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DJ Monitor using B&C 12HCX76

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SteveAATW View Drop Down
Young Croc
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    Posted: 25 August 2020 at 3:33am
So my question on an active mid/top for DJ monitor use in the newbie forum got me thinking. I'm probably going to buy a Yamaha DZR for that purpose still anyway but given lockdown, owning a tracksaw, router etc and not having much else to do I fancied a project to see what a coax monitor could do in this role.

B&C provide plans for a coaxial wedge using the 12HCX76.


Is this tuned slightly higher than might be ideal for a dance music monitor? Or is it giving everything the 12" has low down? I was thinking of putting a 15" sub underneath it anyway but just wondering if this wedge plan is a good starting point or whether to look at the design from scratch.


Second question (and forgive the potential ignorance here). In terms of the tuning, do I understand correctly that the main component is the port size in relation to the cabinet volume? So if the volume remained the same and the port size remained the same but the cabinet shape was changed the tuning should in theory be the same?

The only other thing that struck me was the ports are just cut outs in the front baffle of the cabinet, there's no extension to them. From memory believe the baffle on our old Res 1's was similar but just wondered if this was simply how the ports work at this frequency range or something else that I'm missing (I've only built subs before).

This will be driven 2 way active.
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csg View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote csg Quote  Post ReplyReply Direct Link To This Post Posted: 25 August 2020 at 6:54am
Yes, in broad terms if cabinet volume and port dimensions remain constant, but cabinet proportions change the tuning will remain constant. In practice the cabinet may sound different due to the effects of internal reflection, panel resonance and other energy transmission/reflection effects.

A port that is simply a cut out on the baffle still has depth, the thickness of the material. Provided that air velocity remains acceptable then this is perfectly valid, reduces construction costs and complexity etc. If air velocity is too high, port cross section can be increased by also increasing depth whilst tuning remains constant.
“The fact is this is about identifying what we do best and finding more ways of doing less of it better”
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SteveAATW View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SteveAATW Quote  Post ReplyReply Direct Link To This Post Posted: 26 August 2020 at 3:48am
So, I took the B&C monitor plan and figured out it's volume as 37.4 litres

Putting the driver and cabinet into WinISD though, I get port lengths that don't connect with the B&C reference design - WinISD is suggesting a vent length of 15cm whereas in the plans it is the depth of the baffle - 15mm

Am I missing something?
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smoore View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote smoore Quote  Post ReplyReply Direct Link To This Post Posted: 26 August 2020 at 2:45pm
Maybe your vent dimensions are out by a factor of 10?
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SteveAATW View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SteveAATW Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 4:43am
I thought that but HornResp and http://www.mh-audio.nl/Calculators/ReflexBoxCalculator.html both give similar figures. The tuning frequency could affect it but I've tried everything from 80Hz down and nothing gets close to the a 15mm port length

I've found this thread which mentions triangular ports being reduced by ~10% from what is calculated to account for the position of the port, but not a 10x difference

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JonB67 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JonB67 Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 7:34am
Possibly a typo?

Have you tried simming it with the same volume but a port of 15mm to see what it does to the tuning?
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SteveAATW View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SteveAATW Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 1:34pm
Good idea - for a port length of 15mm its tuning frequency is 106.85Hz, which seems a little high? Unless B&C have tuned it primarily as a vocal and instrument monitor?

Accounting for a 10% difference in port length due to triangular ports brings the tuning down to 105Hz






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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 1:52pm
Port position in the corner and enclosure shape , expanding from the port, and floor postioning will make it acoustically longer than measured.

Still not a full range box by far with 60hz at -12dBs. You will need some subs to get a proper dj monitor. But subs are also excellent stands!

Look also at 18sound monitor enclosure and at the "smithers wedge" plans online.
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SteveAATW View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SteveAATW Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 3:19pm
I guess that is my ultimate question - where should it, as a 12" driver, be tuned to for this application? Driver params are below



I just took the B&C design as a starting point, expecting they would have tuned it lower, but I will be building it as a more conventional mid/top style box with a separate (probably single or twin 15" bass reflex) sub-underneath so I can start from scratch on the tuning.

I did spot the smithers wedge when I was looking into this, however, in attempting to register on soundforums despite trying two separate email addresses the activation email never arrives so can't activate the account to download the smithers wedge plans Disapprove Will have a look for the 18sound one now - thanks for the pointer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 9:40pm
uploads/20832/Smithers_WedgeConstruction2_1.pdf

Link should work.
While a traditional wedge was more aimed at piercing tru the mix in the midrange for voices and instruments to be heard by performers, a dj monitor needs to be more fullrange, you don't need to feel the 30hz wobbles but you need the boom and the tiz for mixing. 
Classic wedges rely also on being on floor for 2pi loading, this rounds the bass, if you plan to use this at ear height this has to be compensated.
So if you need to use it standalone for dj monitors  you need to tune lower than regular wedges, and loose some efficency. 
Using separate subs will allow to compromise less on the midtops and put less strain on them, so they will sound better at same level.

Crossover will play a crucial role, you need a specific one, or dsp and amplifiers for every channel. Proper designed filters, either dsp or passive, will result in better feedback control, this is a monitor after all.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jo bg Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 9:45pm
As simulating ports is always a little tricky for enclosure efgects, build in a way that you can adjust ports afterwards for fine tuning.
Either build longer and cut or start short and add pieces until you are happy. 

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SteveAATW View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SteveAATW Quote  Post ReplyReply Direct Link To This Post Posted: 27 August 2020 at 10:22pm
Thanks - appreciate the info + advice.

Any guide point on where to start with the tuning frequency? Adding is probably going to be easier - especially with triangular corner ports - can just take the driver out and glue, pocket screw and clamp more sections on
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