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Joespride
03-26-2011, 02:44 AM
I'm new to building speakers and alot of what I read seems Greek to me. I have 20 years experience with woodworking. I'd like to get a reccomended volume size in lay terms (ie... I need X amount of internal volume in cubic feet or inches ) for 2 different boxes I want to build. The first box will be for midbass and will house 2 altec 421-8lf drivers, the second box will be for low end bass I'd like it to go down into the mid 20 hz range and the driver will be the 18" eminence bass driver or jbl equilivant (depends on what I can find ) I don't have all the specs of the drivers (this is the Greek part for me) I would appreciate any help. I also don't have access to a computer most of the time I rely on my iPhone so web site reccomendations are welcome but not always practicle. Thanks in advance. Joe

Lee in Montreal
03-26-2011, 05:44 AM
Hi Joe

Make the bass cabinet as big as you can. 7 cubic feet up to...
Include a port to increase the low reach. Port can be round or square. Port volume is substracted from enclosure volume. So is driver's volume.

http://www.psp-inc.com/tools.html

Plan here for a 25Hz sub.
http://www.audioheritage.org/vbulletin/showthread.php?24178-2245-Subwoofer-Box-12-Cubic-Ft&p=242923

Port area is related to cone surface and excursion. That's why JBL's B460 had three 4" ports. You can use one 6" cardboard duct. Personally, I like using ABS tubing and finish off the surface with a router. You can also make a square port.

50597

A simple construction if you want a very low but narrow frequency band is to build a tapped horn (or "air coupler" according to JBL's terminology). A simple tube or box, that can be folded in two. You can easily reach 20Hz if your driver can do it (the 2245h can).

Replace the heavy square box by a round sonotube with a 3/4" MDF panel in the center and MDF top and bottom, and you have a lightweight sub that can be built in one afternoon without making much dust and costing less than $50 of material.

50598

Ruediger
03-26-2011, 10:09 AM
What kind of crossover are You thinking about?

Ruediger

DavidF
03-26-2011, 10:23 AM
I'm new to building speakers and alot of what I read seems Greek to me. I have 20 years experience with woodworking. I'd like to get a reccomended volume size in lay terms (ie... I need X amount of internal volume in cubic feet or inches ) for 2 different boxes I want to build. The first box will be for midbass and will house 2 altec 421-8lf drivers, the second box will be for low end bass I'd like it to go down into the mid 20 hz range and the driver will be the 18" eminence bass driver or jbl equilivant (depends on what I can find ) I don't have all the specs of the drivers (this is the Greek part for me) I would appreciate any help. I also don't have access to a computer most of the time I rely on my iPhone so web site reccomendations are welcome but not always practicle. Thanks in advance. Joe

Having the woodworking skills is a plus for DIY but so is some general knowledge of speaker design. You don't mention how you will approach the electric side of the project, ie crossover and final driver complement. If you are building a project around a known design is one approach. Pulling drivers together in your own custom systems is a whole different ball game. Many folks would like to help but most often unless someone has actually gone through a design process with your specific drivers, etc., we can only answer in general terms. If you are thinking along the lines of a custom system I recommend a publication such as Vance Dickason's Loudspeaker Design Cookbook to lay out some of the challenges and provide some ideas on how to approach solutions.

Ruediger
03-26-2011, 12:32 PM
I found TS params on the net. Altec 421-8LF: fs = 27.5 Hz, Vas = 20.34 cuft, Qt = 0.25

Thiele alignment #3:

Qt = 0.259
f3 = fs * 1.77 = 48.7 Hz
fb = f3 / 1.25 = 38.9 Hz
Vab = Vas / 4.46 = 4.56 cuft

ruediger

Joespride
03-26-2011, 01:27 PM
Thanks so much for the responses my project is not based on an available design but the components are basically common in several systems my plan with electronics is for the low end build a pair of sub boxes housing either jbl or eminence 18" drivers and use my active crossover (marshand xm9) set at 40 hz powered by behringer 1500, then have a second pair of cabinets holding 2 altec 421-8lf drivers in each use my marshand xm26 active crossover and cut this cabinet off at 300 to 350 hz powered by a muse model 160, on top of these I will use my altec 1003b horns with 291-16c drivers running full range powered by a pair of welborne labs drd 45 amps. I'm also thinking of running a tweeter like the jbl 2405 off the welborns using a cap to cut off at 7000 hz. Any more info needed. Joe

Ruediger
03-27-2011, 05:35 AM
I found TS params on the net. Altec 421-8LF: fs = 27.5 Hz, Vas = 20.34 cuft, Qt = 0.25

Thiele alignment #3:

Qt = 0.259
f3 = fs * 1.77 = 48.7 Hz
fb = f3 / 1.25 = 38.9 Hz
Vab = Vas / 4.46 = 4.56 cuft

ruediger

The total Q "Qt" is determined by the "generator resistance" seen by the loudspeaker. It is the sum of dc-resistance in the crossover series coil, cable resistance, and amplifier output impedance.

The total Q ("Qt") can be calculated as follows:

1/Qt = 1/Qa + 1/Qe [ Re / (Rg + Re) ]

The Q values for the Altec 421-8LF are: Qm = 2.75 (Qm is identical to Qa), Qe = 0.28.

The above equation tells us that the ideal Rg is 0.12 Ohm. If You are going active, then there is no crossover coil. If Your amp has a damping factor of 200, then it's output impedance is 8 Ohms / 200 = 0.04 Ohms. The rest is the resistance of normal loudspeaker cables. Bingo!

f3 = fs * 1.77 = 48.7 Hz <-- this is the -3dB frequency of Your system (box + speaker)
fb = f3 / 1.25 = 38.9 Hz <-- this is the tuning frequency for Your box
Vab = Vas / 4.46 = 4.56 cuft <-- this is the volume of Your box

The calculator to which Lee has set a link works for the tunnels of that particular company. If You calculate a tunnel with another program don't forget the end corrections. Ask for help in this forum if You don't know what that is.

Ruediger

Joespride
03-28-2011, 04:58 AM
Sorry for the spelling Ruediger. So the 4.56 cu feet volume is per driver I assume this means 9.12 cu feet for 2 drivers in 1 cabinet ??? Joe

Ruediger
03-28-2011, 05:17 AM
Sorry for the spelling Ruediger. So the 4.56 cu feet volume is per driver I assume this means 9.12 cu feet for 2 drivers in 1 cabinet ??? Joe

Yes, but why would You want to have two drivers in one cabinet? How do You want to arrange the drivers?

ruediger