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DJ Monitor using B&C 12HCX76 |
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JonB67 
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Joined: 22 April 2016 Location: UK Status: Offline Points: 1373 |
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 Posted: 28 August 2020 at 8:35am |
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Personally i would retune that as a full range, even with the floor effectively extending the ports its still tuned to give nothing significant lower down. I suspect jo bg is bang on the money with that design's intended use.
Once you're in the business of tuning the port length you may as well modify the design to whatever you need. I would use tube ports that are easier to adjust the length
Id make something more like this: |
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SteveAATW
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Joined: 04 September 2007 Status: Offline Points: 1173 |
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Posted: 02 September 2020 at 12:33am |
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So, I've been messing around with this in WinISD and am coming to the conclusion I should probably stop fiddling and build a few prototypes.
That said, I feel I'm a little out of my depth on a few counts: 1. Tuning frequency, Fb, what should this be for a full range box? Taking this any lower than 50Hz seems to produce excessive port lengths, or when increasing the port CSA to decrease the lengths, port velocities in excess of 20m/s. In most scenarios this will be run with a sub but it would be good to use it standalone as well, does this have any impact on the choice of Fb? 2. Box Volume, is 50l a good starting point? Most designs in this area appear to be around 35-50l for a 12" mid/top box. Reducing the volume increases the port length and decreases the LF response. 3. Does WinISD take driver + port volume into account, or do you need to add those onto the cab volume when building 4. Do I need to be worried about the apparent exceeding XMax at rated power (350W) The pics below are from a 50l box tuned at 55Hz which seems to give the flattest response up to 100Hz   |
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JonB67
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Joined: 22 April 2016 Location: UK Status: Offline Points: 1373 |
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Posted: 02 September 2020 at 7:58am |
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Im not an expert in designing mid tops but the graph looks like you're too big and tuned too low to me, ill have a play in winisd later if i can make time. Id start by looking at the dimensions of the boxes the driver is used in, or any manufacturer recommendations. Id also start at round the 60hz point.
3) nope, got to add driver, ports and handles before calculating box dimensions. 4)yes, but add your high pass filter in and it should be fine. Tickling the xmax line is ok, sailing way past isnt. 5) check your port speed, there's a tab for this, it should be <30m/s to avoid port noise |
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smitske96
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Joined: 16 February 2016 Location: The Netherlands Status: Offline Points: 1039 |
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Posted: 02 September 2020 at 10:07am |
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For that B&C 12" coax, I would go for something like 65 Hz is you want a somewhat fullrange solution.
PS: In your plot, the HPF in place is definately to low, set it to 45 Hz and max excursion should be far less.
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DMorison
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Joined: 14 March 2007 Location: Aberdeen Status: Offline Points: 1617 |
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Posted: 02 September 2020 at 1:32pm |
Re your point 1, this depends a lot on who will be using the box and their expectations. In general, as long as they can hear some bass it should be ok, but you can easily do some experiments for yourself. Playback some typical program material that you think the box will be used for, and add in a high pass filter at various frequencies to see just how high you can cut it off and still hear when the kick is happening. You can do this easily on any digital sound desk these days or in software - Audacity is free and can do it no probs. (Think of this stage as "virtual prototyping" if you want  )As you're finding out, trying to get the LF flat down low in a smaller box leads to a cascade of other compromises, so I'd recommend accepting the highest cutoff (& thus tuning frequency) you can get away with. Remember a monitor (in a live gig setting) isn't first and foremost about pristine hifi sound quality; rather it's about making sure the performer can hear what the need in order to deliver a great performance. Hearing when the kick is happening makes perfect sense for a DJ monitor, feeling the full impact of it, not so much. My hunch is that somewhere in the 60-70Hz range as suggested by smitske96 will turn out to be better than trying to get down into the 50's. HTH, David.
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kipman725
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Joined: 02 September 2020 Location: Warrington Status: Offline Points: 231 |
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Posted: 02 September 2020 at 2:01pm |
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If your using DSP to EQ the box sometimes tuning lower than would deliver the most flat extended pass band is advantageous as although you loose a bit of unequalized bass extension and a bit of efficiency compared to the 'optimal' tuning you gain the ability to EQ the box flat lower in frequency with a loss of efficiency. For example my tops are tuned to 40Hz but the end of the pass band unequalised is around 80Hz, however if I am using them without subs at lower volume levels I can just EQ them flat to 40Hz and have a full range (ish) speaker. If I had tuned them to 70Hz I would extend the passband down the 70Hz but they wouldn't have the ability to play 40Hz with EQ.
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SteveAATW
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Posted: 20 September 2020 at 9:54pm |
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Finally found some time to start building a prototype for this. Ended up with a 50L cab tuned to 60Hz, though will put the port in with just a couple of dominos on the baffle and putty to seal it to the cab sides initially so I have a play with the length after I've measured (I've made a change to the calculated length as the ports are triangular and in the corner).
Couple of things 1/ will the baffle will need bracing or is the cab sufficiently small and rigid to not need it? 2/ do I need a gasket on the mounting flange? Pics of dry fit attached - need to domino the top and rout in the handles and speakon plate before I can glue it up and line it.    |
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SteveAATW
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Posted: 21 September 2020 at 7:58pm |
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One other thing on this - the speakon plate is a metal recessed dish type and relatively large. Do I need to put an MDF backer box on the inside of the cab where the cut out has reduced the density of the cab wall from 18mm of MDF to ~1mm of steel? Ta!
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SteveAATW
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Posted: 10 October 2020 at 10:48pm |
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All assembled!
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SteveAATW
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Posted: 10 October 2020 at 10:57pm |
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First test of the response is below. I've made a few port lengths that can push onto two dominoes sitting on the back of the baffle and be sealed temporarily so I can test the differences in tuning. Cabinet has some polyester wadding in it.
It's driven actively via a DCX2496 and then both HF and LF are running off two channels of a NX4-6000 LF 0dB 44Hz 24dB Butterworth - 1.21kHz 24 dB Butterworth HF -8dB 1.21kHz 24dB Butterworth - 20kHz 24 dB Butterworth Expected tuning at this port length was 60Hz Any thoughts?  |
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SteveAATW
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Posted: 11 October 2020 at 7:51pm |
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Done some more testing today and also had the chance to get it up loud - very impressed by the B&C driver, the intelligibility of both the drums and vocals is very good and it still has enough low end extension to run full range on it's own.
Messed around with the various port lengths as well 64mm, 53mm and finally removing the extensions and just using the 18mm baffle thickness. Also ran a Genelec 8030A I had handy through the same test as a comparison (appreciate running a studio monitor vs. a PA speaker is not a like for like test). Noticing quite a bump at the x-over point - is this down to filter type selection or does it need moving or EQ'ing out? Haven't yet run the drivers independently with the same test.     |
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Jo bg
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Posted: 11 October 2020 at 11:01pm |
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You are using butterworth filters centetred at same frequency. Butterworts are - 3db at that frequency so you get a hump.
Tou can move to linkwitz riley filters wich are -6 at crossover and sum flat to try remove the hump
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SteveAATW wrote: