DIY 3015 cab questions
I have a few questions about port tuning. First, can port tuning be done before putting in the damping insulation? Does damping effect the tuning? Second, is there a way to check port tuning besides the salt test, say on a rear-loaded cab with the grill clothe on?
Finally what is best to use for speaker wire? gage, material, brand? etc.
1. You ought to model your cab in WinISD instead of tuning your ports the hard way.
2. Damping minimally affects internal volume and tuning. Think of damping foam/insulation as honeycomb; it appears large, but in reality has empty space in it. It takes up a fraction of what a solid, space-less version of itself would take.
3. I don't even know what the salt test is, can't help ya there.
As for wire, I bought a 410 that had simple solid-core copper wire inside it (probably 16, 14, or 12 gauge), and I've also seen cabs with huge speaker wire meant for 4000w car power amps, ie 2 gauge. Really, it's up to the amount of power you're putting through the cab.
EDIT: see this page for amperage appropriate for each wire gauge.
Find Johnk_10 thread of his Starliner cab. Fine tuning for 3015 is discussed.
There are plenty of other threads, no need to reinvent a wheel, Eminence even publishes verified tunings, or do you have unusual goals?
No reason not to verify with salt. Prop up cab, don't put the grill on.
Take a look at the tinsel wire that connects the terminals on your speaker to the voice coil.
These are the small silver looking wires. Note the small gauge.
These small leads are sufficient to handle the power rating of your speaker, over that short distance.
The use of larger wire is to reduce voltage drop over distance.
This is the distance between your amp and cab.
I would use uncooked rice kernels, not salt.
Easier to clean up, and much less tendency to get stuck in the cone that is salt.
The tuning frequency for a vented box, is the point where they jump the least.
Foam insulation, i.e. mattress topping burns quite well.
I suspect mattress topper affects (reduces) internal volume, but have not specifically tested for it.
The intent of the damping material is for high frequency only.
It has no effect on on LF, and a whole lot of effect on HF.
An inch of lining, fiberglass bat or similar, on all internal surfaces will do just fine.
Keep your port 1 diameter distance from any surface and you will be fine.
That's why bracing has the same effect as dampening, no?
Also, doesn't dampening need to be placed on half of the walls, as in, for each wall that faces another, one of the two is covered?
All stuff I read here (tb), I just want to make sure i'm not entirely mistaken...
The addition of damping material to a cabinet lowers the tuning frequency; or put another way, it reduces the port length required for a given tuning frequency. Some modeling programs take it into account, and some do not. An attempt to model the effects of damping material on the tuning frequency will be an approximation at best, but imo it's a good idea to take as much into account as you can even if it's an approximation.
Compared with a bare-wall cab, lining the walls with 1" convoluted foam will probably drop the tuning frequency by ballpark 2-3%, or allow a reduction in port length of ballpark 6-10%.
Boom is the result of a too-high Qtc caused by running the driver in a too-small internal volume.
Hollow-core polyester fiber fill controls boom by reducing the Qtc by making the box acoustically 'larger' by Stuffing the box.
BP102 Sealed Qtc Comparison
This chart shows the Eminence BP102-8 Qtc in various sealed box volumes.
Note how the Qtc increases as the box volume gets smaller.
The Qtc=1.5 has a noticeable boom in the 150 Hz range.
The intent of damping material is to absorb short wave lengths that would otherwise cancel internally after being reflected from an opposing wall.
The Allison Effect is a cancellation (suckout) that occurs from a 180 degree out-of-phase reflection from a wall.
(Allison, Roy: "The Influence of Room Boundaries on Loudspeaker Power Output." JAES, June, 1974)
For example, you have a cab that is 11.5" deep from the speaker cone to the closest wall.
11.5" is one-quarter of a 3.83 foot wave length, or (1128/3.83) = 294.5 Hz.
This frequency will cancel due to the Allison effect.
Reflections bouncing around inside the cab will vary in wave length depending upon where they reflect in the cabinet.
Example of undamped cabinet
Notice the nasty response pattern in this raw sweep of an unlined cabinet.
Bracing prevents panel vibration (buzzing).
Acoustic power is wasted when expended on making the panels buzz.
When all reflective surfaces are covered, more control is exerted over high frequency reflections.
Thank you to everyone that replied.
So, I guess there is not another way then the sugar/salt/ rice test? Really don't want to take the cab apart. (It's screwed together) The rice idea was a great one.
The other answer seems to be that the denim insulation I'm using will effect the tuning so I'll have to put it in before testing.
I'm using the 6.86" length suggested by Eminence (+1" for the Precision Port flares) but my ports are 18mm closer to the sides and bottom then the 4" diameter so the ports may have to be a little shorter.
Does anyone have any idea how much?
Vb =2.483 cu.ft
V(total) =2.825 cu.ft (Mine may be a little bigger, say: 2.830 cu.ft. How much difference would that small volume really make?)
Fb =49.42 Hz
QL = 7
F3 = 64.54 Hz
Fill = 2" denim insulation (squashed to 1" by the packaging) on every side but the baffle.
No. of Vents = 2
Vent shape = round
Vent ends = one flush
Dv = 4 in
Lv = 6.86 in
Duke what type of wire do you use?
i know nothing about this. Could you tell me more?
To go the computer route, you typically just need a decent soundcard and a simple jig using a few cheap resistors. I've been using an old orphaned bit of software, but I think Room EQ Wizard will also do the job, and it is freeware. Parts Express sells a whole all in one solution too, and there are plenty of more expensive ones out there as well.
I highly recommend going to Diyaudio.com and/or the Parts Express tech forum for more thorough discussion of methodology. ;)
The True RTA free version has a nice signal generator built in.
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