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Tuning a cabinet to extend response beyond & below resonance of the driver

@mijostyn +2 @mahgister +1 @erik_squires I have thoroughly benefited from the thread of two weeks ago about making adjustments to the Altec speakers. It was a fascinating read and I am impressed with all the sharing of knowledge. I have an 18" driver (normally I don't go beyond 15", but this has a BL 31, so I figured I had adequate cone control), and maintains a SPL above 96dB between 100Hz and its Fs 38, which is my relevant range for this sub. The curious thing is that its free-air response is flat enough that it stays above 93dB beyond its Fs and down to 25Hz, and I would like to build a cabinet that takes advantage of this, a cabinet that can extend its response from 38 to 25. Its Vas calls for a 4.13 cu.ft. cabinet, and its compliance is fairly stiff with a Cms of 0.05mm/N. With its Qts right at 0.40 I am undecided between a ported and a sealed. It moves a lot of air with a Vd at 1286 cc, and of course, that's without much excursion, so I feel it should have full, solid and tight, punchy bass. So, my question is two fold: how can I build a cabinet to drive the response down to 25Hz? And if so, does the cone stiffness hinder or help, and how can I best take advantage of the driver's properties? Are there resources or apps for me to consider that would help me? Any help would be appreciated! 
hsbrock
hoosierinohio
162 posts
 

https://www.firstwatt.com/pdf/art_elpipeo.pdf  Some good reading on Nelson Pass El-Pipe-O.

mijostyn's avatar
mijostyn
8,570 posts
 

You always want to go with a sealed cabinet. You limit your low end reach by using a port. The trick is to lower the Q and control the driver by using a digital crossover with EQ capability Then you can make the driver do anything you want as long as you have the power to do it. I would make an enclosure around 6 cubic feet plus the volume of the driver. The problem is building an enclosure that does not resonate. Most enclosures are musical instruments. Large flat sides will resonate the most. With bracing, you just raise the frequency. The stiffest structure is a sphere, but a sphere is very difficult to work with. Next is a cylinder. You can easily place a driver in the end of a cylinder. You can get 20 - 24" pipe in any number of materials, the best would be aluminum. With a driver that large you want an enclosure that weighs at least 200 lb or you could place a driver in both ends and make a balanced force subwoofer. I made the cylinder with a decadron cross section using 1.5" plywood placing a 12" driver in both ends. 18" drivers would require a huge enclosure, double the volume of a single driver enclosure. My guess is that would be impractical. How you make the enclosure also depends on the shop capability you have available. 

Good luck. If you have any questions message me.  Mike

hsbrock OP
36 posts
 

I have followed your advice, except with a Bi-Horn cabinet to get the efficiency hi enough that the 24 dB Xover and 31-band EQ allows me adjust the Below-Fs response to keep it flat to 24. The cabinet weighs about 235 lbs. (Sits on casters), the two 12" drivers are used dipole with a 12" Tractrix horn in front and a rear folded Tractrix horn 6' 6", with the horn using the corners of the room. The drivers have a high Qes .81, a Qts .71, Cms .12. I had them built 4 ohm so I could wire them in series, giving them BL 17. They have an extreme Xmax capability of 23 mm, enabling the two 12" to move plenty of air, providing cushion to the low end, able to handle the EQ. With the combined effects of the 2 drivers on both right & left channels, dipole, with horns, it uses little power. Top cutoff is 120 Hz where AMT-Planars, 2 each side, handle the mids. The cabinets are 38" wide by 38" deep by 30" high, but that’s mostly due to the horns. The horns are heavy because the horns have 2" thick walls of solid MDF cut by laser, covered in walnut veneer. The size and weight do not include the upper dipole bi-horns of the midranges and tweeter, which are similar in outer size though the horn measurements are quite different. They give a spacious sound, tight, highly defined, sharp, but mellow. (Using a 300B tube amp.) I recognize that the big difference is the weight killing those resonances and the EQ. I’m thanking you for your help, and always curious/ interested in any feedback?

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