Port Dementia! A plea for help. - diyAudio
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Old 12th March 2009, 08:05 PM   #1
Beach is offline Beach  United States
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Default Port Dementia! A plea for help.

Can somebody please help me with this?

I'm trying to complete a cabinet for full-range amplification of electric guitar. This will be used in a LOUD live band. I have the cabinet and the speaker, but I am going nuts trying to figure out the port dimensions.

I would be sooo grateful if somebody would provide the port dimensions for me. I can probably use a single port, but I think two smaller ones might fit better.

Cabinet and baffle are 3/4" birch ply. Internal volume of the unloaded box is 2.85 cubic feet. A single Eminence Delta Pro 12a will mount from the inside (I don't know how much the cab volume is reduced by the speaker). No other drivers in the box. I plan to use fiberglass on the interior surfaces.

The speaker specs are below:
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Old 12th March 2009, 11:16 PM   #2
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Guitar cabinets are not hifi cabinets - don't port.
Either go fully sealed if you like more bottom (a la Marshall)
(also more power handling with sealed)
or open back (a la Fender).
Don't put anything on the interior surfaces - those reflections are part of the sound.

Protect your hearing.
The problem with most guitar cabs is that they point to the playerís knees Ė this leads to a volume war with players turning up when they canít hear themselves.
Better to angle the cab back so itís pointing at your ears and keep the volume down.
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 13th March 2009, 01:42 AM   #3
Beach is offline Beach  United States
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Thiele & Small Parameters - Eminence Delta Pro 12a
Resonant Frequency (fs) 51Hz
DC Resistance (Re) 5.71
Coil Inductance (Le) 0.84mH
Mechanical Q (Qms) 7.56
Electromagnetic Q (Qes) 0.37
Total Q (Qts) 0.35
Compliance Equivalent Volume (Vas) 81.7 ltr/2.9 cu. ft.
Peak Diaphragm Displacement Volume (Vd) 242cc
Mechanical Compliance of Suspension (Cms) 0.21mm/N
BL Product (BL) 15.3 T-M
Diaphragm Mass inc. Airload (Mms) 48 grams
Efficiency Bandwidth Product (EBP) 138
Maximum Linear Excursion (Xmax) 4.6mm
Surface Area of Cone (Sd) 532.4cm2
Maximum Mechanical Limit (Xlim) 13.7mm

Thanks!
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Old 13th March 2009, 01:42 AM   #4
Beach is offline Beach  United States
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I appreciate your reply, Pete. However, in this case exactly what I need is flat response and high volume. This cab will be used with a POD X3 Live which digitally emulates guitar amps. These emulations require a good, flat playback system.

Drummer is always too loud and my hearing protection is always in!
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Old 13th March 2009, 02:04 AM   #5
infinia is offline infinia  United States
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Hi

This driver is low Qts and not suitable for open back cabs!
using WinISD
51 liters fb =f-3 =60Hz
vents 2x 3''dia x 4.4" long

edit>
That's a tiny 1.8 cu feet stage monitor size

I ran it with your 80 liter box and the response to tuning was extremely sensitive! So I would recommend very strongly to go with the smaller box and tunings above. You can add an internal shelf to limit the volume. Should go very loud with the smaller configuration tho.
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Old 13th March 2009, 04:25 AM   #6
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Shove an old woolen blanket into the kick drum when he isn't looking, should rob him of 3dB
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Old 13th March 2009, 02:37 PM   #7
Beach is offline Beach  United States
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Thank you, Infinia. The thing is, I already have the box and I'm struggling to get it ready by tonight. What do you mean when you say my box is "extrememely sensitive?" I have an RTA and tone generator, so I'd be able to cut a few port lengths and experiment if I could only get close to the correct dimensions.

Moondog55: I think we'd have to cover the drummer completely with multiple wet blankets to achieve the desired result!
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Old 13th March 2009, 03:10 PM   #8
infinia is offline infinia  United States
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Quote:
Originally posted by Beach
Thank you, Infinia. The thing is, I already have the box and I'm struggling to get it ready by tonight. What do you mean when you say my box is "extrememely sensitive?" I have an RTA and tone generator, so I'd be able to cut a few port lengths and experiment if I could only get close to the correct dimensions.

Moondog55: I think we'd have to cover the drummer completely with multiple wet blankets to achieve the desired result!
Withe bigger box if I tune the ports a few Hz either way, the response is non flat (goes from large peaks to severe drooping) So to get it right would take massive tweeking and lots of time and still change later after the driver warms up a bit.

Probably look at moving the cleats and back panel further in the box by around the depth of the ratio of the volume change.

Time wise for a temp solution you could thro a bunch of styro blocks 1.3 cu ft or so in there. ( 25% more styro volume cause it's not that rigid.) with two new ports?
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Old 13th March 2009, 04:01 PM   #9
infinia is offline infinia  United States
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From the web
This is closer to what you have and if you don't like the bass then you can blame Eminence.
Fine Tune after some break-in time. ReTune the ports to fb = 55Hz ie In box find the minima impedance between the 2 peaks. note the freq is Fb which is related to Lv. Trim Lv to make Fb=55Hz.

Also note the need for the high pass filter and 150 Watt limit.
The smaller cab is not tuned so low to the hilt, so should be louder if doing nothing to protect the woofer excursion.


DeltaPro12 Larger Vented Cab, Med Pwr, Full Range Use
By McJerry, Eminence Speaker LLC
Displacement limited to 150 watts. Must use a 24 dB per octave high pass filter set to 50 Hz or higher
to protect woofer from overexcursion. Very flat response.
Box Properties
--Description--
Name:
Type: Vented Box
Shape: Prism, square (optimum)
--Box Parameters--
Vb = 3 cu.ft
V(total) = 3.139 cu.ft
Fb = 55 Hz
QL = 7
F3 = 51.53 Hz
Fill = minimal
--Vents--
No. of Vents = 2
Vent shape = round
Vent ends = one flush
Dv = 4 in
Lv = 3.006 in
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Old 15th March 2009, 03:37 PM   #10
Beach is offline Beach  United States
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Well, Infinia, thanks for your efforts on my behalf. I guess I'm going to have to go with your idea of stuffing the cabinet to reduce the volume.

I really thought that it would be simple for somebody knowledgeable to calculate the required port area for a given box volume with a specific driver, but I see that none of the free software available on the web has a provision for that. It's their box or no box!

Anywayze, thanks again. The volume reduction idea is very helpful!

thBeach
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