully daisy chained together, and setup was super quick and easy!
 
Hi Max:

Thanks for the links; that's exactly the kind of info I'm looking for. I'll probably make them out of 1/2" baltic, as I want to keep them as low profile as possible. Also the idea of a coax-point source appeals to me, rather than trying to build a double 6.5 +1". They will mainly be used for vocal intelligibility in the first few rows, as instruments are usually loud enough off the stage. I'll post some pictures when I get them off the ground.

-thanks, Mark
 
I built four double 6 X 1 boxes with the horn between the 6's. I can rotate the horn depending on how the box is going to be used. They can lay flat horizontal or I can flip them over on the angled side for small stage monitors and I put a pole socket in the end to use them for small speakers on sticks events. Those work well for me and get lots of use.
 
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Mark Oakley said:
Hi Folks:

I'm considering building 4 8" coax boxes to use as front fills. I'll be running them off one channel of a Powersoft M50Q-DSP+Eth (1250 w, 4 ohm/ch. ) so I'm looking at making them 16 ohms. Fulcrum Acoustics makes the exact box I'm looking for (the "P"), but the price tag is a little high. I have no doubt it's an excellent box, but I've done a few DIY projects over he years (cases and wedge monitors) and don't mind earning some knowledge by doing it myself.

B&C used to make a 16 ohm version of their 8CXN51; I'm not sure if it's still available. I'm also considering buying a design program like BassBox pro to help with proper cabinet volume and porting, although with a 8" front-fill I'm not too concerned with bass response. Any advice on do's or don'ts?

-Mark
Click to expand...

I think a coaxial 6” or 8” would make a great front fill. Do you have a passive crossover design for that driver?
Building the correct crossover and getting the amplitude and phase correct is not easy.

Here are a couple of 8" + 1" designs (not coaxial)
http://www.eighteensound.com/Portals..._8%202ways.pdf
http://www.bcspeakers.com/media/W1si...2ZpbGUiXV0.pdf
 
Hi Peter:

Thanks for the plans. I don't have a crossover yet, but a "pro" sound company that I work with has a head technician that is familiar with crossover design, and had told me to bring him the boxes (with drivers) once built, and he can design and build a suitable crossover. Correct amplitude is a matter of getting a flat magnitude response between the low and hi drivers, is it not? Correcting phase with a passive crossover is going to be much more difficult. I'm hoping that by starting with a coax driver my phase (at least through the crossover region) will be good to start with.

I'm also looking into a box design program to help with proper box dimensions and port design. I took the SMAART course and have been using the program for two years now, so I can make basic measurements and verify how well the drivers are working together. Also the M50Q-DSP has a full suite of tools (including all-pass filters) which I'll be using to tweak the results.

-thanks, Mark
 
I just started a similar project: 4x 5" coax mini monitors / infills. I'm using the Beyma 5CX200Nd with a simple, passive crossover at 3kHz (6dB/oct LP on the 5" and 18dB/oct HP on the 1"). I have to see how the time alignment will work out... I'm also using the M50Q with dsp (2x). So, amplitude and phase reponse corrections can be performed by the dsp, but time alignment must be approx ok with the passive x-over. It will be a closed box design BTW. I don't care about low extension (< 200 Hz) and rather have more smooth phase response.

My plan is to mount them using Manfrotto 035 Super Clamps. This should allow me to mount them on mic stands for brass musicians etc. At least, that is the plan... ;-) [IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/uploads.tapatalk-cdn.com\/20170419\/dfdb67f3a86d42474b13d6f8c53476f9.jpg"}[/IMG2]



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Hardware (almost) done. As expected the filter needs some work :)

4bcc38cef9c7eeef4a92bc1ed68aed87.jpg


fa780ef5af69170825d8dfe605f44515.jpg




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I don't have a scale to weigh them, but the mic stand is quite stable. The weight is within the base, so it even helps to keep the stand on its feet.

You don't want to see my measurements yet ;-) There is a deep dip at 4kHz due to cancellation: the low driver falls off too slow. So, I have to adapt the passive x-over.

If you would use the Beyma fd-2cx x-overs, you would get the response from the Beyma documentation.

The sound is very good (except for the 4kHz dip). ;-)


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Mark -

I'd suggest biamping the coax drivers rather than building a passive crossover. I've recently put something together with a Eminence Beta 12 CX and a Dayton CD. With DSP, you can use a 4th order XO, and achieve a flat phase in the XO region. Assuming you have a working measurement system (I use freeware REW), tweaking DSP just seems far easier and more flexible than building a passive XO. With passive, you'll either have a phase flip in the XO region with 2nd order filters, or you'll need a slew of parts (cost) to build a 4th order passive XO. Too much trouble... Another channel of M50q is likely overkill for the CD's -- not sure if you have something else suitable on hand. Also not sure how snug you are on rack space. My two cents.

- Robert
 
Hi Robert:

Your two cents are most welcome.

I'm totally sold on the power of active DSP (I use a Xilica for Mains and have the Powersofts). However, I'm trying to build something that is simple to implement on a gig and can be run off one spare amp channel. I use SMAART 7.5 and try to measure and optimize before every show, which (depending on time) usually works out to about 80% of them.

At the moment, my (passive) monitor rack is a pair of M50Q DSP+Eth's. I've been thinking of adding a pair of M28Q DSP+Eth's for Hi's and bi-amping all my wedges, which would certainly give me more control. The problem with running Hi's and Lows from separate DSP amps is that any front-fill alignment delay needs to be applied separately to each passband. Instead of spending the money on extra amps to bi-amp the monitors, it might be smarter to invest in 4-8 IEM sets, or even a couple of small mixer or headphone amps, and trying to get most of the musicians I work with to bring ear-buds.

I'm hoping that by using coax speakers, the phase alignment through the crossover region will be close. If the pattern flips, I could try reversing polarity on the Hi's or working with All-Pass filters. I may also spring for 4th order filters on the crossover-there's no downside to doing things properly.

-Mark
 
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The dsp in the M50Q has sufficient all-pass filters to flatten the phase of a 2nd order passive x-over. There will be even enough eq to flatten the amplitude response as well. So, you only need to make sure that the x-over is ok-ish: it does not need to be perfect. Mine is currently not ok, because I have an almost full cancellation at 4kHz, so an eq won't help. ;-)


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Your 4kHz dip may be from enclosure geometry and not crossover problems. The dimensions of your box might be around 5/4 wavelength causing a null from internal reflection. Have you measured the driver free-air to see if the dip is indeed from the crossover?
 
Igor Pinski said:
Your 4kHz dip may be from enclosure geometry and not crossover problems. The dimensions of your box might be around 5/4 wavelength causing a null from internal reflection. Have you measured the driver free-air to see if the dip is indeed from the crossover?
Click to expand...

4kHz is above the x-over frequency and the high driver is isolated from the box volume.
The low pass on the low driver is not steep enough, so there is still interference at 4kHz, but I do not expect this to be due to box dimensions: if the low driver would get full cancellation at 4kHz, the high driver should still produce output.

But indeed, good idea to check it in free-air. Could be a combination of 2 effects...


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Peter van Gils said:
4kHz is above the x-over frequency and the high driver is isolated from the box volume.
The low pass on the low driver is not steep enough, so there is still interference at 4kHz, but I do not expect this to be due to box dimensions: if the low driver would get full cancellation at 4kHz, the high driver should still produce output.

But indeed, good idea to check it in free-air. Could be a combination of 2 effects...


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Click to expand...

Peter,

FWIW Beyma's crossover is 24 dB Low pass and 24 dB Hi pass @ 2K6. The HF is attenuated about 5.5 dB and there is also a little bit of roll-off from 10Khz down to 3Khz ... about 2 dB. I suspect this would fix your problem :)~:)~:smile:
 
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Okay... after some experiments with a passive x-over my conclusion is that you need at least 24 dB/oct slopes for a smooth response (on- and off-axis). With a lower slope (12dB/oct) you can only get a flat respone or on-axis, or off-axis (not both): you can not overcome the time delay between the high and low driver, and at the same time keep the phase the same for both drivers...

The Beyma filters won't fit in my boxes. So, I will stick with a somewhat less than optimal response. I will post measurement results later. :)


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